SU‐GG‐T‐78: Influence of Material Heterogeneities for Brachytherapy Dose Calculations
Purpose: To quantify the influence of material heterogeneities for brachytherapydose calculations. Method and Materials: Current brachytherapytreatment planning systems (TPS) use the TG‐43 dose calculation formalism. This formalism uses the superposition principle with single‐source dose distributions in liquid water, and is used for all conventional brachytherapy sources (both low‐energy and high‐energy photon emitters). In the high‐energy regime, dosimetric sensitivity to media heterogeneities in comparison to water is less significant than for low‐energy sources. To quantify these effects as a function of photon energy, a series of Monte Carlo(MC) based radiation transport simulations was performed using the MCNP5 code. For simplicity and generalization, brachytherapy sources were approximated as point sources in spherical phantoms, with isotropic mono‐energetic photon‐emitting point sources to approximate photon energies from common radionuclides. The first simulation series was a homogeneous water phantom used as the reference geometry, and was compared to two different heterogeneous phantoms selected with consideration to photon energy. The low‐energy heterogeneous phantom used breast tissue for r≤3cm, cortical bone for 3.0<r<3.3cm, and water for 3.3≤r≤15 cm. The high‐energy heterogeneous phantom used lung tissue for 6<r<10cm and iron for 15.0<r<15.5cm with water everywhere else out to 40cm. Results:Dose to medium‐in‐medium (DMM) for low‐energy sources differed by −20% in breast and by a factor of 4–7 in bone, while dose to water‐in‐medium (DWM) differed by +30% and −50, respectively. For high‐energy sources, DMM differed by −21% to +8% in lung and similar amounts for DWM. In iron, DMM differed by a factor of 20−1.8 for 50–662keV, but by <30% for DWM. Using spherical geometry maximized the influence of material heterogeneity due to scatter. Conclusion:Brachytherapydose calculation accuracy using the TG‐43 formalism is challenged by material heterogeneities, resulting in errors exceeding an order of magnitude under certain conditions.