TOTAL SCALP IRRADIATION – DOSIMETRIC COMPARISON BETWEEN DIFFERENT TECHNIQUES

Introduction: Scalp skin malignancies usually have poorly defined borders and multifocal dissemination. Extensive basal cell carcinomas, especially the desmoplastic and angiosarcoma subtypes are the most frequent etiologies. Extensive dissemination in this topography present a challenge in planning, due to their convex surface and adjacent organs at risk (OAR). Available modalities include high dose rate brachytherapy (HDR), conformational teletherapy with photons and electrons beams (3D) and photon beams with modulated intensity (IMRT). Objective: To identify the best therapeutic option for total scalp irradiation, several techniques have been tested. The goal of this study is to perform a dosimetric comparison between different techniques, aiming at the interpretation of treatment volume coverage (PTV) and dose in OAR. Methods: A phantom was used, in ventral decubitus, with a bolus attachment of 0.5 cm of wax throughout the scalp and a thermoplastic mask with 18 catheters distally 1.2 cm between them. The fusion tool with magnetic resonance (MRI) and computed tomography (CT) of the skull was used for the design of CTV, PTV and OAR. In the HDR option was used 192Ir source. In the conformational plane (3D), a combination of 6MV photons and 9 MeV electrons (2 lateral) was performed. Static IMRT Planes (IMRT-7 coplanar fields), modulated volumetric arch (VMAT-2 arches) and tangential modulated volumetric arch (TVMAT-4 half arches) were generated. Dose measurements were performed with dosimetric films in 5 different positions on the surface of the phantom, under the bolus. Results: The TVMAT plan granted better PTV coverage and increased dose homogeneity within it. The HDR plan did not significantly spare the OAR, as the 3D planes, IMRT and VMAT did, but the 3D plane presented the highest maximum dose. The dose in the OAR was lower in the TVMAT plane when compared with the others. The values measured through the radiochromic film, within the uncertainty interval, are coincident with the values calculated by the planning system, which demonstrates its reliability. Conclusion: IMRT planes, especially TVMAT, are suitable for targets with convex surface, offering better PTV coverage and higher dose distribution, with the advantage of offering lower punctual doses in the central nervous system OAR.