Purpose: Commercial brachytherapy (BT) treatment planning systems (TPSs) rely on TG-43 calculations that treat all materials as water and fail to accurately accommodate shielded BT sources and applicators. While this works fine for conventional applications such as HDR prostate and GYN BT, treatment of superficial lesions and those in proximity to organs-at-risk will benefit from more sophisticated treatment delivery techniques. This includes the cylindrical AccuBoost breast and Leipzig skin BT applicators. Model-based dose calculation algorithms and applicator models for these devices are not readily available. Innovations using the TG-43 formalism make it possible for BT TPSs to approximate dose distributions in the vicinity of these shielded, high-Z applicators.
Methods: This study created models for the AccuBoost and Leipzig BT applicators to function within a conventional BT TPS. Published Monte Carlo-derived dose distributions in Cartesian coordinates (along-away) were transformed into polar coordinates (radius and polar angle) for derivation of TG-43 dosimetry parameters through customized values of the dose-rate constant, radial dose function (RDF), 2D anisotropy function, active length, and calibrated source strength. RDF and 2D anisotropy function values were chosen to have sub-millimeter/sub-degree changes, permitting characterization of high-gradient dose regions (i.e., >40%/millimeter). These models were formatted as XML+MD5 files for TPS entry (Oncentra v.4.6.0, Elekta), then used to generate treatment plans for source/applicator commissioning. DICOM RD files were prepared and exported for direct comparison with published Monte Carlo-derived dose distributions.
Results: Though the TPS limited the 3D dose calculation grid to voxels of at least 1.0 mm, high accuracy of absolute dose was observed with agreement typically within 0.2% and greater than 99.9% of the dose points being within 2%. Voxels outside 2% agreement were located in the high-gradient dose regions where volume averaging was not insignificant.
Conclusion: This innovative method produced dosimetry results that were deemed acceptable for clinical use.