Presentation Model evaluation of normal tissue response by mini-beam radiotherapy with ion beams

Matsufuji, Naruhiro

Bragg peak characteristics of ion beams makes ion-beam radiotherapy advantageous over conventional X-ray. The localized dose delivery to the Bragg peak enables to control deep-seated tumor cells while sparing surrounding benign tissues. The Bragg peak gets shaper for heavier ions; additionally, the biological effectiveness of carbon and its vicinity also increases gradually in accordance with increasing energy loss. Idea of the mini-beam radiation therapy (MBRT) was initially studied with X-ray as a method to spare proximal normal tissues by irradiating through submillimeter slit collimator. Recent studies try to apply the MBRT concept for proton beam by interlacing multiple sparse heterogeneous fields. The combination of the MBRT with ion-beam RT may lead further reduction of normal tissue toxicities.This study aims at assessing the efficacy of the MBRT with ion beams with commonly-used normal-tissue complication probability (NTCP) consideration.A Monte Carlo code PHITS was used to simulate the spatial dose distribution of carbon 276 MeV/n and proton 145 MeV pencil beams in water. Incident energy was selected to reach 15 cm depth. The beam was aligned in parallel at interval of the full width at half maximum at the Bragg peak layer. Accordingly, Equivalent Uniform Dose (EUD) widely used in common NTCP models was assessed from dose deposited proximal to the Bragg peak layer by changing the organ-structure parameter. The result revealed that carbon-ion beam is advantageous over protons in reducing EUD to the proximal normal tissues, however, the advantage of the mini-beam delivery was found only in case the normal tissue has extremely parallel-like structure therefore sensitive to minimum dose within the volume. The result is compared with the other NTCP models.
World Congress on Medical Physics and Biomedical Engineering 2018 (IUPESM2018)

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