E. Pedroni Paul Scherrer Institute SWITZERLAND PSI...

E. Pedroni CPT - Paul Scherrer Institute - Erice 20 -04-2009

PSI

25.01.2009

Session: delivery systems and gantries

E. Pedroni

Paul Scherrer Institute

SWITZERLAND


First presentation: Scholz GSI

• Biological treatment planning (Local Effect Model L EM)

• Conformal scanning provides variable modulation of the range

– this gives laterally-varying mixtures of LET and fragmentation depth-profiles

– Need to adjust physical dose to obtain an homogeneous equivalent dose

• Model constituents

– Alpha-beta model response to X-ray doses

– Statistical fluctuations of the microscopic dose around the single ion tracks

– Average of dose effect give the effective RBE at a point

– Biological data were presented, which confirm the method

• The LEM model is a big step forward for ion therapy

– The established standard for ion therapy in the context of scanning beams

– With scanning one can obtain a more homogeneous equivalent dose than with scattering

• Why not starting using this model for protons as well ?


• Ballistic properties of ions are superior to protons• Comment: for discussion later

– Gain in healthy tissue sparing (in terms of dose distribution)

• Step going from photons-to-protons = factor 2.5

• Step going from protons-to-carbon = factor 1.2-1.3


Second presentation: Pedroni PSI

In room

Sliding CT

New proton

scanning

gantry for

treating

moving targets

New gantry 2


• Second generation scanning gantry – based on Gantry 1 ex perience

– Combination of proton treatments with a sliding CT

• For image guided proton therapy

– Option to use BEV X-ray simultaneous to proton beam delivery

• For QA of moving targets

– System designed for developing very fast dose painting

• Spots – lines –contours ( > 1 cm/ms)

• Repainting and gating

– Fast energy changes (80 ms)

– Fast 2d parallel scanning

– Simulated parallel scattering with variable range modulation

– The complement to this talk was given in another session by David Meer


• Link to other sessions on scanning advancements

– An excellent presentations on scanning and scattering was given by J. Flanz

– Impressive: the work on advanced developments on scanning with the NIRS synchrotron

in Chiba by Dr. Noda

• Synchrotrons can provide complex delivery patterns during slow extraction


45° dipoles

90° dipole

scannermagnets

treatmentroom

absorber

Third presentation: Weinrich GSI Gantry optics


The new point of reference for ion gantries

• Ion optical properties of the ion gantry at GSI

– The first ion gantry of the world – quite impressive to see the system rotating (video)

• Major problem encountered: cabling drum (a solvable problem)

• Description of the ion optical parameters

– Tolerances were very stringent - what has been achieved is probably OK

– Coupling of vertical and horizontal plane in order to achieve a round beam at the

isocenter

• By equalizing the divergence of the beam at gantry entrance

– Corrections as a function of the gantry angle

– Parameterization for 5 different spot sizes

– Commissioning work: A lot of work – in principle: no essential differences to the

commissioning work for a proton gantry

• Link:

– the very informative HIT presentation of T. Haberer in another session


Fourth presentation: Trbojevic BNL

FFAG permanent magnets gantry

Workshop on Hadron Beam Therapy of Cancer, Erice 9

78o

r=2.71 m

h1=1.58 m

13 cells - 25 cells

150o

h2=2.42 m

Orbits magnified10 times

From a density of the No-Fe-B 11.7 gr/cm3

The weight of the whole gantry ~ 500 kg.

(Eberhard Keil)

h3=4.29 m


• FFAG light weight gantry – static solution – no power consumption– Only 500 kg structure (effective bending radius of 1.7 nm) for protons

– Alternating permanent magnets (Halbach round cells of 18 cm)

– Repeated very strong focusing over very short distances (60 cm)

• A gantry dealing with any energy [70, 250 MeV] • and with any time structures of the injected beam

– Scanning can be made very fast in all 3 dimensions;

• in range and laterally at wish

• Ideal to track organ motion laterally and in range - equally quick

• Solution for ions (superconducting windings) – superconducting windings around a pipe – homogeneous resultant field

– 1500 kg

• Pictorial description– The “fiberglass cable” for charged particle beam?

• When the first magnet prototype?


• A New Tracking Gantry-Synchrotron Idea

– Basis of the new idea is use of the following:

• simple, synchrotron and gantry magnet lattices (small aperture)

• series connection of magnets for 5 Hz tracking

• one power supply for main ring/gantry magnets

– Scanning idea (H and C)

• Scan full length tumour on a line in depth at each 5 Hz cycle

– stripping foils for wide energy-range extraction ( I(t) within a pulse)

• Extract slowly beam while accelerating

• the full beam current is delivered in every scanning cycle

• Transverse motion and depth-scans in subsequent cycles

• Fast scans for least effect of tumour-movement (10x10x10 in 20 s) 80s?

• Scanning started upstream of triplet?

Fifth presentation: Rees, RAL, UK.


Conceptual gantry design for C 6+& H+ ions

(300°)

~10 m

H+

C6+

small aperture

low rf volts

5 Hz rings

foils

~5 m

2 nA (aver), 250 MeV H +

0.1 pnA, 400 MeV/u C 6+

10 x 10 x 10 cm 3 tumourscanned in ~20 secondswith a dose of 2.5 Grays(for one gantry position)

C = 43.68 m & 49.92 m

H¯

C4+

(small apertures)

(elliptically shaped central support )


“CONTROVERSIAL POINTS” FOR DISCUSSION


TOPIC 1: TYPE OF PARTICLE


Rationale for using protons

• Low LET

– Same radiobiology as photons (fractionation to protect vital tissues within the target)

– Good for treating large targets

• Advantage: avoid the dose bath of the photons outside the target …

– Not so good for small tumors (AVMs > 2-3 cm)

• Due to MCS in the Bragg peak –> less good lateral dose fall-off

– Carbon has a better ballistic property

• Needs to be compared with Gamma knife etc.

• The decision to use protons can be decided solely on the comparison of the protons- with the photons dose distribution pla ns

– Decision within treatment planning

• In principle: protons always superior to photons (a question of costs)

– Gain: lower integral dose outside the target: by a factor of 2 to 5

• A question of cure, complications (and quality of life after surviving cancer)


Rationale for using carbon ions

• Arguments in favor of carbon

– The high LET of heavy ions should be an advantage for treating

• Radio-resistant tumors

• Poorly oxygenated tumors (OER oxygen enhancement ratio)

– Better precision of the beam - Less MCS - Sharper Bragg peaks (He – Li –C )

• But with fractionation tails – Dosage uncertainties due to the varying RBE (LEM)


X-rays

neutrons

X-rays

neutrons

• Difference between high-LET and low-LET is expressed at low doses

– At high doses the differences tend to disappear

– Should one then uses carbon with low doses (high fractionation)?


Possible disadvantages of high LET

• Too simplistic

– Carbon is better because is “more effective” (higher RBE)

• What matters is the ratio of surviving healthy cells to tumor cells (target and plateau)

– Therapeutic ratio T

– Potentiation with fractionation Tn use protons if T> 1 (within target)

• High LET can be a contra-indication

– Absence of repair of radiation damage in the healthy tissues within the target

• important for large tumors (cases infiltrating vital structures)

– Risk of late effects (important for pediatric treatments)

• Poor experience with neutrons (and pions) in the 80’s

– Bladder shrinking after 2 years pion therapy delivery at PSI

• Higher magnetic rigidity of the beam (x3)

– Size of facility doubled -> Costs doubled (huge gantry or no gantry)

– Needs substantial investments in radiobiology (should be the main interest)


Controversial – a too simple statement

• Carbon therapy “needs” few fractions – is therefore less expensive

– Nothing forbids to deliver protons with a low number of (high dose) fractions

• In fact

– Good experience with small tumors

• radio-surgery

• eye treatments (melanomas)

• At a very high dose per fraction protons behave similarly to high-LET radiation

• Repair of radiation damage in the cell is suppressed

• Double Strand Breaks gets enhanced

– If we deliver carbon ions with only a few fractions of high doses we loose the beneficial

effect of having low LET in the plateau region of the beam

• We shoudl not sell the argument twice


Research issues for ion therapy

• Boost therapy?

– Carbon boost followed by photons (IMRT) ?

– Why not carbon ions followed by proton therapy? (the best possible boost treatments)

• But HIMAC has shown very good results with carbon ion s

– With hypo-fractionation (lung tumors)

• Comparison with protons is however missing

• May by there is really something new there - when using high LET with ions

• Helium

– As the low-LET radiation source for highest precision?

• We have learned from A. Brahme

– Today we have to look also at the new molecular radiobiology endpoints

• Lithium and others light ions – best for apoptosis within the Bragg peak region

• We certainly need much more research with ions and protons!


TOPIC 2: GANTRY or NO GANTRY?


The rationale for using a gantry ?

• To apply several fields (“beam incidences”) typically 3 (1-3)

– To distribute the plateau dose over several tissues (in the same session)

– To stay below organ tolerance

• Due to the Bragg peak protons need a lower number of fields as photons (3-9)

• With the patient treated in supine position

– To keep the position of internal organs unchanged (soft tissues in the body)

• Same position as at the time of CT-data-taking for treatment planning

– For best comfort of the patient

• To offer maximum flexibility of choosing the beam di rections

– Avoid beam going through sensitive organs

– Avoid beam going through complex density heterogeneities in the patient body

• Dose errors due to interplay of MCS and range

– Shadows at interfaces parallel to the beam (bones and metal implants)

– Select possibly angles with low ranges (keep the energy – plateau dose length - small)


• Disadvantages ?

– None - A gantry can do more than an horizontal beam line

– With a gantry we can deliver many fields (2-3) in sequence without entering the room

(and without changing the room)

• But it implies additional costs

• For protons: the established standard solution

– Which is the proper mixture of gantries and horizontal beam lines?

– How many horizontal beam lines do we need, if we don’t treat mainly prostate? eyes?

– If we choose a few fixed angles

• How severe are the logistics problems?

• For carbon facilities

– If too bulky…

– Why not having few dedicated proton gantries in the facility?

– To avoid suboptimal treatments with protons (Medaustron poster)


• If you use a gantry;

– Don’t forget space for new future in-room diagnostics

– Leave enough space in the room to adapt your system to the future developments

coming from conventional therapy

• CT

• CT-PET?

• Out-of-room positioning with MRI?

– (Position reproducibility?)


• If you use a gantry for carbon ions and protons togethe r

– You should optimize the nozzle first for the protons

• The most sensitive in terms of beam quality (MCS)

– To obtain a small enough beam size


TOPIC 3: SCANNING


Why scanning ?

• Best way to automates treatment

– Multiple fields in one go (avoid changes of individualized hardware) (gantry)

• Flexibility to deliver non-homogeneous dose distribut ions

– IMPT ( = simultaneous optimization of dose fields) – compete with IMRT

– Biological targeting (image guided proton-ion therapy)

• Variable modulation of the range (conformity)• Best use of the beam

– Less neutrons (~ factor of 5?)

– Less activation

• Better dose fall-off (in theory)

– Dose edge enhancement with sparse scanning

– Collimation as an option

• The only major problem ; organ motion sensitivity

– Fast scanning helps


Do we need to have scattering as well?

• Dose precision

– Scanning alone is superior for most deep seated tumors

– Scattering is in practice better for lower ranges (but not necessarily simpler – Optis 2)

– Scanning with optional collimation is superior everywhere

• More equivalent to single scattering than double scattering

• Best strategy?

– Build facility for scattering and then add scanning …

• Doubling of the efforts …

– Or rather build a facility for scanning and then just simulate scattering …?

• Simulated scattering (very fast - uniform scanning)

– With high repainting number

– Parallel scattering no dose errors from compensators

– Variable modulation of the range shrinking shape of iso-energy-layers

– Less neutron dose


Organ motion errors with scanning

• Disturbance of the lateral dose fall-off(same problem scattering and scanning)

• Add safety margins

• Reduce with Gating or Tracking

• Disturbance of the dose homogeneity

– Scattering – highly repainted - insensitive

– Single painted scanning - very sensitive

– Repainted scanning

• Alone – for medium motion

• With Gating or Tracking – for large motion

• The experience of treating moving targets with scanning is still inexistent

– WE HAVE TO LEARN HOW TO COPE WITH

THAT


Choice of delivery techniques matters

Fast energy change Slow energy change

- G2 lines, scaled - G2 spots, iso-layer - G2 spots, scaled - G1 spots, scaled

• data points: Median of the RMS spectrum• shaded area upper boundary: 75 % of the dose distributions are above.• shaded area lower boundary: 25 % of the dose distributions are below.


Step 1 - Repainting (repainting strategies)

• Scaled repainting

• Repeat same scan n times with dose divided by n

• Layered repainting

• Deliver partial scans with constant max. partial dose

• Revisit only those spots which need more dose

• Using spots or lines or contours

beam

0 % missing dose25 % missing dose

75 % missing dose50 % missing dose

100 % missing dose

Break possible synchronism of scanning and respiration periods


Step 2 - Gating + Repainting

• Treatment of moving targets

– Which gating signal should we use?

• breath holding ?

• external signal (like thoracic strip ?)

• 3-d stereo viewing of external marks on patient skin?

• Whole energy layer delivered within a gate? (200 ms pe r plane)

Pla

n1

Beam

Target at times ti

Iso-energylayers E at ti

E

Residual motion?

Desynchronize?


Step 3 - Tracking?

Nodule

Rib

Lung

• Lateral shift applied to the beam as a function of t he target displacement

– Given for example by BEV X-ray images

• Problem:

– Density heterogeneities in the beam path

• example bone in front of a lung tumor

– Does tracking need the inclusion of dynamic range corrections?

• Probably - See thesis work of C. Bert at GSI

Beam


Step 3.bis - Why not “Multi-gating”?

• Associate several time- gates with corresponding gated-CT plans – Deliver multiple time-instances treatment plans simultaneously

• Full energy layer of a plan is delivered within its ow n gate– A better alternative to tracking?

• No need for energy corrections - No target motion-deformation model needed

• Repainted by definition – Fast scanning - Good duty factor of using of the beam

t1 t2

Pla

n1

Pla

n3

Pla

n2

Beam

Target at times ti

Iso-energylayers E at ti

E

t3


• If the previous methods don’t work …

– Use scattering as a backup solution for treating moving organs

• Simulation on the scanning nozzle

• Fast uniform scanning is at the moment an important issue (“safety net”)

Step 0 - Simulated scattering


TOPIC 4: ACCELERATOR TYPE


Specifications for the accelerator ?

• Small pencil beam (larger beam are easy to produce)– Good lateral and distal dose fall-off – 3 mm sigma

– Absolute beam positioning precision better than 1 mm

• DC beam– Ideally: 100% duty factor for (repainted) scanning

• 2d - fast magnetic scanning– Motion faster than 1cm / ms

• Continuous fast scanning with dynamic beam intensit y modulation?– For 1cm wide pulsed beam - corresponds to a repetition rate of 1 KHz

• With a few % precision of the control of the dose per pulse

• Dose control precision per spot– 1 % spot dose precision of average spot 0.2 % for absolute dosimetry

– a challenge for pulsed systems? (Control of the spot dose within the pulse)

• Dose spot dynamics for conformal therapy: 30:1 (dista l proximal)– Within a layer of a conformal treatment we have a non-homogeneous proton fluence

Really Needed?


• Fast energy changes? How much?

– For volumetric repainting (necessary?)

• Sychrotrons – 1 s

• Cyclotron + degrader + gantry - 80 ms (gantry limited)

• Cyclinac - 1ms

– But gantry limited (if one uses a gantry) … unless

• Gantry with a large dp / p acceptance (U. Amaldi - good for tracking)

• FFAG – energy pulse by pulse – not gantry limited (if a FFAG gantry)

• There is no experience up to yet in treating moving t argets with beam scanning – we have to work on it

• The new dream machines?

– Dielectric wall accelerator (a few meters proton linac) could be the revolutionary invention

for the whole field – and steer radiotherapy away from photons to protons in general

– Laser based acceleration – probably a longer perspective ?


• THANK YOU

A very exciting field also in the future…

Based on a beautiful idea

E. Pedroni Paul Scherrer Institute SWITZERLAND PSI...

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