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External radiotherapy is one of the main treatment modalities for brain tumors. However, irradiation of structures adjacent to the target volume is unavoidable, particularly in procedures such as whole-brain radiotherapy (WBRT). Among the most frequent adverse effects is localized alopecia, resulting from dose deposition in the scalp [1]. Although a well-established correlation exists between absorbed dose and the incidence of alopecia [2], accurate estimation of superficial dose by treatment planning systems (TPS) presents limitations. The scalp region is characterized by the absence of electronic equilibrium, the presence of the build-up region, and tissue–air interfaces, factors that compromise the accuracy of conventional dose calculation algorithms, especially within the first millimeters of the skin. Therefore, this study aimed to determine the dose distribution in the scalp during brain radiotherapy, to support dosimetric parameters associated with the risk of alopecia. Monte Carlo simulations were performed using the TOPAS code [3] with a 6 MV Varian TrueBeam photon beam and the MRCP anthropomorphic phantom [4]. The scalp was defined as the region extending from the skin surface to a depth of 5 mm. The absorbed dose in the medium was calculated. The results indicated a dose variation from 17.51 Gy at the surface to 22.65 Gy at 5 mm depth, demonstrating the characteristic build-up effect of megavoltage photon beams. The scalp's dose presents a steep gradient within the first millimeters, which may not be adequately estimated by conventional TPS algorithms. The Monte Carlo method proved essential for characterizing this critical region, contributing to a better understanding of alopecia risk and to the improvement of brain radiotherapy treatment planning.
[1] Sawada et al 2020 Br J Radiol 93 20200159
[2] Moghaddam et al 2024 28 650-656
[3] Faddegon et al 2020 Phys Med 72 114-121
[4] Kim et al 2020 Ann ICRP 49 13-201