LAUSR

Purpose Recent studies have identified and proposed gamma emitting radionuclides (75Se, 169Yb, 153Gd) with intermediate energy (50 keV  Materials and methods Post implant treatment plans were simulated with a Geant4-based Monte Carlo dose calculation engine, BrachySource, coupled to a column-generation based optimizer, for a prostate brachytherapy case. The patient was treated with a brachytherapy boost with a dose of 15 Gy in a single fraction. Simulations were performed using 60Co, 192Ir, 75Se, 169Yb, and 153Gd as the active cores of the source. The plans were independently approved by two radiation oncologists. Two MC calculation protocols were performed for each radionuclide: (1) dose calculations for which patient anatomy is modelled as unit density water and (2) dose calculations for which patient anatomy is modelled with accurate chemical composition of tissues and densities are obtained using the HU values from CT scan. Results With 90% of the planning target volume (PTV) receiving over 15 Gy, plans can reduce the PTV V150 to 19.8%, 18.0%, 18.5%, 13.7% and 11.6% for 60Co, 192Ir, 75Se, 169Yb, and 153Gd, respectively, without sacrificing the urethral D10, the bladder V75 and the rectum V75. In general, dose homogeneity within the PTV increased with decreasing average photon energy. The inclusion of tissue composition and density corrections resulted in a reduction of the PTV D90 (urethral D10) by 0.0% (0.0%), 0.8% (0.7%), 1.8% (1.6%), 3.0% (4.7%) and 4.5% (4.1%) for 60Co, 192Ir, 75Se, 169Yb, and 153Gd, respectively. Conclusion Intermediate-energy sources have the potential to increase dose homogeneity within the PTV while reducing hot spots in the urethra, bladder, and rectum. This work shows the importance of accurate MC-based treatment planning engine, which can account for tissue composition and heterogeneities, for the dosimetry of intermediate-energy sources.