Scanning Beams (Clinical Physics)

Transcription

1 Scanning Beams (Clinical Physics) Stefan Both, PhD PTCOG 53 June, 9 th, 2014

2 Outline Why Scanning Beams? PBS and Clinical Planning. What matters? -Minimum Energy/Beam Degraders/Spot Size - Planning Techniques: SFUD, IMPT, DET - Robust Optimization and Evaluation - Interplay Site Specific Implementation; Technical Protocols. Summary. 2

3 Proton Therapy Technologies Passive Scattering PBS Techniques SOBP SFUD IMPT 3DCRT/IMRT IMRT/VMAT 3

4 Pencil Beam Scanning PBS Magnetically scan p beam (X,Y) and control depth switching Energy (Z) A target is divided into many layers, a layer is divided into many spots, Spots are scanned one by one; layers by layers, from distal to proximal. A full Some A set, few proton more pencil with pencil beams a homogenous dose together. conformed Beam Spot... distally & proximally. Image by E.Pedroni PSI, Switzerland 4

5 Why Pencil Beam Scanning? PBS: Lowest Integral dose(2 to 3 times vs XRT). PBS 3D HIGH DOSE Conformality (1-4Fs )~ IMXT (4-9Fs). PBS can treat large fields, comparable with XRT. PBS penetrates at larger depths if no beam modifiers present. PBS produces less neutrons contamination outside of the patient vs PS has no beam modifiers are required(less nuclear interactions). 5

6 Pencil Beam Scanning Challenges It is difficult to generate very narrow pencil beams which result in optimal dose lateral fall off. PBS Minimum Energy (70 to 100 MeV). PBS has a time structure, can be more sensitive to motion than PS. The main difference from XRT: Range Uncertainty. 6

7 Outline Why Scanning Beams? PBS and Clinical Planning: - Minimum Energy/Beam Degraders/Spot Size - Planning Techniques: SFUD, IMPT, DET - Robust Optimization and Evaluation - Interplay Site Specific Implementation; Technical Protocols. Summary. 7

8 PBS and Clinical Planning What matters? - Minimum Energy - Shallow Target/ Beam Degraders. Spot Size vs. Air Gap: Small => High Conformality/Low HI Large => Less sensitive to motion - Planning Techniques - Plan Robustness: Optimization Evaluation Motion Management/Interplay Site specific implementation 8

9 Beam Degrader/Minimum Energy Beam Degrader Options: Machine Specific: Range Shifter on the snout. Problems: Size of Air GAP & Collision, Beam data commissioning workload. Solutions: Telescopic Arms Bolus, Collision Detection. 9

10 Beam Degrader/Minimum Energy Beam Degrader Options: Patient Specific: Bolus, Thick table top, Bridges, etc. Problems: Treatment Table Weight limits, Patient Positioning &Collision, Impact on Imaging. Solutions: Use of Universal Removable Devices, Automated Collision Detection, homogeneous & less dense material. PENN/ Qfix 10

11 Spot Size Integrity: Bolus vs. Range Shifter X direction Y direction No RS 2cm Bolus 8cm Bolus RS No RS 2cm Bolus 8cm Bolus RS Spot size (mm) Beam Energy (MeV) Beam Energy (MeV) S. Both, J.Shen et al, IJROBP 2014 in press 11

12 Beam Degraders Collision Detection -CAD /MATLAB ray casting algorithm. -Incorporated during the proton treatment planning phase=>improved QA processes and clinical efficiency. A B Green Body contour points; Red gantry polygon W. Zou, S Both et al. Med Phys J

13 PBS Treatment Planning A single field uniform dose (SFUD) technique, combines one or more fields optimized individually (SFO) to deliver a relative uniform dose across the target volume. Note: SFUD is the PBS equivalent to open field treatment. An intensity modulated proton therapy technique (IMPT), combines fields with heterogeneous dose distributions optimized together(mfo) to deliver a relatively homogeneous dose across the target. Note: IMPT is the PBS equivalent to patch fields passive scattering and IMRT. 13

14 SFO/SFUD vs. MFO/IMPT Techniques SFUD RT LT IMPT RT LT 14

15 PBS Optimization Treatment plans are optimized using inverse planning techniques which allow for variation in intensity and energy of each pencil beam (increases the degree of freedoms vs IMXT=>better dose shaping)?? 3D Forward planning PBS: Inverse planning 15

16 PBS Optimization Pencil beam weights are optimized for each beam direction using inverse planning techniques => beam weight maps for different energy layers. 16

17 Degeneracy of solution in inverse planning In general, based on the input the optimization problem may pose many equivalent solutions. It is difficult to decide whether a result produced by a planning algorithm can be further improved (e.g. adding more beams, reducing #of spots, etc.) SFUD IMPT 17

18 PB PBS Techniques 2.5D SFUD 2.5D uses poli-energetic SOBP pencil beams( different weights by different colors) individually adapted distally and proximally to TV=> uniform dose per field. 3D IMPT 3D uses poli-energetic pencil beams, non-uniformly distributed and adapted distally and proximally to TV => non-uniform dose per field. DET DET uses pencil beams distributed individually only on the distal TV edge => high non uniform dose per field. A. Lomax, 1999 PMB 18

19 PBS Optimization: PTV? For protons in addition to lateral margins, a margin in depth has to be left to allow for range uncertainty, therefore each beam orientation would need a different PTV, beam specific PTV(bsPTV). Generally, in practice, the dose distribution is determined based on a Optimization Volume(OV) derived from CTV adding lateral and range margins: DM = (0.035 x CTVdist) + 1 mm PM = (0.035 x CTVprox) + 1mm LM based on setup, motion, penumbra. Patient WED PM CTV LM DM PTV OV 3.5%- uncertainty in the CT# and their conversion to relative proton linear stopping power 1 mm - added to correct for range uncertainty Note: it works for PS, SFUD-not necessarily for patched fields and IMPT

20 PBS Plan Robustness Plan robustness measures the differences in quality between planned and delivered dose in the presence of uncertainties( e.g. setup and range uncertainties) o Robust Plan Optimization Account for uncertainties in the optimization function o Robust Plan Evaluation Worst case scenario : Standard robustness test For example: systematically simulate setup error by shifting isocenter in 6 directions, introduce HU # variation M. Engelsman, M, Schwartz, L.Dong, SRO

21 PBS Beam Angle Optimization Choosing the right beam orientation Shortest and most homogeneous path Ex: Pelvis (3 to 9 o'clock, CW) Choosing the best geometry of irradiation Limited numbers of beams, avoid serial OARs Coplanar vs. non-coplanar beams. 21

22 SFUD Optimization: Beam Specific PTV( bsptv) Avoids geometrical miss of the CTV through lateral expansion. DM, PM margins calculated from the target in beam direction for each Ray Extra margins based on local heterogeneities, using a density correction kernel LM Ray Tracing/ WED BEAM CTV DM Physical Margins corrected for SETUP&Range based on local heterogenities P.Park, L.Dong et al, IJROBP

23 SFUD Optimisation: bsptv Dose distributions conforming dose to CTV using PTV(>30%)&bsPTV(<5%). CTV CTV CTV CTV P.Park, L.Dong,et al IJROBP

24 PBS Robust Optimization Unkelbach et al, Med Phys 2009 Probabilistic Optimization: The range of each pencil beam is a random variable, the quantity to be optimized is the residual cost over the possible setup errors and range uncertainties weighted based on their probability. No overshoot 5mm overshoot 24

25 Multi-Criteria Optimization (MCO) In MCO: - a database of plans each emphasizing different treatment planning objectives, is pre-computed to approximate the Pareto surface. - robustness can be integrated by adding robustified objectives and constraints to the MCO problem. Chen et al, PMB 2012 Minimal set of absolute constraints D(GTV) > 50 Gy(RBE) Specify competing objectives minimize max OAR vs maximize min GTV dose H.Kooy, MGH 25

26 HN(PO vs. PO+AP): Robust Evaluation 26

27 HN :PBS vs. RA LPO+RPO RA AP+LPO/RPO) Init Init Init Verify Verify Verify 27

28 HN Target Robustness:CTV54Gy(RBE)/CTV60Gy(RBE) 100 Verification study for CTV 54Gy(RBE), PT EE 100 Verification study for CTV 54Gy(RBE) with ANT field, PT EE Ratio of Total Structure Volume [%] PO 20 Initial plan Shift plans Verification plans Ratio of Total Dose [%] Ratio of Total Structure Volume [%] PO+AP 20 Initial plan Shift plans Verification plans Ratio of Total Dose [%] 100 Verification study for CTV 60Gy(RBE), PT EE 100 Verification study for CTV 60Gy(RBE) with ANT field, PT EE Ratio of Total Structure Volume [%] PO 20 Initial plan Shift plans Verification plans Ratio of Total Dose [%] Ratio of Total Structure Volume [%] PO+AP 20 Initial plan Shift plans Verification plans Ratio of Total Dose [%] 28

29 HN OAR Robustness: Cord/Oral Cavity Ratio of Total Structure Volume [%] PO Verification study for Cord, PT EE Initial plan Shift plans Verification plans Total Dose [cgy] Ratio of Total Structure Volume [%] Verification study for Cord with ANT field, PT EE 2PO+AP Initial plan Shift plans Verification plans Total Dose [cgy] The shift plans may not always reveal potential problems and therefore verification volumetric imaging should be used. We can learn from robust evaluation how to come up with the clinically relevant cost functions for robust optimization. 29

30 PBS&Interplay Effect Prostate Motion PBS delivers a plan spots by spots; layers by layers. Each layer is delivered almost instantaneously. The switch (beam energy tuning) between layers takes about 1 to 5 s. Prostate motion during beam energy tuning causes interplay effect. 30

31 PBS: Interplay & Dose Distribution Worst Case Scenario Patient Worst Single Fraction A-P Drift (mm) C-C Drift (mm) 2 1 A-P Drift (mm) C-C Drift (mm) 8 9 Tang, Both et al, IJROBP

32 Outline Why Scanning Beams? PBS and Clinical Planning: -Minimum Energy/Beam Degraders/Spot Size - Planning Techniques: SFUD, IMPT, DET - Robust Optimization and Evaluation - Interplay Site Specific Implementation: Technical Protocols. Summary. 32

33 Site Specific Implementation: Technical Protocols Site specific technical protocols have to be developed within the limitations of existing technology for robust proton treatment clinical implementation as proton dose distribution may change with: different machine parameters. treatment planning algorithms. patient's anatomy changes in the beam (weight loss, air pockets, ports, etc) or misalignments of the proton beam relative to the patient. the way we manage uncertainties. 33

34 Site Specific Implementation: Technical Protocols To determine meaningful technical protocols it is important to: work with the physicians and the treatment team to acquire and analyze prospective patient data within the treatment environment. move consistently from simple to complex treatment sites( prostate, brain, pelvis (GI, GU, GYN), HN, CSI, etc) enrolled and analyzed 10 to 12 on treatment patients CT data sets including weekly scans. recognize potential problems, address them as necessary. perform dose accumulation studies for moving targets review the literature and one s institutional data within the limitations imposed by differences in technology, patient population, etc. 34

35 Lower GI: 2POs SFUD vs.rapid Arc 35

36 PBS CSI Field Geometry Two lateral PBS fields are used to treat the brain One or more posterior fields are used to treat the spine Fields overlap for 5-8cm A shallow dose gradient is created between the fields in order to create a safe, smeared field match H..Lin, S. Both et al..ijrobp,i n press, 36

37 Target volumes for planning PBSTV_Compo Field specific target volume PBSTV_compo obrain ospine Optimization target volume PBSTV obrain ospine PBSTV_Compo up gradient volume o 80brain_20up o 60brain_40up o 40brain_60up o 20brain_80up low gradient volume o 80upp_20low o 60upp_40low o 40upp_60low o 20upp_80low H.Lin. S. Both et al, PTCOG 53 37

38 CSI film measurements on field matching Sagittal dose profiles comparison for the (a) spinal-spinal and (b) craniospinal junctions. The blue lines indicated the location to draw the dose profiles. H.Lin. S. Both et al, PTCOG 53 38

39 Patient related uncertainties More complex Image by B.White, UPENN 39

40 Dose Accumulation on IACT Dose accumulation over 8 phases(left) vs. nominal dose distribution(right) Left Right Red contour: ITV CT image shown: IACT C.Zeng, S.Both et al. PTCOG53 40

41 Large Spots Small spots: σ ~ 5 mm; Big spots: σ ~ 10 mm 41

42 Summary Dose Conformality is best for PBS techniques and worst for passive scattering techniques(except patching). High dose region conformality will improve relative to IMRT as PBS matures. Dose Heterogeneity in general is best for PBS techniques relative to passive scattering and IMRT. Plan Robustness will improve as robust optimization becomes available, however the problem of motion requires additional forms of management(minimize motion, tracking, rescanning). Technical protocols have to be developed for each clinical site implementation as not like in IMRT, the patient and machine are decoupled and therefore geometry does not equals dose. Communication between the planning team and clinical team is crucial during treatment for a safe and accurate proton treatment. 42

43 Thank you Acknowledgements: Penn Radiation Oncology & Collaborators 43

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