Estudo experimental e computacional de dosimetria utilizando o gel dosimétrico MAGIC-f

Abstract

Dosimetry with polymeric gels combined with Monte Carlo simulations results in an extremely reliable analysis for quality control in radiotherapy. This study aims to investigate the MAGIC - f dosimetric gel use for dose measurements due to high energy photon irradiation from a linear radiotherapy accelerator through experimental studies and Monte Carlo simulations. We produced two batches of MAGIC-f gel and placed into test tubes. The irradiations were carried out in a Varian IX linear accelerator, using photons with energy of 6 MeV, field size 20x20 cm², and source-object distance of 94 cm. Two batches of MAGIC - f dosimetric gel were evaluated, with planned doses of 0, 2, 4, 6, 8, 10, 20, and 40 Gy for the first batch and 0, 2, 4, 6, 10, 12, 14, and 16 Gy for the second batch. The Chimera system, from Geant4, was used for the Monte Carlo simulations, modeling the Varian IX linear accelerator with the opening of collimators for different field sizes. We performed all simulations in clusters at the PUCRS High-Performance Laboratory. To validate the linear accelerator simulation, we modeled deep dose percentage (PDP) acquisitions using a cubic phantom of 48 x 48 x 40 cm³ filled with water. Magnetic resonance images (MRI) were acquired with the Fast Spin Echo (MSE) and Spin Echo (SE) sequences, using 11 echoes. For the MSE sequence, the dose sensitivity values obtained were 0.27225 and 0.22717 Gy-1s-1 with TR = 3 s for the first and second batch. Using TR = 10 s, dose sensitivity values were 0.28127 and 0.22978 Gy-1s-1 for the first and second batch. For the SE sequence, dose sensitivity values were 0.41967 and 0.41158 Gy-1s-1, with TR = 3 s for the first and second batch. The experimental PDP values were used to validate the simulation, showing maximum differences of 2.9% (10 x 10 cm²), 2.3% (20 x 20 cm²), 1.9% (30 x 30 cm²) and 1.5% (40 x 40 cm²), in relation to the experimental values. Concluding, the MRI SE sequence has a greater dose sensitivity compared to MSE. The sensitivity and linearity of the MAGIC - f gel agree with previously published studies. There is a linear trend of the dose deposited in the tubes with the increase in the number of simulated particles, requiring the splitter technique to obtain higher doses.