Dosimetric studies through Monte Carlo methods in surface high-dose-rate electronic brachytherapy

Non-melanoma skin cancers are among the most common human malignancies, presenting a continuous increase in their incidence worldwide. Surface electronic brachytherapy (eBT) has become an effective treatment in this context, achieving excellent control rates and good cosmetic results. Surface eBT systems consist basically of two components: an x-ray tube and an applicator attached. The x-ray tube accelerates electrons in the energy range between 50 and 70 kV, producing flattened x-ray beams. The eBT applicator serves as a collimator positioned directly in contact with the skin, producing conical beams of 10 to 50 mm diameter. The use of low-energy photon beams introduces several experimental and theoretical challenges to the absorbed dose determination (e.g., depth dose measurement, beam calibrations, etc.). This thesis aims to characterize a clinical surface eBT device of 69.5 kV, using Monte Carlo (MC) methods while attempting to ameliorate some of the drawbacks appearing in the absorbed dose determination of low-energy photon beams. The MC model of the eBT unit here used generates depth-dose curves and dose profiles with differences, generally, < 5% with respect to the available experimental data. Such differences are within those published by other authors for different eBT units and MC systems. The data here reported show that some of the most relevant quantities used in radiation dosimetry of low-energy photons can be obtained with combined uncertainties within 0.5% (k = 2). These results represent a significant improvement regarding the uncertainties reported in other widely used dosimetric datasets, such as the TG-61, with 3% uncertainty (k = 2). This improvement can reduce by 40% the final uncertainty of a beam calibration procedure. The experimental uncertainties in the depth-dose measurements, here defined as the combination of the system alignment (eBT unit and detector), the manufacturing tolerances, and the detector response dependencies, have been obtained to be about 3%, when the recommended detector, the T34013 parallel-plate ionization chamber, is used. This value is lower than those reported elsewhere for beams of 50 kV, using the same detector. Several organizations recognize that using depth-dose measurements in low-energy photon beam calibrations, replacing the half-value layer (HVL) as beam quality index, is a desirable research path. Using the data obtained in this thesis, we have found that the use of depth-doses measurements avoids the downsides of the experimental HVL determination, with a negligible or nonexistent effect in the final calibration uncertainty