Dosimetria computacional de parâmetros do eixo central em campos pequenos

Abstract

The concept of small fields in radiotherapy is found mainly in techniques that modulate photon flux or in the treatment of small lesions, such as radiosurgery or stereotactic body irradiation. The use of small or irregular fields poses major challenges for dosimetry. Therefore, research should be conducted with the aim of studying the dosimetry of small fields. One way to investigate, analyse and understand the possible parameters that may affect dose estimation in small field radiotherapy is to use the different calculation codes available in the literature. Here, the Monte Carlo N-Particle eXtended (MCNPX) computational code was used to model and simulate the different and possible scenarios in small-field radiotherapy. The computational modelling allowed to investigate the different levels of complexity of the geometry of the detectors used in small-field dosimetry. Percentage values of dose in depth (PDP), the measure of beam quality for regular and small fields, and correction factors for dose in small fields were estimated. Two X-ray spectra with an energy of 6 MV, found in the literature, were used to simulate the 10x10, 5x5, 4x4, 3x3, 2x2 and 1x1cm² fields, using different detector geometries. The geometries of the modelled detectors ranged from a simple model represented by water spheres to a more complex geometry consisting in the replication of the real model of the PinPoint 3D 31016 ionisation chamber. In addition, experiments were conducted in a hospital to obtain the PDPs and validate the computational models developed. The difference found for the PDP20,10 value when comparing the experimental and computational results, for the spherical model and with the source spectrum provided by Braualla, was 5.81%. The correction factors estimated with the results of the simulations for the small fields were calculated. It was possible to see that the complexity of the geometry of the detector modelled with MCNPX and the energy spectrum used directly influence the results for the small fields. The calculated correction factors showed deviations of 5.55% and 9.97% for fields of 2x2 and 1x1 cm², respectively, in the modelling and computational simulation using the spectrum of the MCMEG group and the PinPoint 3D 31016 as detector, compared to the correction factors published in the literature.