Patricia Van Marlen (Netherlands)
Amsterdam UMC RadiotherapyAuthor Of 1 Presentation
Treatment Planning And Dose-Rate Distributions For Conventionally Fractionated Head And Neck Cancer Using Uhdr Transmission Beam Proton Therapy
Abstract
Background and Aims
Transmission beam (TB) proton therapy (PT) uses single, high energy beams with Bragg-peak behind the target, sharp penumbras and simplified planning/delivery. TB facilitates ultra-high dose-rates (UHDRs, e.g. ≥40Gy/s), which is a requirement for the FLASH-effect. FLASH may also require a dose threshold, but this remains uncertain and UHDR-distribution investigation is of interest for head-and-neck cancer treatment.
Methods
We investigated (1) plan quality for conventionally-fractionated head-and-neck cancer treatment using spot-scanning proton TBs, intensity-modulated PT (IMPT) and photon volumetric-modulated arc therapy (VMAT); (2) UHDR-metrics. VMAT, 3-field IMPT and 10-field TB-plans, delivering 70/54.25Gy in 35 fractions to boost/elective volumes, were compared (n=10 patients). To increase spot peak dose-rates (SPDRs), TB-plans were split into three subplans, with varying spot monitor units and different gantry currents.
Results
Despite the lack of Bragg-peak advantages, average TB-plan OAR-sparing was comparable to IMPT: mean oral cavity/body dose were 4.1/2.5Gy higher than IMPT (9.3/2.0Gy lower than VMAT); most other OAR (salivary glands, larynx, pharynx) mean doses differed by <2Gy (2.0-12.1Gy lower than VMAT). Average percentage of dose delivered at UHDRs was 46%/12% for split/non-split TB-plans and mean dose-averaged dose-rate 46/21Gy/s. Average total beam-on irradiation time was 1.9/3.8s for split/non-split plans and overall time including scanning 8.9/7.6s.
Conclusions
Conventionally-fractionated proton TB-plans achieved comparable OAR-sparing to IMPT and better than VMAT, with total beam-on irradiation times <10s. Splitting TB-plans increased the UHDR, demonstrating the advantage of gantry current variation per spot. If a FLASH-effect can be demonstrated at conventional dose/fraction, this would further improve plan quality and TB-protons would be a suitable delivery system.