Comparing electron therapy with low energy photon therapy for the treatment of small superficial target areas

Introduction

A typical isodose distribution is presented in figure 1, showing the isodose lines from 90% to 10% of the maximum central axis dose. This includes all the typical features:

An almost uniform dose region from the surface of the skin to the therapeutic range,

A steep fall-off of dose beyond this depth.

This is fairly consistent across the central part of the field. However, nearer the edges of the field the penumbra is quite distorted. This is due to scatter. At the surface, the penumbra is quite narrow however with depth it spreads. Lower value isodose lines bulge outwards, causing significant overdosing outside the field edge. Respectively, the higher value lines draw in towards the middle of the field, accounting for the loss of dose from the electrons that scattered outwards. This results in patients being underdosed in the outer regions of treatment area and overdosed in the region surrounding the treatment area.

Furthermore, the width of the 90% isodose depends on beam energy and field size. As the field size reduces, the width of the uniform central portion of the isodose distribution reduces; until for small field sizes the two penumbral edges join to effectively exclude the central maximum dose region. This means no part of the treatment area receives a uniform

dose. Figure 2 illustrates the isodose curve for a small field. Similarly, any dimension that is particularly small will produce such a

distribution: For example small applicators, small shaped fields and even larger fields where one dimension is small enough.

The International Commission on Radiation Units and Measurements (ICRU 1993) recommends that at least 95% isodose should be used to cover the entire planned treatment area; however as is explained above in the case of electron therapy this would result in a large extent surrounding the treatment area also receiving a considerable dose. Obviously, this is far from ideal, either dangerous tissue is left underdosed or healthy tissue is exposed to a dangerously high dose.

Therefore, other techniques are now being explored. It is believed that, in the case of small superficial treatment areas, photons provide a much more uniform dose to the majority of the area

Figure 1: Typical isodose diagram, note the flat region in the centre and narrow penumbra.

Figure 2: Isodose diagram for a small field, note the narrow flat region in the centre and wide penumbra.

with a much narrower penumbra.

In this report I will investigate the differences between the use of 6MV electrons and low energy photons for small superficial treatments, focusing particularly on the width of the penumbra and the isodose received in the central region of the treatment area. Using ETB3 film to measure the dose rate at the treatment area and small lead cut outs to sculpt the various electron fields.

Investigation

Calibration

In order to calibrate the film, we exposed 2x2cm squares of film to various doses, keeping all other factors the same. In order to ensure the squares were exposed to the dose and scanned at the same orientation a small letter ‘F’ was printed in the top right hand corner of each piece before cutting, as in figure 3.

The films were then placed on an epoxy resin ‘solid water’ (water substitute) phantom to mimic the back scattering and other similar effects of treating a human body.

The films were then scanned using the Epson Scan software. (Note that the calibration of the film is only valid provided the time between exposure and scanning is the same as that for the calibration to within around half an

hour).

The scanned images were then analysed in ‘Image J’ software to find the pixel value in the centre of each piece of film, producing figure 4 the calibration curve for the superficial machine. The colour channels were split and only the red one used as it is highly dose dependent. Note the small difference in pixel value between the 3 hour and the 24 hour

calibration at high doses.

This could then be used to find the dose a film has received after measuring its pixel density, simply by drawing up to the line of best fit and tracing that back to the x-coordinate of that point as the dose. However a more accurate method would be to use a known equation for the relationship between dose and pixel density and then use our results to calculate the specific constants in that equation for our particular film and components. The equation relating dose to pixel density is as follows:

Figure 3: Scanned films vary in saturation having received different doses.

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Figure 4: Calibration data for superficial machine using ETB3

Dose (Gy )= cPixelValue−ab−Pixel value

(1)

This was rearranged in order to find the dose received on our experimental films from the measured pixel density as:

Pixel value=bDose+aDose+c

(2)

In order to find the specific constants for our set up we used the solver function in excel which minimised the square difference between the measured pixel values and the pixel values calculated by the above equation. This resulted in our final calibration conversion for the superficial machine:

Dose (Cy )=3.65960 . Pixel value−1616814977.62−Pixel value

(3)

This same method was then repeated for the 6MV electrons, producing a final calibration conversion for the linac:

Dose (Cy )=2.99928 . Pixel value−1320435547.16−Pixel value

(4)

Testing penumbra at different depths

Next we moved onto testing the penumbra of circular treatment areas. We used three different sized circular lead cut outs to test a range of field sizes. The set up was as displayed in figure 6. Again we used ‘solid water’ boards (made of epoxy resin) below the film to mimic the effects of the human body underneath the treatment area. However, we also placed these on top of the film to simulate treating an area slightly deeper into the body, in this case this was 5mm and 10mm deep. In the case of electrons we also use a 10mm board above the film to mimic the bolus that would be used on patients to achieve high dose at the surface. The films were then radiated for a minute each and scanned as in the calibration.

Initially we simply looked at how the central dose in each circle varied, by using the command Ctrl M to measure a central square portion avoiding any unusual marks on the film that may be scratches or dust. Below figure 7

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Figure 5: Calibration curve shows how well eq. 3 fits the measured pixel values.

Figure 6: Photo shows the lead cut-out positioned centrally over the piece of film which rests of the ‘solid water’ board.

shows the dose received in the central region of the treatment area. Both types of therapy abide by the depth dose theory: Increasing depth resulting in decreasing dose. However, it is clear that in the case of electron therapy the dose also drops at the surface as the field size decreases. Particularly, the 1.5cm diameter the central area isn’t receiving the recommended 95%+ dose.

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In order to calculate the dose as a percentage of applied dose, we first had to know the exact dose we had applied. For the electron therapy this was simple as the machine gives dose in monitor units and 1 Gy = 100 MU. However, for the superficial machine the operator simply chooses how long they want to apply the dose, this must therefore be calculated using the doserate. The equation used for calculating the output doserate for the superficial machine is as follows:

Figure 7: These graphs show the proportional dose received by the centre of the film as a percentage of the dose applied.

Output Doserate=Applicator Doserate× ¿

where BSF(CutOut) is the back-scatter factor for the lead cut out and equally the BSF(Applicator) is for the applicator attached to the head of the machine, SSD is the source to skin distance in cm for the applicator in use and SO is the stand-off in cm between the skin-surface and the plane through the base of the applicators.

We then moved onto focusing on penumbra. Figure 8 is the original scan of the films for the 1.5cm cut out at 10mm depth, for photons and electrons respectively. The more saturated the film, the greater dose it has received. The photon film appears to have a similar saturation over the whole treatment area, unlike the electron film which is gradually fading. And as figure 7 demonstrated, the saturation in the centre of the film is less than in the centre of the photon film, indicating less central dose.In order to find a quantitative gradient at the edge of the exposed region, we used Image J to produce a profile of the pixel value across the widest part of the circle. The pixel values were then converted to dose to produce figure 9 below.

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Figure 8: Original scans of the 1.5cm diameter cut out at 5mm deep.

Figure 9: Graphs illustrating the proportional dose received by the film as a percentage of dose applied across the profile of the treatment area.

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These graphs display the measured dose as a percentage of applied dose across the diameter of the film. There is a clear difference in the penumbra of the two films: With the electron dose gradually decreasing radially from the centre, as opposed to the photon treatment that has a much more uniform dose across the majority of the film and only drops suddenly at the edge of the treatment area. In addition to this with the smaller field, even the central region of the electron dose is below the recommended 95%, indicating that the entire treatment area would be underdosed, as in highlighted in figure 10. Furthermore, figure 10 also indicates the healthy regions beyond the treatment area that would receive unnecessary and potentially hazardous dose. (We can also use these graphs to verify the depth dose distributions we model our less superficial treatments on.)

Figure 10: The superposition of the electron therapy profile and the photon therapy profile highlight the misdosed regions.

Finally, in order to produce a clear diagram highlighting the areas of various dose directly over the treatment area, we used a macro math processing tool to convert our pixel densities directly into dose on our films and then convert this into percentage dose of

applied dose. This way we could see exactly which regions would

receive what dose as we would during the planning stage of treatment. Figure 11 displays these figures and highlights the difference in width of penumbra of the two forms of treatment. The lighter the shade of brown, the higher the dose. The electron film clearly shows a gradual stepping down of the dose as it nears the edges of the treatment area, as oppose to the photon film which is a uniform shade until the very edge of the treatment area. This also makes it clear just how much of an electron therapy treatment zone is underdosed.

Conclusions

In conclusion, I believe there is strong evidence to suggest that the photon emitting superficial treatment machine provides a more uniform and therefore more effective and safe treatment, than the electron emitting linac, when treating small superficial areas.

References

Electron Beams in Radiotherapy, Radiotherapy Physics, Medical Physics Dept, Nottingham City Hospital NHS Trust.

Radiation Measurement and Detection, Radiotherapy Physics, Medical Physics Dept, Nottingham City Hospital NHS Trust.

Gafchromic ETB3 film, Scan Handling Guide TSET Film Calibration & TSET Film Preparation and Scanning, Physics Equipment, Radiotherapy

Physics Dept Work Instruction, Oncology, Nottingham University Hospitals

Mayles, P., Nahum, A., Rsenward, J-C., Handbook of Radiotherapy Physics, 20.2.3, 24.3 34.3, 2007

Electron Treatments, Treatment Planning, Radiotherapy Physics Dept, Oncology, Nottingham University Hospitals

Superficial Unit Xstrahl 100, Dosimetry, Radiotherapy Physics Dept, Oncology, Nottingham University Hospitals

Figure 11: Films converted form pixel density to dose display the exact dose rates reaching each point of the treatment area, with photon therapy on the left and electron on the right.