Moderator of 1 Session
Presenter of 2 Presentations
Physics
Treatment Planning
Author Of 5 Presentations
Physics
Treatment Planning
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.
OPTICALLY STIMULATED LUMINESCENCE DETECTORS (OSLDS) FOR ULTRA-HIGH DOSE RATE PROTON DOSIMETRY
Abstract
Background and Aims
The objective of this study was to assess the use of optically stimulated luminescence detectors (OSLDs) to support radiobiological experiments for ultra-high dose rates (FLASH) proton beams.
Methods
Two experimental setups were tested to accommodate either biological samples or multiple mm2-sized Al2O3:C OSLDs. The OSLDs were read out using a protocol with a reference irradiation under known conditions to account for material differences. The experiments were conducted in a single pencil beam at the PSI Gantry 1 at a wide dose rate range of (1-3800) Gy/s. A third experiment assessed the spot reconstruction at 9000 Gy/s.
Results
The OSLDs were demonstrated to be dose rate independent with a negligible signal fading. The OSLD evaluated doses were on average (n=66) within 1 % of the nominal dose for (3 – 33) Gy for dose-rates (1 – 1000) Gy/s. The discrepancy between the OSLDs and the nominal dose was higher for the (3800-9000) Gy/s dose rates due to averaging effects of the narrow pencil beam over the OSLD surface, where a correction was demonstrated. An OSLD dose measurement was overall found to be reproducible within 1 %. The use of an OSLD grid enabled an estimation of the beam spot size and position in agreement (deviation < 2%) with radiochromic film measurements.
Conclusions
The results demonstrate that the almost point-like OSLDs are applicable for accurate proton dosimetry in ultra-high dose rates and suitable to support radiobiological experiments in water and air.
ULTRA-HIGH DOSE RATE DOSIMETRY FOR PRE-CLINICAL EXPERIMENTS WITH MM-SMALL PROTON FIELDS
Abstract
Background and Aims
To present alternatives, which rely on a Faraday Cup (FC) as reference, to the use of ionization chambers (ICs) for dosimetry in mm-small proton beams at ultra-high dose rates (UHDRs). Indeed, significant ion recombination combined with the volume averaging effect severely challenge the use of ICs in UHDR small-field dosimetry. Three distinct applications of a FC are presented: i) Prediction of the delivered dose; ii) Response characterization of field detectors up to UHDR; iii) On-line verification of delivered dose to biological samples.
Methods
250MeV transmission pencil beams can be delivered to small biological samples and detectors at currents up to ~700nA (~9000Gy/s on beam axis). For i) FC, beam width and integral depth-dose measurements are used to model the delivered dose. For ii) and iii) the FC is positioned downstream from the detectors or samples to be examined, which are then exposed to a wide range of dose rates. Following detectors have been studied: PTW IC 7862, PTW microDiamond 60019, EBT3 Gafchromic films, scintillating screens.
Results
EBT3 films and scintillating screens are dose rate independent, as well as microDiamond detectors (within +/-0.7%) over the range considered. The PTW IC 7862, though reproducible, exhibits a drop in response larger than 30% at ~9000Gy/s. Reproducibility of delivered dose for the proposed setup better than 1%.
Conclusions
FC are versatile dosimetry instruments that can be employed for dose prediction, field detector characterization and on-line dose verification for pre-clinical experiments at UHDR. microDiamond detectors showed promising results for their suitability for UHDR experiments for proton beams.