Implementing a Proton Beam Scanning System within an Operating Clinical Facility

Transcription

1 Implementing a Proton Beam Scanning System within an Operating Clinical Facility Ben Clasie Many thanks to Hassan Bentefour, Hanne Kooy, and Jay Flanz for their help preparing this presentation 1

2 Francis H. Burr Proton Therapy Center 235 MeV Gantry 1 Gantry 2 DS DS PBS PBS Stereotactic Eye Room 3a Cyclotron manufactured by Ion Beam Applications 2

3 Delivery techniques in Gantry 2 Passive Scattering Broad beam Apertures and range compensators protect healthy tissue and conform dose to the target Pencil Beam Scanning A narrow pencil beam is scanned over the target Apertures and range compensators are optional Universal proton therapy nozzle interchangeable between DS and PBS 3

4 Nozzle in Double Scattering (DS) mode Ion chambers Proton beam Ti window Fixed scatterer Dipole magnets Light-field mirror X-ray box Range-modulator Second Ion chambers wheel scatterer Collimating jaws Snout p D 4

5 Nozzle in Pencil Beam Scanning (PBS) mode 5

6 Treatment room (DS equipment) Gantry * Isocenter First scatterer & Range modulator Ionization chambers Second scatterer 6

7 Treatment room (PBS equipment) Gantry Quadrupole and scanning magnets Ionization chambers * Isocenter 7

8 8

9 In an ideal world. 9

10 Changing modes from DS to PBS Proton beam Range modulator + fixed scatterer Ion. chamber Nozzle quadrupole magnets (optional) 10

11 PBS checklist Date: Initials: Install Clean-up Item Retract RM drawer Change IC cables, HV cables Power to Pyramid devices in the gantry Power to A500 s Power to SMPS Switch B1G2 power supply card to PBS mode Turn DCEU key Install snout Change BeamDeliveryMode and restart TCU2 11

12 General rules for embarking on PBS development within an operating clinical facility 12

13 Rule #1 PBS development can not interrupt normal treatments Example #1 Work is scheduled outside of normal hours Morning Afternoon Evening 6:30-7:30am 6pm-7pm 7pm-8pm 8pm-10pm 10pm-Mid Overnight Mid-2am 2am-4am QA Tx Tx Scanning Ben et. al. Example #2 All work is set up and packed up on each shift without damaging the system Frequent: Time for setting up and packing up Verify the system is operational before morning QA Infrequent: Training Example #3 Upgrades to software and hardware do not affect normal treatments Less Frequent: Design test procedures Perform tests Documentation Compromises: Not all PBS upgrades are compatible with double scattering (Eg extra vacuum chamber in the nozzle) 13

14 Rule #2 Optimize time with the beam Example #1 Priorities, files and devices are prepared before shifts Example #2 When a project is finished at 2am, we start a new project Example #3 Data analysis and reports are prepared in parallel with data collecting, or after shifts If there is time left over, there are also meetings, research projects, , and sleep 14

15 Scanning techniques 15

16 Scanning techniques PBS involves moving a charged particle beam of particular properties and/or changing one or more of the properties of the beam For example: Spot scanning Position is stationary when beam is on. Beam is off between spots. Raster scanning Position is moved from spot to spot but the beam is always on (during and between spots) Continuous scanning Beam is always moving and the beam current is modulated from 0 to 100% Velocity can be modulated Jay Flanz, Beige paper, 2007 Time-driven mode Commands to move the position and change the beam current is synchronized with time Charge-driven mode Commands to move and turn the beam on/off is synchronized with charge collected on the monitor chamber Many names, but still just PBS For example, Spot scanning in the limit of many mini-spots becomes continuous scanning The best choice of operating conditions depends on the condition of the accelerator 16

17 The Pros and Cons of each Regime Spot scanning In addition to the beam-on time, more time is needed for moving the beam and waiting for magnets to settle Simple model of dose deposition Raster scanning Avoids some time delays Needs a model for the dose deposited between spots which will have an uncertainty Continuous scanning May still need time delays to wait for magnets to settle before turning the beam on Safety and clinical requirements may limit beam current and/or scanning speed, especially in fields with large doses Repainting is common Time-driven mode Can be faster than charge-driven mode Requires stable and accurate beam current Charge-driven mode Can be more accurate than time-driven mode There is a small time delay between collecting the appropriate charge and turning the beam off 17

18 Repainting, N times Random Error Systematic Error Constant 1/sqrt(N) N Proportional 1/[N*sqrt(N)] Even Effect of different spot weight errors on the dose distribution uncertainty Repainting may or may not deliver a more accurate dose distribution, it depends on the system and the dominant type of uncertainty Furthermore, repainting usually increases the irradiation time (it may be possible to break even in some system configurations) One can use repainting to reduce interplay effects with organ motion. This benefit is negated, however, if layer-by-layer repainting is done too quickly 18

19 Scanning MGH 19

20 PBS System Schematic Scanning controller, Pyramid Technical Consultants PTC Scanning magnet power supply, JEMA Proton beam Proton source, IBA Beam transport system Ionization chamber Scanning magnets (nozzle) Ionization chambers (nozzle) * Isocenter Hall probes, PTC Electrometers, PTC 20

21 An example of spot scanning and continuous scanning in a single map 21

22 An example of spot scanning and continuous scanning in a single map 22

23 Tolerances in PBS 23

24 Beam Performance requirements for PBS Version 1 (no quads) Requirements for an endometrium tumor Random (peak-topeak) Layer-tolayer (pk-to-pk) Tol: Gamma Index / Dose Prostate Range (mm) ±1.5 ± 1.5 Position ± 20% of σ ± 20% of σ H&N Beam width (%) ± 25 ± 25 Spot weight (%) ± 10 ± 5 Trofimov, Hubeau, and Flanz 24

25 MGH beam performance for the spot scanning technique 25

26 Nozzle and Beam Transport System Gantry magnets p Beam line magnets affect the pencil beam position and width at isocenter Magnets are cycled to reduce hysteresis affects. Proton range is always decreasing or we cycle the magnets Some power supplies overshot requested currents I Some power supplies turn off for safety reasons and have a long settling time when they turn back on t 26

27 Beam Position Performance Our requirement for beam position accuracy is 20% of sigma (2mm) Beamline algorithms are: (1) Set beamline magnets and verify the beam position during irradiations (dead-reckoning), (2) Tune the beamline magnets using position feedback at the first energy layer and use this information to correct subsequent layers, (3) use online position feedback to correct the position when required, and/or (4) Tune beamline magnets at every layer using position feedback. We use a combination of (1), (2) & (3) Horizontal Vertical Position Y accuracy at accuracy isocenter [mm] at X accuracy isocenter at [mm] Range [g/cm 2 ] 270 deg 0 deg 90 deg 180 deg 270 deg 0 deg 90 deg 180 deg 27

28 Pencil Beam Size Performance Beam width, 1 sigma [mm] Horizontal Proton Vertical Range [g/cm2] Proton Range [g/cm2] G270 G000 G090 G180 G315 G045 G135 FIT +/- 15% G270 G000 G090 G180 G315 G045 G135 FIT +/- 10% Observed stability in the beam width is better than 15% Beam Size at Isocenter (1 sigma) without quads is about 13mm. Beam widths (1 sigma) of 6 mm have been achieved by installing a quadrupole doublet and a short vacuum chamber in the nozzle and adjusting the magnet settings in the beam line. This will be implemented for treatments in Phase 2. 28

29 Charge delivered Accuracy Timing and Sensitivity Possible techniques for controlling the spot weight are (1) time driven, (2) charge driven, (3) use feedback from the charge rate to control the beam We choose to use chargedriven mode with charge rate feedback for our first PBS version This choice allows us to meet our requirements for our system with two caveats: accuracy (Gp) Spot weight 4.E-04 2.E-04 0.E+00-2.E-04-4.E Proton current (Gp/s) 1. Lower spot weights require lower proton currents 2. The best spot weight accuracy is 0.8x10-4 Gigaprotons which determines the smallest deliverable spot weight 29

30 Time to Achieve desired Range Ra ange in MLIC - target Range [g/cm2] g/cm2 20 g/cm2 7 g/cm Time from transition [s] The dipole magnets of the energy selection system are set without iterating with a field probe. The field probe verifies the magnets are set correctly and will stop an irradiation if they are outside the allowable tolerance Further improvements to reduce the transition time are planned 30

31 Irradiation time for 2Gy/1L Number of Layers 15 Total number of spots 1,500 Total time changing energy (3 s/layer) Total beam-on time Additional time moving Additional time settling Additional time from total delay at end of spots Sum of all contributions 42 s * 20 s 1.7 s 8.3 s 1.5 s 74 s *Will be upgraded to 1.5 s/layer See more results in a forthcoming paper by Stephen Dowdell and Anatoly Rozenfeld in collaboration with MGH 31

32 What is proton PBS? Summary Arbitrary scanning patterns in (X, Y, Z=E, Q, I, σ, ) PBS offers distal and proximal dose conformation At MGH, PBS is being developed in a Universal nozzle while treating patients with DS Two Rules were described 1. PBS development can not interrupt normal treatments 2. Optimize time with the beam which adds complexity and inconvenience to the development of PBS Further refinements to our PBS system are planned 32

33 Thank you! 33