Quantifying the effect of seed orientation in postplanning dosimetry of low-dose-rate prostate brachytherapy

Purpose: Radioactive seed orientations are usually ignored in clinical brachytherapy dosimetry for prostate implants. Associated with the anisotropic dose distribution of seeds, these orientations could cause dose differences between the planning configurations and the clinical postplanning dosimetry. This study will quantify the impact of seed orientation on the dosimetry. Methods: 3D seed positions and theta and phi polar angles were obtained using five independent fluoroscopic images for 287 patients. Five dose calculation methods are compared: TG43-1D (1), TG43-2D parallel to implant axis (2) and with orientations (3), Monte Carlo (MC) simulations parallel (4), and MC simulations with orientations (5). GEANT4 v4.9.6 MC simulations were made in 1 mm(3) voxelized geometries based on the DICOM-RT information. Materials were assigned using thresholds based on the HU number, as recommended in TG186 reports. Seed voxels are overridden with prostatic materials and the layered mass geometry [Enger et al., Phys. Med. Biol. 57(19), 6269-6277 (2012)] allows subsequent placement of the source geometry. 500 million histories were used per patient. 3D dose and DVHs for each structure were calculated. Results: The various seed orientations do not result in statistically significant differences on the dose metrics for the clinical target volume (CTV) or the urethra, based on the Student t-test p-value. Difference as low as -0.238% and 0.059% has been seen on the CTV D90, respectively, for the MC and the TG43. The difference between parallel and oriented calculations for the organs at risk (OARs) can differ by 2% on average. Conclusions: Based on the results from this study, seed orientations have no significant impact of CTV and urethra dose metrics but can affect OARs that are external to the CTV.