Abstract
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., ≥40 Gy/s), which is a requirement for the FLASH-effect. 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 70/54.25 Gy 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. Average TB-plan organs-at-risk (OAR) sparing was comparable to IMPT: mean oral cavity/body dose were 4.1/2.5 Gy higher (9.3/2.0 Gy lower than VMAT); most other OAR mean doses differed by <2 Gy. Average percentage of dose delivered at UHDRs was 46%/12% for split/non-split TB-plans and mean dose-averaged dose-rate 46/21 Gy/s. Average total beam-on irradiation time was 1.9/3.8 s for split/non-split plans and overall time including scanning 8.9/7.6 s. Conventionally-fractionated proton TB-plans achieved comparable OAR-sparing to IMPT and better than VMAT, with total beam-on irradiation times <10s. 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.
Highlights
Standard intensity-modulated proton therapy (IMPT) for head-and-neck cancer (HNC)places the Bragg-peak in the target
For subplans with spot-monitor units (MU) ≥40, 20–40 and 10–20, the average percentage of dose delivered at ultra-high dose-rates (UHDRs) was 69%, 49% and 17%, respectively
Better than photon for beams, achieved comparable OAR sparing to IMPT and better than photon volumetric-modulated arc therapy (VMAT) for fractionated HNC treatment, (2) UHDR-metrics for low fraction-doses increase with apfractionated HNC treatment, (2) UHDR-metrics for low fraction-doses increase with appliplication of MU-based beam-splitting (3) irradiation times of 10-field Transmission beam (TB)-plans are short, cation of MU-based beam-splitting (3) irradiation times of 10-field TB-plans are short, with with
Summary
Standard intensity-modulated proton therapy (IMPT) for head-and-neck cancer (HNC). It is possible to use high energy transmission beams (TBs) where tissue is irradiated with the beam section proximal to the. Bragg-peak, which is itself preferably located outside the patient. Investigation of proton intensity-modulated TB delivery is limited [1,2,3], even though this technique has some advantages. TBs increase robustness in the beam direction, eliminate uncertainties due to density changes, have sharper penumbras and require only one beam energy. High energy TB-plans can be delivered at ultra-high dose-rates (UHDRs), unlike current Bragg-peak plans (Bragg-peak plans require energy modulation, which lowers the beam intensity). Pre-clinical research dating back more than 50 years supports the hypothesis that
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