Presenter of 1 Presentation
FLASH PROTON THERAPY FOR WHOLE BREAST IRRADIATION: EXPLORING MACHINE REQUIREMENTS
Abstract
Background and Aims
With a sizeable patient population, a target largely comprising healthy tissue, and clinically relevant late effects, FLASH whole breast irradiation (WBI) merits consideration. Transmission beams provide a practical way to deliver ultra-high dose rate proton therapy. However, the large WBI volumes make it harder to achieve FLASH dose rates with pencil-beam-scanning (PBS). We therefore performed a simulation study to identify PBS machine characteristics needed for such treatments.
Methods
For a left-sided breast case (861cc) a single-field spot-reduced plan was generated using 250MeV transmission beams. ‘PBS dose rates’ were calculated, considering the total time (including dead-times) to deliver 95% of the dose in each voxel. We varied maximum beam current at isocenter (200, 400, 800nA), energy-layer-wise or spot-wise current, minimum spot duration (0.5, 1, 2ms), and fraction dose (5x5.7Gy, 2x9.74Gy; equivalent BED3). The percentage of dose delivered above FLASH thresholds was evaluated, considering dose rate thresholds of 40Gy/s and 100Gy/s, and dose thresholds of 4Gy and 8Gy.
Results
For 40Gy/s dose rate threshold, spot-wise currents generally provided >70% of dose delivered above FLASH thresholds, with little dependence on beam current and spot duration (Figure 1). When using energy-layer-wise currents, comparable FLASH dose was achieved only for 9.74Gy fraction dose and 0.5ms minimum spot duration. For 100Gy/s dose rate threshold, substantial FLASH dose was obtained only with extreme machine settings (i.e. 800nA, spot-wise, <=1ms).
Conclusions
Assuming large fields do not necessarily preclude a FLASH effect, FLASH WBI is theoretically achievable, but may require large fraction sizes, and may be (too) demanding for current PBS machines.
Author Of 3 Presentations
TREATMENT PLANNING TOOLS TO EVALUATE THE FEASIBILITY OF FLASH THERAPY WITH PROTONS
Abstract
Background and Aims
To exemplify the potential and limitations of two approaches to FLASH treatment planning with protons by testing them on two clinically realistic scenarios and different FLASH-specific parameters
Methods
We selected two planning approaches to be delivered with a cyclotron: 3D range modulator (3DRM) and transmission beams(TB). (See Table below for details on the beam delivery parameters.) We associated each planning technique with a disease site and a clinically applied hypofractionation protocol ( 3DRM - liver - 3x25Gy, TB - lung - 3x20Gy). We evaluated the resulting dose distributions for different beam currents (200nA and 800nA at isocentre), two dose rate definitions (dose-averaged dose rate (DADR) and a sliding time window), two minimum dose thresholds and two dose rate thresholds for the FLASH effect (4Gy and 8Gy, 40Gy/s and 100Gy/s, respectively).
Results
Both techniques achieved acceptable dose distributions with a limited number of fields (liver - 1 field, lung - 3 fields) for FLASH proton plans. All combinations of beam intensity, dose rate definition, dose and dose rate threshold we investigated were associated with some level of FLASH dose, suggesting that these disease sites and dosimetric protocols are reasonable candidates for FLASH proton therapy. The figure below shows an example of the results for 200nA and 4Gy and 40Gy/s thresholds.
Conclusions
Treatment planning studies are a useful tool to test candidate disease sites, protocols and planning techniques for proton FLASH. The next step will be to include additional combinations of beam production systems, planning techniques, and patient anatomies.
FLASH PROTON THERAPY FOR WHOLE BREAST IRRADIATION: EXPLORING MACHINE REQUIREMENTS
Abstract
Background and Aims
With a sizeable patient population, a target largely comprising healthy tissue, and clinically relevant late effects, FLASH whole breast irradiation (WBI) merits consideration. Transmission beams provide a practical way to deliver ultra-high dose rate proton therapy. However, the large WBI volumes make it harder to achieve FLASH dose rates with pencil-beam-scanning (PBS). We therefore performed a simulation study to identify PBS machine characteristics needed for such treatments.
Methods
For a left-sided breast case (861cc) a single-field spot-reduced plan was generated using 250MeV transmission beams. ‘PBS dose rates’ were calculated, considering the total time (including dead-times) to deliver 95% of the dose in each voxel. We varied maximum beam current at isocenter (200, 400, 800nA), energy-layer-wise or spot-wise current, minimum spot duration (0.5, 1, 2ms), and fraction dose (5x5.7Gy, 2x9.74Gy; equivalent BED3). The percentage of dose delivered above FLASH thresholds was evaluated, considering dose rate thresholds of 40Gy/s and 100Gy/s, and dose thresholds of 4Gy and 8Gy.
Results
For 40Gy/s dose rate threshold, spot-wise currents generally provided >70% of dose delivered above FLASH thresholds, with little dependence on beam current and spot duration (Figure 1). When using energy-layer-wise currents, comparable FLASH dose was achieved only for 9.74Gy fraction dose and 0.5ms minimum spot duration. For 100Gy/s dose rate threshold, substantial FLASH dose was obtained only with extreme machine settings (i.e. 800nA, spot-wise, <=1ms).
Conclusions
Assuming large fields do not necessarily preclude a FLASH effect, FLASH WBI is theoretically achievable, but may require large fraction sizes, and may be (too) demanding for current PBS machines.
EVALUATION OF PROTON FLASH TREATMENT PLANS USING TRANSMISSION AND RIDGE-FILTER SOBP TECHNIQUES
Abstract
Background and Aims
Recent proton FLASH research focuses on transmission planning to exploit the higher beam transport efficiency for high energies, with some compromises to plan quality. This study explores the plan quality and achievable dose rates for transmission and spread-out Bragg peak using a ridge-filter. The results utilise a FLASH effectiveness factor that incorporates dose and dose rate to facilitate an overall plan quality comparison between plans.
Methods
Three spot-reduced, single-field plans were optimised for three patients: IMPT using down-stream range-shifters with consecutive delivery of energy layers; IMPT using down-stream range-shifters with simultaneous delivery of all energies for each lateral spot position, simulating a personalised variable ridge-filter; transmission using the highest available energy of 229MeV. The fraction dose was 1x22.3Gy (equivalent to 30x2Gy and tumour alpha/beta=10Gy). The potential FLASH effects were estimated by multiplying any dose contribution delivered in FLASH state by a FLASH effectiveness factor FEF=0.67. The FLASH state was triggered in a voxel if any dose contribution was delivered above 40Gy/s and 5/10/15Gy and was assumed to persist for 200ms after the trigger has ended.
Results
Figure 1 shows very similar and pronounced FLASH effects for ridge-filter deliveries compared with transmission plans. Figure 2 suggests a potential clinical benefit of the FEF-weighted ridge-filter plans compared with a clinical reference plan’s physical dose, with positive values indicating a reduced integral dose.
Conclusions
With encouraging FLASH characteristics and excellent dose conformality, variable ridge-filters might be a promising approach to bring FLASH proton therapy to clinics.