Impact of cardiosynchronous brain pulsations on Monte Carlo calculated doses for synchrotron micro‐ and minibeam radiation therapy. Issue 7 (8th June 2018) - Manchado de Sola, Francisco
Abstract : Purpose: The purpose of this study was to assess the effects of brain movements induced by heartbeat on dose distributions in synchrotron micro‐ and minibeam radiation therapy and to develop a model to help guide decisions and planning for future clinical trials. Methods: The Monte Carlo code PENELOPE was used to simulate the irradiation of a human head phantom with a variety of micro‐ and minibeam arrays, with beams narrower than 100 μm and above 500 μm, respectively, and with radiation fields of 1 × 2 cm and 2 × 2 cm. The dose in the phantom due to these beams was calculated by superposing the dose profiles obtained for a single beam of 1 μm × 2 cm. A parameter δ, accounting for the total displacement of the brain during the irradiation and due to the cardiosynchronous pulsation, was used to quantify the impact on peak‐to‐valley dose ratios and the full width at half maximum. Results: The difference between the maximum (at the phantom entrance) and the minimum (at the phantom exit) values of the peak‐to‐valley dose ratio reduces when the parameter δ increases. The full width at half maximum remains almost constant with depth for any δ value. Sudden changes in the two quantities are observed at the interfaces between the various tissues (brain, skull, and skin) present in the head phantom. The peak‐to‐valley dose ratio at the center of the head phantom reduces when δ increases, remaining above 70% of the static value only for minibeams and δ smaller than ∼200 μm. Conclusions: Optimal setups for brain treatments with synchrotron radiation micro‐ and minibeam combs depend on the brain displacement due to cardiosynchronous pulsation. Peak‐to‐valley dose ratios larger than 90% of the maximum values obtained in the static case occur only for minibeams and relatively large dose rates.