ArticleMedical physics2026
Impact of millimeter-scale target motion on the precision of spatially fractionated radiation therapy using proton minibeams.
Article in Medical physics, 2026. The graph could read no effect estimate from its abstract, so it casts no vote on the map. Not yet cited in PubMed.
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Abstract
backgroundWhile pencil beam scanning (PBS) with motion mitigation such as re-scanning, breath hold, or gating is well established for treating moving targets in clinical practice, the precision for Spatially Fractionated Radiation Therapy (SFRT) using proton minibeams introduces presents a unique challenge. The sub-millimeter precision required for SFRT cannot be achieved with typical PBS spot sizes (millimeters), necessitating multi-slit beam collimation, which reduces the dose rate and reintroduces motion as a critical issue. In this study, we report on our experience with minibeam irradiations of chorioallantoic membranes (CAM) of chick embryos and discuss potential solutions to the motion problem.
methodsThe first experiment was conducted at a PBS gantry using 130 MeV protons and a 6 cm thick copper multi-slit collimator with 0.3 mm wide slits and 1 mm spacing. To achieve a target peak dose of 30 Gy, CAM samples were irradiated with more than 10 re-scans. Due to a 91% beam loss in the collimator, the total irradiation time extended to two min.
resultsPost-irradiation analysis using gafchromic film revealed smearing of the minibeam profiles, while biological tissue analysis uncovered unexpected and disrupted dose effects, likely caused by irregular CAM movement during irradiation. A subsequent experiment, incorporating additional setup optimizations such as reduced air gap and increased dose rate, shortened the irradiation time to below one min. This led to sharper dose profiles with negligible variation in valley dose and peak width, although a reduction in peak dose of 8%-13% was still observed. This variation is considered acceptable for our biological experiments.
conclusionsFor clinical applications, minibeam re-scanning is likely to be feasible only under ultra-high dose rate conditions (e.g., those required to induce the FLASH effect), where sufficiently short irradiation times minimize motion-induced dose degradation.
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