Caracterização dosimétrica e estudo de campos pequenos para um sistema de irradiação pré-clínica com feixe de raios X de média energia

Abstract

Major technological advances are being developed for small animal irradiation systems to simulate modern irradiation techniques with therapy purposes, allowing a wide variety of radiobiological studies. However, the gap between radiation dosimetry in the preclinical environment and dosimetry with clinical applications has reduced the possibility of translating biomedical hypotheses validation and the results of researches into randomized trials in humans. The combination of medium energy photon beams generated by the preclinical irradiators, with the millimetric dimensions of the collimators, required because of the sample dimensions, presents enormous dosimetric challenges, for which currently there are not specific codes of practice (CoPs). In this work, a specific methodology was developed to perform the dosimetry of small fields in an irradiator with X ray beams of 220 kVp (SARRP), developed for preclinical research. The methodology was based on the adaptation of the reference dosimetry for unconventional field sizes from the AAPM TG-61 protocol, and on the quantitative analyses of the responses of several radiation detectors that had not yet been extensively investigated for small fields relative dosimetry in medium energy beams in water. Detectors responses were assessed during transitions between the 40 × 40 mm2 and the 3 × 3 mm2 field sizes, adapting different dosimetry concepts from the CoPs for high energy photon beam. From the dosimetric data, acquired with a high-resolution 3D scanning water phantom, a CAT3D treatment planning system (TPS) was commissioned in the factor-based algorithm. Several methods of evaluation and validation were applied. Regarding the relative dose distributions comparison, a Semiflex 31010 (0,125 cm3) ionization chamber (IC) and a PinPoint 3D (0,016 cm3) were accurate within 1,5 to 3% for depth dose distributions (PDDs), when compared against an Advanced Markus parallel plate IC, and a PTW-60019 microDiamond (μD) detector was accurate within 1,6% when compared against the PinPoint 3D IC and within 0,8% when compared against the Monte Carlo MC3D simulations up to 3 × 3 mm2 and without correction factors applications. For the lateral profiles comparison between the solid-state detectors, and the EBT3 data provided by the manufacturer, the gamma analysis indicated an average of 100% passing rate with global gamma criterion of 1 mm/3%. The calculation method based on intermediate field sizes allowed to correctly determinate the output factors up to the smallest 3 × 3 mm2 cone with the μD, reducing the effect of variations in the response of the detector due to the possible energy dependencies. The 2D gamma analyses with the EBT3 data acquired in water, and the “end-to-end” test, validated the measurement and commissioning methodology to obtain complex dose distributions from measured data as well as the sensitivity of the CAT3D TPS with the “fast” algorithm to reproduce dose distributions in water and similar media. Results of the dose maps gamma analysis indicated an average of 96% passing rate with global gamma criterion of 1 mm/3% with increased tolerance to 5% in the low dose regions. The use of intermediate field sizes integrated with all the proposed methods has allowed reducing the need for correction factors for small field dosimetry at SARRP. This work provides an innovative methodology that can be inserted in a dosimetry protocol for preclinical applications.