Dosimetry using a CCD + scintillator system · 4.1.2 Quenching effects in the scintillation...

4

Dosimetr y usin g aCCD + scintillator s ystem

In this chapter we will discuss the development of an instrument for the purpose ofdosimetry and`W of radiotherapy with a scanningproton beam. In this irradiation tech-nique [103] a pencil beam of protons is moved by scanning magnets to cover the targetvolume (see also section 5.2.1) Although the advantage of this method is a larger flex-ibilit y in delivering complicated dose distributions, for example intensity modulateddose distributions, the much larger local dose rates require better beam monitoring andquality control. The instrument might also be of interest for application in dynamicX-ray treatments with intensity modulation.

Our instrument resembles a scintillating-screen basedelectronic portal imagingsystem[92] (jVía), but its purpose is in fact a modern equivalent to 2D film dosimetry.It consists of a scintillator screen, mounted perpendicular to the beam and observed byaWWa-camera via a mirror, see figure 4.1. Contrary to anjVía, our screen is directlyattached to the beam exit side of a stack of water equivalent phantom material. Theidea is that the 2D-dose distribution in a thin layer at the beam exit side of the phantomcan be determined from the observed light distribution from the scintillator screen ina relatively straightforward way. It should be noted that this is an essential differenceto methods which determine the exit dose from a portal image [22,37,72,73].

We have made a study of relevant scintillator properties. Differences and similari-ties with portal imaging systems using this technique are also discussed. We comparetheabsolute yieldand thesignal-to-noise ratiomeasurements with calculations. A de-tailed investigation of the dependence on ionization density (_.*_%, the energy lossper unit length of the beam particle), is presented, since this is important for the appli-cation in proton therapy. In this chapter the emphasis is on the dosimetric propertiesof the system, the measurements described in this chapter were therefore performedat passive beam modalities. In this way proper comparison with standard dosimetrydevices was possible. The application in a scanning proton beam will be presented inchapter 5.

84 Dosimetry using a CCD + scintillator system

4.1 Scintillator theor y

4.1.1 The use of scintillators in radiotherapy

Scintillators have been used in radiotherapy for applications such as portal imaging[78, 139], and brachytherapy dosimetry [104]. Our design objectives, however, differin a number of ways. First of all, as mentioned before, our screen is a part of the phan-tom. Furthermore, in portal imaging the largest emphasis is on high sensitivity, becausea small signal and small signal variations shouldgive an image with sufficient contrastin order to make anatomical details visible. In portal imaging a metal plate is usuallyattached to the scintillating screen to enhance the detection efficiency. High contrast isnot the main issue for our application. Rather the main emphasis is on dose measure-ments and sufficient spatial resolution under various circumstances such as differentdose-ratesand ionization densities.

Because of the similarity between the processes that result in light emission inthe scintillator and the radiation induced processes in tissue, scintillators have beenof interest for dosimetry since a long time [108]. A frequently encountered problemis caused by the change of scintillator properties due toirradiation damage. For rela-tive dosimetry this problem is less important, since the device has to be checked andcalibrated anyway.

4.1.2 Quenchin g effects in the scintillation processes

Basically there are two types of scintillators:organic andinorganic. In the inorganiccase the scintillator is available as a crystal or as powder, while organic (‘plastic’)scintillators can be obtained as sheets, fibers, liquids or blocks in any shape, which isof course ofgreat advantage.

CCD

max

.0.

3 m

1.7

m

0.5 m

0.5 m

mirror

2 m

virtualfocus

collimators

tissueequivalentphantommaterial

scintillatorscreen

leadshielding

protons / X-rays

Figure 4.1: Diagram of the setup, not to scale.

4.1 Scintillator theory 85

Materials scintillate when exposed to ionizing radiation because of the producedsecondary electrons. These electrons lose their energy by ionizing and exciting mole-cules along their path. A small fraction of the excited states decays via fluorescencede-excitations. The remainder of the energy is dissipatednon-radiatively. The ratiobetween the energy emitted as visible light and the total energy deposition is referredto as the scintillation efficiency 0. For dosimetry it is essential that the light outputis proportional to the energy deposited in the scintillator. Unfortunately this is notthe case for plastics: these suffer fromquenchingif the ionization density is high(_.*_% 1 MeV cm2 g3�). Several models [20,89,140] exist to explain the quench-ingprocess. Most models follow Birks [20] explanation, which is based on the assump-tion that along the ionization column caused by a particle, the density of the moleculesthat are ‘damaged’ is proportional to the particle’s energy loss_. in the column, withproportionality constantî. It is assumed that a fraction& of these ‘damaged’ moleculeswill dissipate this energy non-radiatively, which leads to quenching of the light yield.The following expression describes the light output energy _u per unit path length _%according to this model:

_u

_%'

0_._%

� n &î _._%

(4.1)

Because& andî cannot be determined separately they are usually treated together.A typical number for protons in the 30 - 200 MeV range is &î = 13.2÷ 2.5 mgcm32 MeV3� for NE102A plastic [12]. Although the model accounts for quenchingas a function of_.*_%, different&î values are reported for different proton energiesand thus for different_.*_% ranges [11,12,34].

For inorganic crystals the quenching model can also be applied [94]: the incomingionizing particle loses its energy in the formation of electrons and holes, a fraction ofwhich recombine promptly to form excitons(strongly coupled electron-hole pairs).The excitons diffuse through the scintillator until they are captured at either a dopingsite or at another unspecified trap. When the capture is at a doping site, this sitegetsexcited and may decay by emission of a light photon or by a non-radiative transition.Each of these sites can capture at most one exciton before decaying. This means thatsaturation occurs when the ionization density is so high that too many excitons areproduced. It also explains that crystal quality and surface treatment strongly affect theoverall scintillator properties, since they strongly determine the number of traps. Tomodel the quenching caused by the saturation a similar formula as for the plastics canbe used. Although the&î has a different physical meaning in this case, it facilitatesthe comparison of different scintillator types. It should be noted that also for quenchingin inorganic crystals, a wide range of parameterizations is found in the literature (e.g.[42,81,88,89]), none of which is accurate enough to cover a large_.*_% range.


Table 4.1:Physical properties of different scintillator screens (from [27,36,77] together with the thick-

ness of the scintillators we have used. Effective potential as used in Bethe-Bloch equation.

NE102A Gd5O5S:Tb CsI:Tl

density (g/cm6) ã 1.032 7.3 4.5

Zig /Aig 0.5 0.41 0.42

eff. excitationpotential (eV) L 6.5ü104 4.9ü105 5.5ü105

scintillation eff (%) % 6.3 18 9.2

light escape fraction (%) í 95 22 22

peak wavelength (nm) è 423 544 580

í$ energy perphoton (eV) .,ð}û| 2.29 2.24 2.14

light decay time (çs) á 0.0024 2.4 1

effective thickness (mg/cm2) | 25 33.7 45

4.1.3 Comparison of scintillator characteristics

We have compared 3 different scintillator types. We have chosen NE102A as an exam-ple of organic scintillators, since it’s widely used in nuclear physics and its propertiesare well known. An advantage of organic scintillators is that the physical properties arevery tissue-like, which makes the conversion to dose in water easier. However, in thiswork we concentrate on Gd2O2S:Tb, which is available as a powder attached to a sub-strate. Normally it is applied as animage intensifier screenfor diagnostic radiology andit is also used for portal imaging. Advantages are the relatively large signal due to thegood wavelength matching with the light-detector system and thegood homogeneityin commercial available screens (Lanex1). We have also tested the inorganic scintil-lator CsI:Tl, because it is reported [11] to have a linear energy dependence. CsI:Tl isavailable as a single crystal but also as a powder connected to a substrate2, which ismore interesting for our application. In table 4.1 we summarize the physical propertiesof these scintillators.

4.1.4 Dependence of scintillator spectrum on ionization density

An interestingquestion is whether the visible light spectrum as emitted by an inorganicscintillator (such as Gd2O2S:Tb) changes under influence of the ionization density.To study this an experiment was performed at the multidisciplinary beam line at theKVI, where 180 MeV protons are delivered by theB}Ni cyclotron. The light spec-trum was measured using a remotely controlled,jítt monochromator (wavelengthsbetween 300 and 600 nm) coupled to a photo-multiplier tube. The wavelengths arecalibrated using the emission lines from a helium lamp. We have measured in a re-gion with relatively low-ionization density (ä 6 MeV cm2 g3�) obtained by a 1 cmpolystyrene slab in front of the screen and in a region with a higher ionization density

1LanexRð, Eastman Kodak Company, Rochester NY, USA2CsI:Tl, Hilger Analytical, Kent, UK

4.1 Scintillator theory 87

wavelength (nm)

inte

nsity

(a.

u.)

500 525 5500.0001

0.01

0.1

at 1 cmin Bragg peak26% of Bragg peak

Figure 4.2: Visible light spectrum of Gs5O5S:Tb scintillator screen measured with LEISS spectrometer,

for 180 MeV protons at a depth of 1 cm polystyrene (solid line) and at a depth of 20 cm polystyrene

(dashed line) and at a depth of 20 cm divided by a constant factor (dotted line)

(ä 23 MeV cm2 g3�) obtained by putting 20 cm polystyrene in front of the screen(corresponding to the Bragg peak). The resulting spectra are shown in figure 4.2.

It can be seen that the only important difference between the measured spectra is aconstant factor, which corresponds to the peak-dose to entrance-dose ratio for 180 MeVprotons with 0.5 % initial energy spread: 3.8 (see section 2.1.6). Thus the light spectraemitted from a Gd2O2S:Tb scintillator are very much the same for ionization densitiesin the range of 6 - 23 MeV cm2 g3�. This indicates that the light production processesin this scintillator are not much affected by varying ionization densities.


4.2 Theoretical performance of the s ystem

4.2.1 Processes contributin g to yield

To compare the results of the experiments and to see if the response to protons differsfrom the response to X-rays we will calculate the theoreticalyield of our system. Inorder to compare our results to calculations onjVía’s (e.g. [5,93,106]), we will followa similar procedure. A quantitative summary of the different steps in the detectionprocess isgiven in table 4.2.

1. The first step in the chain from ionizingparticle toWWa read-out is theenergy lossof the ionizing particle in the scintillator. In the case ofX-rays this is describedby thephotoelectric effect, Compton scatteringandpair-production, of whichthe Compton effect dominates for 6MV X-rays3. In order to maintain the electronequilibrium (this means an equal amount of electrons in and out the detector) thescintillator has to be much thinner than the range of the Compton electrons. Italso is necessary to put phantom material both before and behind the scintillator.TheBragg-Gray cavity principleapplies in this case (see section 2.2.3) and theabsorbed dose can be calculated from the ratio of the electron stopping powersin the scintillator and in water, which we calculated using VûN|WNju4.

For theproton case the energy loss is for the largest part caused by Coulombinteractions and described by the Bethe-Bloch equation (see section 2.1.2). Fora small part protons also haveinelastic nuclearinteractions (see section 2.1.5).With respect to theconversionof dose in water to dose in the scintillator, thisfact can be neglected, since the scintillator thickness is small and the number ofnuclear interactions in the scintillator is small.

Since the average range of the secondary electrons (7.B = 35 keV for 180 MeVprotons, corresponding to a range of 2.3ü103ô g/cm2 in water) is much smallerthan the total thickness of the scintillator (0.1g/cm2) the Bragg-Gray princi-ple applies also for the protons (see section 2.2.3). The dose in the screen canbe calculated using the energy dependent ratio of proton stopping power in thescintillator and in water (see table 2.1 in section 2.1.2).

The backscatter of protons is less than 0.6 % (section 3.5.4), so that it is notnecessary to put phantom material behind the scintillator. For both protons andX-rays the absorbed dose scales with the reduced thickness| (g/cm2) of the lightproducing component in the scintillator (without the plastic substrate, see alsosection 3.5.4).

2. The next step is the conversion of the energy of the secondary electrons tovisiblelight. This applies both for protons and X-rays, but since the ionization density of

3Bremsstrahlung produced by 6 MeV electrons that stop in a tungsten target.4PHOTCOEF, AIC Software 1993

4.2 Theoretical performance of the system 89

protons is larger (in the range 6-50 MeV cm2 g3�), the probability of quenchingis larger for protons. Moreover the energy spectrum of the secondary electronsis different for protons compared to X-rays. Both effects may affect theglobalquantity ‘scintillator efficiency’ 0, which is usually defined for a specific typeof radiation. Despite of these differences, no values of0 have been reported forprotons, so we have used values quoted in the literature [27, 36, 77, 106], all ofthat have been determined with X-rays. The uncertainties quoted in table 4.2reflect the spreading of reported values. The energy of the light photons E,ð}û| iscalculated using the wavelength in table 4.1.

3. From the visible light that is produced, a fractionl will escapefrom the scin-tillator. Because NE102A is transparent, with refractive index 1.581, we assumethe fraction here to be 95 % (Fresnell reflection). For Gd2O2S:Tb and CsI:Tl itis more complicated, since the light can bescatteredmany times before it leavesthe screen. In [106] a model is derived for the transport of optical photons, whichusesFresnell reflection and refraction at the binder-phosphor boundary. Whenwe extrapolate this model to the phosphor thickness we have used, we find thatan optical photon fraction of 22÷ 4 % will leave the scintillator.

4. Theangular distributionof the photons that leave the scintillator is important be-cause it effects the number of photons that will reach theWWa. Because NE102Ais transparent the distribution will be more or less isotropic (anisotropy fac-tor w=1). For non-transparent scintillators aLambertiandistribution is assumedwhich is much more peaked in forward direction (anisotropy factorw=4). How-ever some significant deviations have been reported for several different phos-phor types [47], which result iné 8 % spread inw.

5. The optical photons have to betransportedto the CCD through an optical sys-tem. When the screen is imaged by a lens, with magnification factor6, the op-tical solid angle / yields:

/ '

éZ

e

62

8 2E� n6ä2

è�

eZ(4.2)

In our system we used a large aperture lens (8 /1.3) with a focal length s of50 mm. In the experiments we have limited the lens aperture using a diaphragm,with 8 stop numbers (defined as focal length/aperture diameter) varying from8 = 2 to8 = 5.6.

Due to the relatively large distance from the screen to the camera (typically2.2 m) the maximum angle of the light photons with the optical axis is approx-imately 2.5á. Thereforevignetting, that is the decrease of light collection at theedges of the image, is estimated to beé 0.4 %, which can be neglected here.


6. A part of the light is absorbed in the lens of the CCD camera. We have used atransmission factor that is quoted for a similar system in [5]:u=0.9.

7. In a CCD pixel the conversion of optical photons into electrons isgiven by thequantum efficiency. This efficiency is wavelength dependent, which results inscintillator dependent values, as shown in table 4.2.

Another characteristic of especially inorganic scintillators is that the light emissioncontinues after the ionizing radiation has stopped. The short component of thisafter-glow is integrated during the measurement (integration time usually 30 sec.). As longas the light decay time is independent from the intensity, and this can be expected fromthe atomic physics processes that play a role, then even in the case of rapidly chang-ing scanning beams the light production remains proportional to the absorbed dose.The long component can play a role when the shutter is closed before all light has de-cayed. We have verified experimentally that an eventual long component (ô : � sec.),integrated over 30 sec., is less than 1 % of the integrated signal.

The parameters can be separated in scintillator dependent values, listed in table 4.2and parameters that depend on the experimental setup, such as optical efficiency andpixel area. These are listed in table 4.3. The quoteduncertaintiesare mainly causedby the optical light production processes, the scintillator efficiency and the number oflight photons that escape. The parameter which is most limiting, however, is the opticalefficiency. In section 4.5.1 we discuss to which extent this needs further improvement.

The theoreticalyield, expressed in ADU (the unit of the number readout by theCCD) Gy3� pixel3�, can now be calculated with the numbers in table 4.2 and 4.3 andusing equation (4.3):

yield =ErtUð?| cçäf*R ü | ü 0.*ð}ü|

lwu# ü / ü øTð ü ùMð?

ùi*iUADU Gy-1 pixel-1 (4.3)

where+vtUð?|>z,[@sc |u0.*ð}ü|

c lc wc # are scintillator dependent parameters (listed in ta-ble 4.2),u the lens transmission (=0.9),/ the optical efficiency (listed in table 4.3),øTð the area on the scintillator that corresponds to a single pixel on the CCD chip(listed in table 4.3),ùMð? is the number of pixels that are added before transferring theoutput to the data acquisition,ùi*iU the number of electrons collected for a responseof 1 ADU in a single pixel (= 2.5 according to the CCD specification). Unlessexplic-itly stated in a different way, we have used throughout this thesis a binning factor ofùMð? ' e pixels. This means that from now on wherever the wordpixel is used, infact the binned contents of 4 pixels on the CCD chip are meant. In table 4.4 the resultsof equation (4.3) aregiven for the different experimental setups and tested scintillatorcombinations.

4.2.2 Processes contributin g to spatial resolution

The conversion of absorbed dose to aWWa signal can be distorted by several processes.One type of distortion operates independently on the individual pixel level (described


Table 4.2:Comparison of the scintillator-dependent parameters. The values forw> î andH*ð}ü| can be

found in table 4.1. The source of the other numbers is described in the text. Expressed uncertainties are

1 â


1a ratio scintillator/H5O +vtUð?|>z,[ 0.97÷0.03 0.92÷0.03 0.89÷0.03

dose forX-rays

1b ratio scintillator/ H5O +vtUð?|>z,s 0.98÷0.03 0.59÷0.02 0.59÷0.02

dose forprotons

2 light photons w ü %@ 3.4÷0.3ü1045 1.7÷0.1ü1046 1.2÷0.1ü1046

/cm5/Gy in scint H*ð}ü|

3 escape fraction í 0.95÷0.04 0.22÷0.01 0.22÷0.01

4 angular anisotropy light ë 1.0÷0.05 4.0÷0.3 4.0÷0.3

6 transmittance of the lensu 0.9 0.9 0.9

7 quantum eff. CCD ì 0.050÷0.001 0.36÷0.01 0.40÷0.01

Table 4.3:Step 5: efficiencies of the optical system (calculated with equation (4.2) ) used in the different

setups and the area on the scintillator screen that corresponds to 1 single pixel on the CCD chip

eff. of the opt. system$ pixel areaDsl{ (cm5 (1 pixel)3�)AZG March 1996 1.31ü10ý9 2.27ü10ý6

Louvain April 1996 5.16ü10ý9 2.30ü10ý6

Louvain June 1997 1.19ü10ý9 2.50ü10ý6

Uppsala May 1996 8.64ü10ý: 1.75ü10ý6

Uppsala Nov. 1997 7.88ü10ý: 1.92ü10ý6

PSI Nov. 1996 3.41ü10ý: 1.77ü10ý6

PSI May 1997 2.12ü10ý9 1.39ü10ý6

Table 4.4:Comparison of the estimated yields for the different experimental setups for various scintil-

lator types. The yields are calculated using equation (4.3) and expressed in ADU Gyý4 (pixel)ý4, of

which ADU is the unit of the 15 bits number as readout by the CCD electronics. 1 ADU is equivalent to

ä 2.5 electrons collected charge in a single pixel.Qelq = 4. The quoted uncertainties are 1â.


AZG March 1996 - 2.2÷0.2ü107 -Louvain April 1996 - 5.5÷0.6ü107 -Louvain June 1997 - 1.4÷0.1ü107 -Uppsala May 1996 - 6.9÷0.7ü106 5.0÷0.4ü106

Uppsala Nov. 1997 - 7.0÷0.7ü106 -PSI Nov. 1996 1.5÷0.1ü106 2.8÷0.3ü107 -PSI May 1997 - 1.4÷0.1ü107 -


in section 4.2.3), while the other type has spatial correlations between the signals of thedifferent pixels. The latter contributes to the ‘point spread function’ (Vtu) of the systemwhich is responsible for inducing a dependence between thegeometrical shape of thedose distribution and the magnitude of the signal in a single pixel. Several processescontribute to theVtu.

First the optical photons as produced in the scintillator will be emitted in differentdirections and leave the scintillator at different positions. This causes the first spread-ing and it is strongly dependent on the optical properties of the scintillator (transpar-ent, emulsion or crystalline powder). In the case of X-rays the optical photons have topass theV66B phantom material that is necessary for theelectron equilibrium(seesection 2.2.3). Subsequently they are reflected by the mirror and scattered by the lens-diaphragm system of the camera, which also contributes to theVtu. In [60] it is shownthat the shape of theVtu can be described as a sum of twoGaussiansof differentwidths. The narrow Gaussian has the largest amplitude and is often referred to as ‘spa-tial resolution’. The broad Gaussian can have a width of a few cm and it is causedby light scattering within the screen andV66B. Also light the scatters back from themirror to the screen plays a role. The relative amplitude of this Gaussian is usually notmore than a few percent, but since we are dealing with a two-dimensional function, itsintegral can be a large fraction of theVtu integral. Therefore its impact can be ratherlarge.

The effect of theVtu û on the response} can be modelled by taking the convolutionof the inputs with theVtu û [68]:

}E%c +ä ' ûE%c +äó sE%c +ä ì] "

3"

] "

3"ûE%ý %âc + ý +âäsE%âc +âä_%â_+â (4.4)

For calculating this convolution we can use the convolution theorem [68], whichstates that the convolution in the spatial domain corresponds to a multiplication in theFourier domain:

}E%c +ä ' I3�EIEûE%c +ää ü IEsE%c +äää (4.5)

whereI denotes the Fourier transformation, andI3� the inverse. As an example wehave calculated in one dimension using this equation the effect of only a single Gauss-ianVtu with j=20 on a rectangular input field with varying widths. The result is illus-trated in figure 4.3.

It shows that for smaller fields the maximum response also decreases. This causesa dependence on the response of the field size.

For screen + camera basedjVía’s it is known that theVtu is not invariant of theposition on the screen. Especially there is a dependence on the distance to the mirror,which is usually mounted at an edge of the screen. Light photons can be reflected backto the screen and then emitted from the screen again. The chance that this happensdecreases with the distance of the light emission point to the screen edge where themirror has been connected. Contrary to portal imaging systems, we have placed the


0 100 200 300 400 500 600 700 8000

10

20

30

40

50

60

70

80

90

100

distance along profile (pixels)

rela

tive

resp

onse

(%

)

Figure 4.3: Illustration of the effect of point spreading on the response. The point spread function is

assumed to be a Gaussian with a widthâ of 20 pixels (uçû6 = 47). The width of the (rectangular)

input field varied from 10 pixels to 80 pixels in steps of 10, followed by a field with a width of 210 and

500 pixels.

mirror further away at 0.5 m distance from the screen, so that the possibility that opticalphotons are reflected back to the screen is negligible small. With this arrangement, itis expected that theVtu can be regarded to be invariant over the screen. Furthermorethe contribution of the broad component of theVtu will be much smaller.

It is possible to determine theVtu û experimentally by measuring the response toa point source or a line source, for example a very small collimator [87]. After makinga fit û can be parameterized and used for thedeconvolutionof the response} [60,68]to the original field s :

sE%c +ä ' I3�EIE}E%c +ää*IEûE%c +äää (4.6)

However, due to dynamic range limitations, it is difficult to measure the broad compo-nent with sufficient accuracy in this way. TheVtu û can also be determined from thedependence of theyield as function of the fieldsize [60,80]. TheVtu û is assumed tobe a sum of a narrow Gaussian and a broad Gaussian with a certain amplitude ratio.Using this assumption, the field size dependence can be calculated numerically withequation (4.5) and fitted to the experimentally observed field size dependence.

It should be noted that, even if theVtu can be determined accurately, the deconvo-lution process is very sensitive tonoise. Deconvolution always amplifies noise compo-nents, because the Fourier transform of the response} has high frequency compounds(because of the noise) which are not divided out by the Fourier transform of theVtu û.


Table 4.5:Numerical calculation of the SNR per pixel for Gd5O5S:Tb with t=0.034 g/cm5 and a dose

of 1 Gy of 177 MeV protons. The steps correspond to the steps in the tables 4.2 and 4.3. The uncertainty

(1 â) is due to the uncertainties insf,Qs andQt

step Gd5O5S:Tb

Qs 1 1.46ü109

Qt 2 6.83ü109

sf 3-6 3.4ü10ý<

QsQtsf+4ý sf, (detection term) 3.4ü107

s5fQsQ5t (source term) 1.1ü103

SNR 1.8÷0.1ü105

Although filteringof the high frequencies in the image may facilitate the deconvolutionprocess, much care must be taken to preserve the original dose information (which isrelated to the absolute pixel content). Especially if complex shaped fields are applied,a fixed cut-off frequency would locally reduce the signal. Thereforeadaptive filteringtechniques(e.g. Wiener filtering [68]) should be used. However, also for Wiener fil-tering the signal to noise ratio has a large impact on the deconvolution process. Thusa lot of attention has to be paid to minimization of the noise as well as to the deconvo-lution/filtering method if deconvolution is necessary.

4.2.3 Processes contributin g to si gnal-to-noise ratio

The noise component that acts onindividual pixels results influctuationsbetweenpixel output signals that are exposed to the same input signal. However there are 2separate processes leading to this fluctuation. First there is the statistical variation inthe energy deposition process, which is a inherent element of the dose distribution (seesection 2.1.3). In addition to this there is noise introduced by the measurement itself.In order to know if and where further optimizations are useful, we need to calculatethe expected contribution to thesignal-to-noise ratio(tAi) for the different steps inthe measurement process. This will alsoyield the minimum dose that can be measuredaccurately, which determines for instance whether it will be possible to measure onesingle spot in the spot scanning system (see section 5.4.3).

The real energy deposition{. is determined by the number of ionizing particlesùR that have an interaction with 1 cm2 of the scintillator, each depositing B.:

{. ' B. üùR (4.7)

For both X-rays and protons the spread in{. is given by the Poisson distribution:

j{. 'jB.sùR

(4.8)

Assuming that 177 MeV protons have an energy loss of 6 MeV cm2 g3� at a depth of1 cm polystyrene, a homogeneous dose of 1 Gy corresponds toä 1.05ü109 protons per


cm2. This has to be multiplied by øTð (the area on the scintillator that corresponds to1 pixel) to obtain the number of protons per pixel: 1.46ü106. For such a large numberofùR the spread in{E will be negligible. Each of these ionizing particles will producea number of optical photonsù^ (step 2) that will also be Poisson distributed:

ù^ 'B. ü 0.,ð}û|

(4.9)

.,ð}û| is the energy per optical photon and0 is the scintillation efficiency (see sec-tion 4.2.1). For the cascaded process the expectation value of the process isùR ü ù^and it can be shown [112] that the variance is:

j2R^ ' ùRù2^ nùRù^ (4.10)

Note that the variance is not symmetric forùR andù^, as it would be in the case ofindependent processes. The light photons that are produced in the scintillator have aprobability RS of being detected by theWWa camera (step 3-6):

RS ' l ü w ü / ü # ü u (4.11)

The outcome of the detection process (true or false) is distributed according to thebinomial distribution. The expectation value of detectingùS particles is:

ùS ' RSùRù^ (4.12)

with variance (using general error propagating rules):

j2S ' ùRù^RSE�ý RSä n R2SùRù2^ n R

2SùRù^ (4.13)

Now one can consider two extremes: for large detection probabilitiesjS will be dom-inated mainly by second terms (the Poisson source term) while for small detectionprobabilities also the first term (the binomial detection term) will be dominant. In ta-ble 4.5 the results of numerical calculations can be seen for 177 MeV protons andGd2O2S:Tb. For our case the binomial distribution will be dominant, which meansthat the signal-to-noise ratiotAi is:

7ù- 'ùSjS

'sRSùRù^ (4.14)

After the charge has been collected on theWWa, the signal has to be amplified andconverted to a digital value. This inevitably introduces ‘readout noise’ . It can be deter-mined by lookingat the statistical variations in the image obtained with a closed shutterof the camera. Its magnitude isä 11 electrons (1j) collected charge, independent fromthe signal, which is small compared to the calculated SNR.


4.3 Experimental methods

4.3.1 Experimental setup

Figure 4.1 shows a schematic picture of the setup. The setup has been designed suchthat it allows easy transport to other sites. For measurements at different depths thethickness of phantom material at the beam entrance side of the scintillator has beenvaried from zero up to the proton range by inserting polystyrene slabs. To determinethe actual dose at the screen position we have used calibrated plane parallel ionizationchambers mounted to the distal side of the screen. Compared to portal imagingsystemsthere are two differences: our mirror is mounted further away from the scintillator,because we observed distortions due to light which scatters back from the mirror to thescintillator (see section 4.2.2). Also there is no need for a metal plate backing of thescintillator screen. We use alow dark currentCCD camera5 (dark currentä 2 electrons/ minute at room temperature) which makes long integration times (1-5 min.) possible.TheWWa-chip has 768 x 512 (unbinned) pixels. The optical magnification factor for thetypical distancesgiven in figure 4.1, isä 0.02, so that 1 (unbinned) pixel on theWWa(9x9>m2) corresponds to 0.5x0.5 mm2 on the scintillator. Thedynamic rangeof theWWa signal of 1 pixel is 2�D = 32768 ADU, which is the unit of the number as read outby theWWa electronics. 1 ADU corresponds with 2.5 electron collected charge. As thespatial resolution was no limiting factor in our case, we have used 2û2 binning for themeasurements, which means that the contents of 4 pixels are added before transferringit. As already mentioned before in section 4.2.1, throughout this thesis the termpixelmeans pixel in the acquired image (which is in fact the sum of 4 pixels on the CCDchip) is meant. Therefore we used 384 x 256 pixel images, with a pixel correspondingtoä 1 mm2 on the scintillator screen. The actual data-acquisition was performed withanítB 16 bits interface card connected to aníO6êVW compatible laptop computer.

The properties of our system have been determined in experiments with well-calibrated but passively scattered proton beams. The experiments with 175 MeV pro-tons have been performed at the The Svedberg laboratory in Uppsala. The clinicalproton beam line uses passivedouble foil scattering[55] for the lateral beam spread-ing and apropeller wheelfor depth modulation. The distance from the virtual focusto the screen was 2.5 m. To investigate the effects of a sharper Bragg peak and as a re-sult a larger concentration of_.*_% values at the end of the tracks, also experimentshave been carried out in Louvain-la-Neuve with 80 MeV proton beams. It has a pas-sivescatter foil/annulus occluder system[32] for the spreading in the lateral direction,and a propeller wheel for depth modulation. Here the virtual focus distance was 1.5 m.Since it is interesting to relate the measured protonyields to X-ray yields, because thatmakes comparison with standard dosimetry equipment easier, we also measured with6 MV X-rays from a linear accelerator at the University Hospital Groningen. Because

5Hi-SIS 24 with Kodak KAF-0400 CCD chip, Lambert Instruments BV, Leutingewolde, TheNetherlands

4.3 Experimental methods 97

of the desired electron equilibrium and the X-ray backscatter it was necessary in thiscase, to put 1 cm transparent phantom material (V66B) at the beam exit side of thescintillator.

4.3.2 Back ground si gnals

In addition to the random processes as discussed in section 4.2.3 there are severalother sources thatgive rise to undesired signals, but usually one can correct for thesecontributions.

First of all there is theWWa dark current which depends on theWWa temperature.For the low dark current camera we used, this can be neglected. A much larger sourceof background, however, is the ambient light. Although a lot of effort was put into lightshielding, some remnant contribution always remained. This background contributioncan be determined by measuring with the same integration time as the actual measure-ment, but without radiation. Its size is typically 1000 ADU (3 % of theWWa dynamicrange) and it was subtracted from the measurements.

Non-primary ionizing radiation such as X-rays, ò’s or neutrons produced in thecollimator, can have a direct interaction with theWWa chip. This results in large signalsin isolated pixels (‘spikes’). Their density is proportional to the delivered dose, and aprocedure for correcting these spikes is presented in section 4.3.3.

It should be noted thatV66B, which has also been used as phantom material, scin-tillates. We have observed a contribution up to 10 % of the total response for X-raysand for protons. For protons we have replaced the phantom material before the scin-tillator by polystyrene, which as we have observed, does not scintillate. However forthe X-ray measurementsV66B was used as phantom material at the beam exit side,since it had to be transparent: we have measured theV66B contribution separatelyand subtracted it from the actual measurement.

4.3.3 Back ground correction

For the constant components of the background, it is easy to correct the contents ofeach pixel by subtraction. The stochastic spike component can be decreased consid-erably by shielding theWWa camera, but it will not disappear completely. Especiallyduring the X-ray experiments the number of spikes was large. Therefore it was neces-sary to develop a correction algorithm [80]. The algorithm makes use of the stochasticnature of the spikes, i.e. the probability of a spike at the same position in a secondmeasurement is very low. To apply the spike correction two measurements are takensequentially, each one normalized to the delivered dose and corrected for the offsetcaused by the constant components. Then a ‘radiation spike map’ is obtained by tak-ing the absolute value of the pixelwise subtraction of both images. The corrected imageis then constructed from the ‘spike map’ and both source pictures by setting pixel val-ues according to the following rule:


lateral position ( pixelnumber )

repo

nse

(arb

. un

it)

50 100 150 200 250 300 350

Figure 4.4: Profile of a X-ray field with (continuous line) and without background correction (dashed

lines, see text for explanation). The profiles are plotted with different offsets and 1 pixel corresponds

with 0.9 mm

î If the absolute value of the pixel content in the ‘spike map’ islarger than a certainthreshold, it is assumed to be a spike and the minimum of the correspondingpixels in the source pictures is used.

î If it is smaller it is assumed to be undisturbed signal and the average of the twopictures is taken.

In figure 4.4 two raw and the correspondingcorrected profiles can be seen. It showsthat the overall level of the response is not affected by this correction. The remainingfluctuations are either caused by the noise processes as described in section 4.2.3 orwhen two spikes are accidentally at the same position.

4.3.4 Quantitative analysis of the ima ge set

We have used two methods to obtain the light yield from the image sets. The firstmethod determines the meanyield in a ‘Region Of Interest’ (iNí), which has to be de-fined in an area without largegradients. The area of theiNíwas chosen to be 36 pixelsä 36 mm2.

When the spikes are suppressed it is also possible tointegrate the intensity overthe whole screen. The advantage of the measurement of the total screenyield is thatthe result is independent from the spreading by theVtu and also inhomogeneities inthe field will have less effect in this way.

We define thedose-responseat a certain field size as the slope of theWWa out-put reading as a function of the delivered dose in water at the screen position. Dose-response curves have been measured with different field sizes (varying from 3û3 cm2

to 15û15 cm2) and_.*_% values (in the range of 4 - 50 MeV cm2 g3�).

4.3 Experimental methods 99

4.3.5 Determination of &î

We have investigated the ionization density _.*_% dependence by measuring the re-sponse of the system as a function of depth for monoenergetic proton beams. The depthwas varied by inserting a variable amount of polystyrene slabs. The dose at the screenposition was determined with a plane-parallel ionization chamber mounted at the distalside of the screen. The entrance value of the reference ionization chamber measure-ment was normalized to a dose per flux in MeV cm2 g3� using theV|iBA/analyticalmodel entrance value (see section 3.2). The dose per flux for other depths is then takento be proportional with the ionization chamber signal. We neglect here the flux loss dueto nuclear interactions. The_.*_% values depend on the particle spectrum and thuson the depth. For the calculation of&î we have averaged the_.*_% values. We haveused theV|iBA/analytical model value in spite of the fact that these models simplifythe effect of nuclear secondaries (see section 3.4), because they contain the most up todate stopping powers.

After having normalized the reference ionization chamber signal in this way, it ispossible to determine the&î value for the scintillator. First the peak-to-entrance ratioof the reference ionization chamber signalå.hiu and the peak-to-entrance ratio of thescintillator signal å.tUð?| are determined. Equation (4.1) applied at the depth of theentrance and peakyields: ((Ti@! '

ý_._%

üTi@!

, the dose per flux in the Bragg peak and

(i?| 'ý_._%

üi?|

, the dose per flux in the entrance, both in MeV cm2 g3�):

å.tUð?| '

ý_u_%

üTi@!ý

_u_%

üi?|

'E� n &î ü(i?|ä

E� n &î ü(Ti@!ä

(Ti@!

(i?|(4.15)

from this kB can be solved (å.hiu ' (Ti@!*(i?| ):

&î 'å.hiu ý å.tUð?|

å.tUð?| ü(Ti@! ý å.hiu ü(i?|(4.16)

The entrance value of the scintillator signal can now be normalized by applyingequation (4.1) to the (normalized) entrance ionization chamber value with the&î fromequation (4.16).


4.4 Results

4.4.1 Dose response

In order to investigate thelinearity of the relation between delivered X-ray dose inwater andWWa output, the totalyield has been measured for different doses, but with afixed field size of 15û15 cm2 and at a fixed depth of 3 cm (i.e. at dose maximum). Forthe 175 MeV proton measurements at Uppsala theyield was measured at 1 cm depthin aB =10 cm field and the absorbed dose in H2O was determined with a calibratedABWVêh4 ionization chamber. The measurements have been performed with aconstantintegration timeof 30 s. TheWWa yield was determined in aiNí in the centre of thefield, while the reference dose was determined with a calibrated Farmer type ionizationchamber.

The difference between a linear fit and the results with Gd2O2S:Tb are shown infigure 4.5. Both for X-rays and protons this relation turned out to be linear within 3 %.In addition to Gd2O2S:Tb this applied also for CsI:Tl.

In addition to this measurements described in section 4.3.1, we have also performedmeasurements at the spot scanning gantry of PSI, Switzerland. In chapter 5 the specialproblems involved for spot scanningare described, but in order to facilitate comparisonthe results for theyield are already included here. In table 4.6 the results for scintillatormaterial Gd2O2S:Tb are shown, while the results for NE102A and CsI:Tl are shownin table 4.7.

The theoretical value is without taking into account the quenching. The result oftaking the quenching into account is discussed in section 4.5.1. The reason that thetheoretical values are different is that the efficiency of the optical system varied be-tween the different systems because of different screen-camera distances and differentdiaphragm settings (see table 4.4).

To determine the signal-to-noise ratiotAi for a single pixel, it is either possible tolook at the variations intime of the response of a pixel, or at the distribution of pixelvalues at different positions in a single image which have a constant input, under theassumption that the sensitivity variations along the CCD chip are smaller than the noiseyou want to observe. Because it is very difficult to obtain a low-intensity light sourcethat has a constant output, we have used the single image method. For the image wehave taken a measurement performed at the PSI spot scanning gantry, for which thescreen was positioned at a depth of 1 cm. The dose at the position of the screen was1.4 Gy (see also figure 5.7 in section 5.4.3). The result is shown in figure 4.6, togetherwith the frequency histogram of the pixel values. It can be seen that the distributionof pixel values in a homogeneous image has indeed a Gaussian shape, which meansthat the peak position and width of the distribution can be determined by fitting: themean pixel value is 1.59ü10e ÷ 0.01 with aj of 114÷ 2 ADU (uçû6 = 268). Thiswidth corresponds to a noise percentage of 0.71 %ì 0.01 Gy and atAi of 1.4ü102

for 1.4 Gy. This is in the same order of magnitude as the calculated value 1.8ü102 (see

4.4 Results 101

Table 4.6:Summary of the measured response of Gd5O5S:Tb compared with theoretical yield (see ta-

ble 4.4). Units are ADU Gyý4 pixelý4. In the 3rd column the difference between the theoretical yield

and measured response is shown. In the last column the reproducability for the same beam parameters

is shown.

measured theoretical differ. reprod.

(%) (%)

AZG March 1996 6 MV X-rays 2.33ü107 2.2÷0.2ü107 -6 -

Louvain April 1996 80 MeVprotons 5.44ü107 5.5÷0.6ü107 0.5 h 2.5

Louvain June 1997 1.33ü107 1.4÷0.1ü107 3.0 fUppsala May 1996 175 MeVprotons 6.20ü106 6.9÷0.7ü106 12 h 3

Uppsala Nov. 1997 6.40ü106 7.0÷0.7ü106 9 fPSI Nov. 1996 177 MeVprotons 2.44ü107 2.8÷0.3ü107 14 h 1

PSI May 1997 spot scanning 1.17ü107 1.4÷0.1ü107 15 f

Table 4.7:Summary of the measured response of CsI:Tl and NE102A compared with theoretical yield

(see table 4.4). Units are ADU Gyý4 pixelý4

measured theoretical difference (%)

theor./ meas.

Uppsala May 1996 CsI:Tl 3.60ü106 4.97ü106 38

PSI Nov. 1996 NE102A 1.13ü106 1.47ü106 30

applied dose (Gy)0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

diff

eren

ce w

ith li

near

fit

(%)

-4

-2

0

2

4 X-rays AZGprotons Uppsala

Figure 4.5: Difference between a linear fit and the 6 MV X-ray and 175 MeV proton response (Uppsala

beam) as a function of dose.


U (

mm

)

CC

D light yield (A

DU

)

T (mm) pixel value (ADU)

20 60 100 140

20

60

100

140

0

4000

8000

12000

16000

15000 16000 170000

4

8

12

16

% o

f pi

xels

Figure 4.6: The pixel value distribution (diamonds) from a homogeneous dose distribution (as in fig-

ure 5.8A) together with a Gaussian fit (solid line).

table 4.5).FromjVía applications it is known [60] that Gd2O2S:Tb screens havegood radia-

tion hardness properties for X-rays. Comparison of a screen that has receivedä 900 Gy(50 % protons and 50 % photons) with a new screen also did not show any observabledifferences in sensitivity.

To investigate if a non-homogeneous dose distribution alsoyields a linear dose -response relationship, we have measured the transversal dose distribution of a X-raywedge field at 3 cm depth and compared that with an ionization chamber measurementin a watertank. The ionization chamber used was an IC-10 Wellhöfer ionization cham-ber (which has a 3 mm radius) and the measurement was done at 1.5 cm depth. Bothprofiles have been normalized to the 100 % level. The result is shown in figure 4.7.The curves agree reasonably well, apart from the low dose (÷ 20 %) region, wherethere is deviation which may have to do with the different response of the screen andionization chamber to low-energy, scattered photons/electrons.

4.4.2 Spatial response

Although the slopes in the dose distribution in figure 4.7 already give some indicationof the spatial resolution, we also have used photographic films to compare lateral pro-files of aB=10 cm field from the 80 MeV proton beam. The measurements were takenat 9 mm depth. The 80 % / 20 % penumbra of the film was 3.5 mm, which we assumeto be caused by the actual physical dose penumbra. The 80 % / 20 % penumbra mea-sured with the CCD system was 4.6 mm. The spatial resolution for a large field in thefirst order can be described by thej of the GaussianVtu (point spread function). Sincethe 80% / 20% penumbra is equal to a constantä ü j (ä = 2

s2ihu3�EféSä ä 1.68,

where erf3�(x) is the inverse of the error function:2IZ

U %fe3|

2_|), this means that the

spatial resolution isgiven by the quadratic difference of the two penumbrae divided by

4.4 Results 103

lateral position (cm)

rela

tive

resp

onse

(%

)

-15 -10 -5 0 5 10 150

50

100

150

200ionization chamberscintillator

Figure 4.7: Profile of a X-ray wedge field measured with IC-10 Wellhöfer ionization chamber (courtesy

of A.A. van ’t Veld, Groningen) and a Gd5O5S:Tb scintillator measurement both normalized to the 100

% level.

the constantä: 1.8 mm. For this experiment the pixel size corresponded to an area of0.92 mm2 on the screen (see table 4.3). This means that the pixel size has a significantcontribution to the spatial resolution, therefore in case a better spatial resolution is re-quired (for example for very small fields used in radiobiology) this can be improvedby using a 1x1 binning (yielding a pixel size of 0.23 mm2). For the purpose of spotscanning (with relatively large penumbrae), however, the spatial resolution of 1.8 mmturned out to be sufficient.

The dose-response as a function of the field size has been examined both for X-raysand for protons. For X-rays the result is shown in figure 4.9. The field size is definedby the position of lead collimatorjaws. The actual dose at the screen position has beendetermined for each field size using a calibrated Farmer type ionization chamber, sothat we can correct for the field size dependency of the output. TheWWa response hasbeen scaled with this value. It can be seen that the response integrated over the screenhas no systematic field size dependence, while theiNí (region-of-interest) responsedecreases for small fields. This can be explained as being due to the effect of aVtu.Using the fit procedure from section 4.2.2 we obtained the followingparameters for theVtu: small Gaussian: widthj=2.8 mm, broad Gaussian: widthj=42.4 mm. Amplituderatio small vs. broad Gaussian: 1400:1. The result of the fit has also been plotted infigure 4.9.

To investigate the proton dose-response dependence on the field size we have mea-sured circular fields of the 80 MeV proton beam at a phantom depth of 9 mm. Tochange the field size we used different final collimators. For the determination of theactual dose at the screen position a 0.01 cc ionization chamber6 in the center of thefield was used. We observed that the dose in the center of field was strongly dependent

6Mini A-150 thimble ionization chamber by A.N. Schreuder, NAC South Africa


distance along profile (cm)

rela

tive

dose

(%

)

0 5 10 150

20

40

60

80

100

measured with film

measured with CCD

Figure 4.8: Lateral profile for a 80 MeV, B=10 cm proton beam measured at 1 cm depth with film

(dashed line) and CCD system (solid line).

Field size ( cm 2)

rela

tive

resp

onse

(%

)

0 50 100 150 200 2500

80

85

90

95

100

105

ROI Integral 100 % PSF fit

Figure 4.9: X-ray measurement of screen response as a function of field size. Yield measurements in a

region-of-interest in the center of the image (circles) and yield measurements integrated over the screen

(squares) are plotted together with calculated field size dependence (dotted line).

Field size ( cm 2)0 10 20 30 40 50 60 70

rela

tive

resp

onse

(%

)

0

80

85

90

95

100

105

ROI 100 %

Figure 4.10:Proton measurement of the CCD response in a region-of-interest as a function of field size.

4.4 Results 105

Table 4.8:Results of the determination ofnE using the procedure of 4.3.5 Units are mg cmý5 MeVý4

nE (mg cmý5MeVý4)

Louvain Gd5O5S:Tb 6.2÷0.2

Uppsala Gd5O5S:Tb 5.4÷0.2

Uppsala CsI:Tl 36÷4

PSI Gd5O5S:Tb 4.1÷0.2

PSI NE102A 9÷2

on the field size: a decrease of 12 % for aB=2 cm field was measured. The normalizedresult of the screen divided by the ionization chamber signal is shown in figure 4.10.Clearly for protons there is no field size dependence for fieldsè 3 cm2. This differencewith X-rays is due to the fact that for protons theV66B absorber behind the screen isomitted, as will be discussed in section 4.5.2.

4.4.3 Ionization density dependence

We have investigated the ionization density _.*_% dependence by measuring the re-sponse of the system as a function of depth for monoenergetic proton beams. The depthwas varied by inserting a variable amount of polystyrene slabs. The dose at the screenposition was determined with a plane-parallel ionization chamber mounted at the dis-tal side of the screen. We also measured the depth-dose dependence in an energy-modulated beam (depositing a ‘spread out Bragg peak’), to determine the effect ofquenching in clinically relevant dose distributions. Figure 4.11 shows the results of amonoenergetic beam, measured with both the Gd2O2S:Tb (175 and 80 MeV protons)and CsI:Tl (175 MeV protons) scintillators.

The values for&î, calculated with the procedure described in section 4.3.5, arelisted in table 4.8. The results of the PSI experiment (section 5.3) are also includedhere.

An observation that can be made in figure 4.11, especially in the Louvain case be-cause of the scale, is that the scintillator curve is slightly translated with respect to thereference curve. Since the reference ionization chamber was mounted at the distal sideof the screen, the amount of translation corresponds to the effective thickness of thescintillator screen: 0.98 mm polystyreneì 0.105g/cm2. In figure 4.11 the correctedionization chamber curve (dotted line) has been translated with this amount with re-spect to the original ionization chamber curve (solid line), to allow comparison withthe CCD data (diamonds).

In figure 4.12 the depth-dose profile of a clinically used Spread Out Bragg Peakcan be seen, together with the diode measurements. The translation of the scintillatorsignal with respect to the ionization chamber signal is here also visible. In this case themaximum deviation occurs at the distal edge and isé 8 % for the Uppsala beam andé 10 % for the Louvain beam.


polystyrene thickness for a and b (mm) do

se p

er f

luen

ce

( M

eV c

m

2

/g)

polystyrene thickness for c (mm)

0 50 100 150 200

0

5

10

15

20

0

5

10

15

20

0 10 20 30 40 500

10

20

30

40

a

b

c

175 MeV Gd2 O2

S:Tb

175 MeV CsI:Tl

80 MeV Gd 2 O 2 S:Tb

Figure 4.11:175 MeV proton depth-dose distribution in polystyrene measured with NACP-02 ionization

chamber operated at HV=300 V (continuous line), CCD measurement with Gd5O5S:Tb scintillator

(diamonds) and the quenching formula (4.1) applied to the NACP-02 data withnE as fitting parameter

(dotted line). (b) idem for CsI:Tl scintillator. (c) with 80 MeV protons and Gd5O5S:Tb.

4.5 Discussion 107

It is also possible to measure the depth-dose relation in one image. It can be doneby tilting the watertank so that protons exit the tank at the tilted side. The scintilla-tor has been attached to this side so that one can make an oblique image without amirror. The resulting image is shown in figure 4.13 together with a horizontal profile.There are several reasons that this curve looks different from the depth dose curvesin figure 4.11: one reason is due to the inverse square fall-off with depth caused bythe divergence of the beam, which does contribute to this measurement but not to themeasurement in figure 4.11, since those measurements have been performed at a fixeddepth. Another reason is that the transversal beam profile is not completely flat, asshown in figure 4.13. The difference between the edges and centre isä 5 %, whichcorresponds to the initial decrease of the depth-dose curve.

4.5 Discussion

4.5.1 Yield and noise

To test the reproducibility of the system, it is useful to compare the calculated andmeasuredyields for each experiment separately. Due to the large contribution of un-certainties in scintillation efficiency 0 and light escape probability l the ranges of thecalculatedyield values in table 4.4 are relatively large (ä 10 %). The experimentallydeterminedyields shown in table 4.6 are surprisingly close to the calculated valuesusing equation (4.3) (summarized in table 4.4). The ratios calculated/measured valuefor the same beam parameters is reproducible within 3 %, which indicates that the sys-tem is quite stable and robust. One of the reasons for the differences between differentbeam qualities might be the quenching. We calculated the quenching correction (4.1)to the theoretical expectedyield with parameters from table 4.8. The results are shownin table 4.9. The differences between measured and calculatedyield decrease in somecases by applying the quench correction, but remain for the PSI beam still larger thanthe 10 % uncertainty in the theoretical value.

We found atAi in the same order of magnitude as calculated (table 4.5). This

Table 4.9:Effect of applying the quench correction (4.1) on the theoretical yield (from table 4.4) com-

pared to the measured results (from table 4.6). Units are ADU Gyý4 pixelý4

measured quench entrance theoretical diff.

correction (%) quench corrected measured

Louvain April 1996 5.44ü107 6.2 5.1ü107 -5.7

Louvain June 1997 1.33ü107 6.2 1.3ü107 -3.3

Uppsala May 1996 6.20ü106 3.1 6.7ü106 8.5

Uppsala Nov. 1997 6.40ü106 3.1 6.8ü106 5.4

PSI Nov. 1996 2.44ü107 2.6 2.7ü107 11

PSI May 1997 1.17ü107 2.6 1.3ü107 12


depth (mm)

0 50 100 150 200re

lativ

e en

ergy

dep

ositi

on (

%)

0

20

40

60

80

100

NACP measurementLanex measurement

depth (mm)

0 10 20 30 40 50

rela

tive

ener

gy d

epos

ition

(%

)

0

20

40

60

80

100

NACP measurementLanex measurement

80 MeV 175 MeV

Figure 4.12: Measurement of a Spread Out Bragg Peak in a polystyrene medium with 80 and 175 MeV

protons using a NACP-02 ionization chamber and Lanex - CCD system.

20 40 60 80 100 120 140 160 180 200 220

20

40

60

x (mm)

y (m

m)

0 50 100 150 2000

5000

10000

15000

water

tank

scintillatorbeam

collimators

CC

D

profile

x (mm)

CC

D y

ield

(A

DU

)

Figure 4.13: Image obtained with a tilted phantom shows the depth-dose distribution with the purpose

of range verification.

4.5 Discussion 109

implies that the model as used in section 4.2.3 can be used to evaluate the performance.To improve thetAi (=

sRSùRù^ ) one can increase the number of interacting particles

ùR, the number of produced light photonsù^ or the detection probability RS. The effectfor protons differs from X-rays. For protons it is possible to increaseù^ by increasingthe scintillator thickness and thusB.. For agiven dose however,ùR is fixed. For X-rays increasing the scintillator thickness will cause an increase ofùR, whileù^ willremain almost constant due to the long range of the secondary electrons. Thisjustifiesuse of metal screens for X-rays to improve thetAi in portal imaging. On the otherhand the disadvantage of increasing the effective scintillator thickness is that the depthresolution decreases, which affects the shape of the measured Bragg peak. Also thelight-escape probability l will decrease [106]. Furthermore it is expected that also thespatial resolution willget worse, because of light scattering in the scintillator. The bestmethod to improve thetAi is to enlarge the detection probability RS (i.e. l, /, or #)However, because the quantum efficiency # and the photon escape probability l arealready relatively close to unity, a substantial increase in this way is not possible. Animage intensifier, which in effect compensates for a poor#, will thus notgive largeimprovements of thetAi in our case. Important improvements can be expected byincreasing the optical efficiency /, e.g. by using a larger lens apertures or a smallerdistance to the screen (in which case additional shielding is needed, to avoid an increaseof spikes) . In our case, however, we were limited by the dynamic range of the system,since a larger lens aperture saturated theWWa for dose ranges of 1-2 Gy. In principleone could increase thetAi by increasing the optical efficiency while decreasing thesensitivity of theWWa chip.

In the spot-scanning application, it will be important to detect irregularities in thescanning pattern. To estimate the minimum possible dose variations that can be de-tected with sufficient certainty, we can use the experimentally obtainedtAi per pixel:1.4ü102, corresponding to a noise width of 0.7 % (1j). This means that variations largerthan 1.4 % (2j) can detected with 95 % certainty. Since an input dose distribution willgive a response to a large number of pixels simultaneously, there will be a correlationbetween the pixel values. This correlation can be used to increase the sensitivity. Fora homogeneous dose distribution the noise will reduce with the

snumber of pixels.

For the application in spot scanning, this yields in case of a typical spot area of 100pixels an increase in sensitivity of a factor 10. This will be discussed in more detail insection 5.4.

4.5.2 Spatial response

For dose verifications in irregular and inhomogeneous dose distributions such as pos-sible in spot scanning, it is important to know the location of extrema, high gradientsand possible artifacts. This makes spatial resolution an important parameter. It is char-acterized by the width of the narrow Gaussian of theVtu.

In the case of X-rays V66B phantom material is needed behind the scintillator


screen, which causes a severe point spreading effect. This introduces a field size de-pendence soiNí measurements wouldyield large errors, if not corrected for. Whenthe response is integrated over the screen, the field size dependence disappears, whichconfirms that the light is spread over the image by theVtu. It is especially the contri-bution of the broad component of theVtu which reduces the local light yield in smallfields. Although its amplitude is very small compared to the narrow Gaussian, its (2-D)integral is 16 % of theVtu integral due to the long tail. Because of the small amplitudeof the broad component, it is not possible to measure theVtu directly from a sharpdose distribution within the dynamical range of theWWa chip (2�D). The quality of thefit shown in figure 4.9 is very sensitive to the width of the broad component. Since thenarrow component mainly affects theiNí-responses below 3 cm2, the estimate of itsj was not very reliable as estimation of the ‘position resolution’. Therefore we usedthe value derived from the proton measurements with film: 1.8 mm.

For applications with X-rays the deconvolution of the images with theVtu re-quires further development of a reliable procedure. Studies on methods such as adap-tive Wiener filtering or wavelet transforms are in progress [80]. In case deconvolutionwould not work, an alternative approach to verify a dose distribution may be to com-pare the measurement with a precalculated dose distribution, which has been convo-luted with aVtu.

For protons there isgood agreement between the small ionization chamber mea-surement and the screen response. This confirms the expectation that there is no largecontribution to theVtu from the light spread in the scintillator screen since it is nottransparent. Only in the smallestB =1 cm field there is a deviation, probably becausefor this field size the ionization chamber measurement is not valid anymore.

4.5.3 Ionization density dependence

The decrease in light signal compared to a reference ionization chamber measurementas a function of depth is caused by a number of processes (see also section 3.4):

î integration effects in the detector (section 2.2)

î change of stopping power ratio with depth (section 2.2.3)

î light quenchingcaused by change in light production efficiency due to increasingionization density (section 4.1.2),

Although equation (4.1) was derived for the last effect, our measurements showthat it fits the experimental data for agiven set of beam parameters with quitegood ac-curacy, yielding an effective constant&î that accounts for all these processes together.The&î value found for the 175 MeV Uppsala datayields a signal decrease of 8 %in the Bragg peak. This value corresponds with the results of the simulation of stop-ping power and integration effects in section 3.5.4, indicating that the light quenchingcomponent is very small.

4.5 Discussion 111

The effective&î value for Gd2O2S:Tb is smaller than the quoted experimentalvalue on NE102A in literature [12]. The effective&î value for CsI:Tl was much largerthan the other values, but it should be noted that the sample we obtained was very smalland very inhomogeneous, which made the analysis quite difficult.

For 80 MeV the decrease of the signal in the Bragg peak is 19 %. This larger sig-nal decrease might indicate that the light quenching component does play a role forthese low-energy beams. This can be qualitatively understood in the following way:for beams with a lower initial energy, the energy straggling in the Bragg peak is less,which means that the Bragg peak is concentrated over a smaller depth region. Conse-quently the ionization density will be higher (see also figure 3.17 in section 3.5.3). Inprinciple it is possible to simulate also the light quenching component with MC codes,however there is still disagreement on the physical theories behind quenching [89].

To correct for the depth dependent quenching of the signal, knowledge of the beamquality (energy and energy spread) is needed to use the measurements for dosimetricverifications. Checks on the beam energy (spread) can be performed easily by measur-ing the range (figure 4.11) and the peak-to-entrance ratio of the depth-dose curve mea-sured with an unmodulated beam. However, the measurement in the spread-out Braggpeak (figure 4.12) shows that the quenching occurs mainly at the most distal part of thedepth dose curve, due to the increasing contribution of large _.*_%’s (Bragg peaks)with depth. If the measured depth-dose curve is normalized at the entrance dose andno assumptions on the beam energy or any quenching are made, the maximum erroris 8 % for 175 MeV protons and it occurs at the most distal part of the dose distribu-tion. In principle this can be corrected for by applying the quench correction (4.1) tothe individual proton beams that make up the total dose distribution. From this pro-cedureisolight contours in a treatment planning system can be calculated instead ofthe isodose contours, allowing direct comparison between treatment plans and CCDmeasurements.


4.6 Conclusions

We have shown that light measurements from a Gd2O2S:Tb (commercially sold underthe name Lanex) scintillating screen, mounted at the distal side of a stack of phantommaterial, is a useful tool for quality control of proton beams. Due to thegood sig-nal to noise ratio small doses can be measured accurately and it allows detection of1.4 % errors in the dose distribution. A full 3D measurement, separated for differentbeam energies (with typical numbers 10 energies, 10 depths, 30 seconds measurementtime),yields a total measuring time of less than 1 hour. It is possible to perform a quick2D-measurement of the absorbed dose in a phantom, but because of quenching in thescintillator and point spread effects of the optical system, off-line corrections are neces-sary. Since the quenching is a monotonic function of the depth, it is possible to correctfor the quenching effect if the beam energy is known for each spot. However, for aclinically relevant spread-out Bragg peak, this correction is only about 8 % at the mostdistal part of the plateau in the spread-out Bragg peak. For checks where this deviationis known and can be accepted, the measuring time equals the time of the dose delivery.For measurements in proton beams theVtu effect seems sufficiently low to neglectthe field size dependence. so that, even without applying the off-line corrections, thedevice allows a quick and reliable verification of the correctness of a dose distribution.

The method shown in figure 4.13 allows a quick verification of the range and thusof the beam energy.

We expect no serious problems from long-term irradiation damage, since experi-ences from portal imaging systems support our observation, that degrading of the lightoutput due to irradiation damage is negligible. However, possible effects from largerdoses with proton beams should be investigated.

Due to the time-integration capabilities of theWWa camera it can be expected thatthis system is especially useful for dosimetric verification at dynamic treatment modal-ities, such as scanning proton beams and intensity modulated photon treatments.

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