Dosimetry of Large Field Valencia applicators for Cobalt-60-based brachytherapy.

BACKGROUND: Non-melanoma skin cancer is one of the most common types of cancer and one of the main approaches is brachytherapy. For small lesions, the treatment of this cancer with brachytherapy can be done with two commercial applicators, one of these is the Large Field Valencia Applicators (LFVA). PURPOSE: The aim of this study is to test the capabilities of the LFVA to use clinically 60Co sources instead of the 192Ir ones. This study was designed for the same dwell positions and weights for both sources. METHODS: The Penelope Monte Carlo code was used to evaluate dose distribution in a water phantom when a 60Co source is considered. The LFVA design and the optimized dwell weights reported for the case of 192Ir are maintained with the only exception of the dwell weight of the central position, that was increased. 2D dose distributions, field flatness, symmetry and the leakage dose distribution around the applicator were calculated. RESULTS: When comparing the dose distributions of both sources, field flatness and symmetry remain unchanged. The only evident difference is an increase of the penumbra regions for all depths when using the 60Co source. Regarding leakage, the maximum dose within the air volume surrounding the applicator is in the order of 20% of the prescription dose for the 60Co source, but it decreases to less than 5% at about 1 cm distance. CONCLUSIONS: Flatness and symmetry remains unaltered as compared with 192Ir sources, while an increase in leakage has been observed. This proves the feasibility of using the LFVA in a larger range of clinical applications.

Quality assurance tool for determination of position and transit time of a Co-60 source in high dose rate brachytherapy.

In this study, three holders were designed, constructed and characterized to perform quality assurance on the source position and transit time in remote afterloading systems with Co-60 sources for high dose rate brachytherapy. The holders design focused on achieving accuracy, low cost, and a time efficient tool for use in clinical settings. Sensitivities greater than 0.6%/mm and maximum precisions better than 0.14 mm for the source position were obtained. The transit time was determined for the holders with a relative precision better than 19%.

Brachytherapy treatment planning commissioning: effect of the election of proper bibliography and finite size of TG-43 input data on standard treatments.

The aim of this work is to evaluate the performance of a commercial brachytherapy treatment planning system (TPS) with TG-43 Vendors Input Data (VID), analyze possible discrepancies with respect to a proper reference source and its implications for standard treatments, and judge the effectiveness of certain widespread recommended quality controls to find potential errors related with the interpolations of TG-43 VID tables. The TPS evaluated was a BrachyVision 8.6 loaded with TG-43 VID for a VariSource high-dose-rate 192Ir source (Vs2000). The reference data chosen were the TG-43 data published in the literature. In the first step, we compared TG-43 VID with respect to the chosen reference data. Next, we used percent dose-rate differences in a point array matrix to compare the outcomes of the TPS on standard treatment setup with respect to an in-house developed program (MATLAB R2009a-based) loaded with the chosen full TG-43 reference data. The cases with major discrepancies were evaluated using the gamma-index analysis. The comparison with the reference data indicated a lack of sample in the angles between near to the tip (between 165 < θ < 180) and cable (0 < θ < 15) of the F(r,θ)(VID), which causes a dose underestimation of approximately 17% in the investigated points due to inaccurate interpolations. The differences over 2% encompassed approximately 17% of the surrounding source volume. These results have special relevance in treatment using one applicator with a few dwell steps or in Fletcher treatments where 10% dose underestimates were identified within the tumor or in organs at risk, respectively. Our results suggest that the differences found in the TPS under study are created by a lack of information on the angles in high-gradient zones in the F(r,θ)(VID), which generates important differences in dosimetric results. In contrast, the gamma analysis shows very good results (between 90% and 100% of passed points) in the analyzed treatments (one dwell and Fletcher). Further studies are required to exclude the possibility of finding noticeable effects in the DVH of treatment plans caused by the discrepancies here described. To achieve more strict control over the TPS dose-rate calculation, we recommend using QA test thinking in a source with nonaxial symmetry, adding a control point on the angles of the high-dose gradient zones (e.g., between 0° and 15° and between 165° and 180°). More studies are required to achieve full understanding of the clinical implication of such discrepancies.