Comparison and Evaluation of the Effects of Rib and Lung Inhomogeneities on Lung Dose in Breast Brachytherapy using a Treatment Planning System and the MCNPX Code

Introduction: This study investigates to what extent the computed dose received by lung tissue in a commercially available treatment planning system (TPS) for 192Ir high-dose-rate breast brachytherapy is accurate in view of tissue inhomogeneities and presence of ribs. Materials and Methods: A CT scan of the breast was used to construct a patient-equivalent phantom in the clinical treatment planning system. An implant involving 13 plastic catheters and 383 programmed source dwell positions were simulated using the MCNPX code. Results: The results were compared with the corresponding commercial TPS in the form of isodoses and cumulative dose–volume histogram in breast, lung and ribs. The comparison of Monte Carlo results and TPS calculation showed that the isodoses greater than 62% in the breast that were located rather close to the implant or away from the breast curvature surface and lung boundary were in good agreement. TPS calculations, however, overestimated dose in the lung for lower isodose contours and points that were lying near the breast-air boundary and relatively away from the implant. Discussion and Conclusions: Taking into account the ribs and entering the actual data for breast, rib and lung, revealed an average overestimation of dose in lung in the TPS calculation.