Proton Therapy: Application and Quality Assurance

Transcription

1 Proton Therapy: Application and Quality Assurance IMRT IMPT Tony Lomax Centre for Proton Radiotherapy, Paul Scherrer Institute, Switzerland

2 Treatment delivery 1. Basics 2. Passive scattering 3. Active scanning 4. Treatment gantries 5. Clinical and practical considerations 6. Summary

3 Why protons? R.R. Wilson, Radiology 47(1946),

4 Dose per proton (ngy) PAUL SCHERRER INSTITUT The Bragg peak E.g. Depth-dose curve for 177 MeV protons Peak-to-plateau ratio 4-5 (depending on width of energy spectrum) Dose plateau where velocity is high Depth in water (cm) Dose peak where protons slow down and stop Width of peak dependent on range straggling in medium and initial energy spectrum Dose concentrated in small volume in Bragg peak.

5 The problem of lateral field size and coverage in depth A mono-energetic proton Bragg peak is not very useful for treating anything other than the smallest tumours. σ~3-5mm Target volume FWHM~9-11mm σ~5-8mm To make protons useful, we need to spread out the dose laterally and along the beam direction.

6 Treatment delivery 1. Basics 2. Passive scattering 3. Active scanning 4. Treatment gantries 5. Clinical and practical considerations 6. Summary

7 Extending the dose in depth The Spread-Out-Bragg-Peak target

8 Extending the dose in depth The range shifter wheel A rotating wheel with varying thickness The range shifter wheel in action RS wheel rotates continuously in beam at ~3000 rpm

9 Extending the dose laterally Single scattering Scattering foil Source size Drift space Target ±2-3% Useable protons Beam broadened in Gaussian form by insertion of scattering foil Drift space to patient required for beam to broaden to desired field size (typically 20cm) Flat dose achieved by working in top (flattest) part of Gaussian Very poor efficiency (very few protons contribute dose to target) Good penumbra after collimation (source size similar to initial beam size)

10 Extending the dose laterally Double scattering 1 st scatterer Contoured scatterer ±2-3% Second, composite and contoured scatterer, flattens beam For similar drift space, larger field sizes are possible (to 40cm) Target Useablepr otons Better homogeneity of dose across field Source size Drift space Much better efficiency (many more protons contribute dose to target) However, worse penumbra due to large virtual source size

11 Passive scattering in practice Collimator Compensator Range-shifter wheel Scatterer Target Patient The collimator 2D Projection of target along beam direction Field specific aperture (collimator) matching projected shape of target Each delivered field (incident direction of irradiation) requires specific collimator and compensator As range shifter is up-stream of scatterer(s), extent of SOBP is fixed across field

12 Passive scattering in practice Computer-milled compensators (examples from Orsay) Example collimator Eye irradiation Generally, a compensator and collimator are required for every field in the passive scattering approach

13 Single passively scattered field Passive scattering in practice Three passively scattered fields 10cm Fixed extent SOBP leads to poor sparing of normal tissue proximal to target Conformation of dose can be improved through the use of multiple fields

14 Passive scattering in practice Field patching Matching distal and lateral field edges to improve sparing of neighbouring organs. E.g.

15 Treatment delivery 1. Basics 2. Passive scattering 3. Active scanning 4. Treatment gantries 5. Clinical and practical considerations 6. Summary

16 Active scanning Magnetic scanner Proton pencil beam Target Range shifter plate Patient

17 Active scanning a treatment planning example Spot definition selection Spot weight optimisation Optimised dose Dose Calculation Incident field

18 The active scanning system at PSI Magnetic sweeper Monitor chambers Range shifter plates X T Z Y Motorised table

19 Passive scattering and active scanning compared Passive scattering Spot scanning 1 field 1 field 3 fields 3 fields

20 Passive and scanning delivery compared. Passive Mature technology + Insensitive to + organ motions Relatively inflexible - Field specific - hardware required Large gantries - required Integral dose - Scanning New technology - Very sensitive to - organ motions Very flexible + No field specific + hardware required Smaller gantries + Integral dose +

21 Intensity Modulated Proton Therapy: The simultaneous optimisation of all Bragg peaks from all incident beams. E.g. F3 F4 F1 Combined distribution F2 Lomax, Phys. Med. Biol. 44: , 1999

22 Passive scattering PAUL SCHERRER INSTITUT The three orders of proton therapy compared Spot scanning IMPT 1 field 1 field 1 field 3 fields 3 fields 3 fields

23 Two examples of clinical IMPT plans delivered Sacral chordoma, 10 year old girl at PSI Skull-base chordoma 3 fields 4 fields

24 Treatment delivery 1. Basics 2. Passive scattering 3. Active scanning 4. Treatment gantries 5. Clinical and practical considerations 6. Summary

25 Proton treatment gantries - challenges Linear accelerators (LINACS) mounted on rotating gantries are standard equipment in conventional radiotherapy However, protons are 1800 times heavier electrons (the particles actually accelerated in LINACS) Much larger magnets are therefore required for bending the proton beam onto the patient In addition, for passive scattering, a large drift space is required from the last scatterer in order to achieve large fields Thus, for the foreseeable future proton gantries will always be larger than conventional LINACS

26 Proton treatment gantries The Loma Linda passive scattering proton gantry Bending magnets 12 meter diameter Drift space to patient Side view Front view

27 Proton treatment gantries E.g. Gantry 1 at PSI 90 o bending magnet Patient table 3.5 meter diameter Treatment nozzle 3.5m diameter, eccentric gantry +/-180 degree rotation Discrete scanning system Start of patient treatments 1996

28 Proton treatment gantries PSI Gantry 2 E. Pedroni PSI gantry 2 7m diameter, isocentric gantry +/-90 degree rotation Discrete/continuous scanning system Start of patient treatments planned 2009

29 Treatment delivery 1. Basics 2. Passive scattering 3. Active scanning 4. Treatment gantries 5. Clinical and practical considerations 6. Summary

30 Chordomas and chondrosarcomas of the base of skull: PSI results 1.8 Probability Median doses 74 and 68CGE for chordomas and chondrosarcomas respectively Time (months) Complication free survival chordomas/chondrosarcomas (n=29, median follow-up 29 months) Weber et al, Int. J. Radiat. Oncol. Biol. Phys, 63, 2005

31 Comparison of reported results for chordomas of the base of skull n Radiation Mean dose LC 3-yr LC 5-yr LC 10-yr Castro, Helium, Neon Terahara, PT, RT Hug, PT, RT Noel, PT, RT Schulz-Ertner, Carbon, RT 60* 87 Igaki, PT, RT Weber, PT Ares, 2007* 64 PT (81*) *unpublished and provisional

32 Range and robustness: potential problems The advantage of protons is that they stop. The disadvantage of protons is that we don t always know where 10% range error

33 IMPT Plan degeneracy (non-uniqueness) A B Two, 5 field IMPT dose distributions 3D IMPT DET Corresponding spot weight distributions from field 2

34 Recalculation of plans simulating 3% overshoot of all protons 3D(1) DET 3D(2) Diff % +10 Diff % +21 Diff % Dose difference 3% overshoot Dose difference 3% overshoot Dose difference 3% overshoot

35 Sources of range uncertainties Limitations of CT data (beam hardening, noise, resolution etc) [Σ ~ 1%] Uncertainty in energy dependent RBE [Σ ~ 2%] Calibration of CT to stopping power [Σ ~ 1-2%] CT artifacts [Σ] Variations in patient anatomy [Σ,σ] Variations in proton beam energy [σ] Variations in patient positioning [σ]

36 Chordomas and chondrosarcomas of the spinal axis 1 3-yr 85%, 5-yr 71% Local control (n=26) median follow-up 35 months 3-yr 85%, 5-yr 71% Rutz et al, To be submitted to IJROBP Months

37 Chordomas and chondrosarcomas of the spinal axis 1 3-yr, 5-yr 100% Without implant (n = 13) yr 69% 5-yr 46% Implants (n =13) Months Rutz et al, To be submitted to IJROBP

38 Chordomas and chondrosarcomas of the spinal axis More advanced initial tumour at diagnosis? Problems in defining CTV due to artifacts? Problems in dose calculation due to artifacts? Proton therapy may only be as good as we are at estimating the range in the patient

39 Summary Proton therapy is a mature technique, with more than 50,000 patients treated world wide Spot scanning/impt provides improved flexibility and conformality in comparison to passive scattering IMPT is being delivered routinely at PSI with about 40% of all patients receiving IMPT as part of their therapy Initial clinical results with spot scanning are very favourable c.f. other particle therapy centres (including C-12) However, protons bring their own challenges e.g. detecting and dealing with range uncertainties

40 Particle therapy is fast becoming mainstream Some stat s from this year s AAPM meeting in Minneapolis (July) ~ 20% of RT oral presentations were related to proton therapy (3 from 10 in the young investigators session) Two 2 hour symposiums dedicated to the subject At one symposium, an estimated delegates attended (20-25% of total)!