Heterogeneity and scatter effects on Ir-192 brachytherapy dose distribution

We studied the effects of the presence of an air cavity and scatter due to patient size on dose distribution near an Iriduim-192 brachytherapy source (Ir-192). The source was modeled using Monte Carlo (MC) code MCNP5. The Radial dose, g_L(r), and the anisotropy function, F(r,θ) specified by the AAPM TG-43 have been determined and compared with the consensus data (AAPM report No. 229). We compared our MC results to the measured dose distribution using an EBT3 Gafchromic® film measurement. The dose was determined in the presence of an air cavity of 3, 5, and 7 mm diameters located at 2 mm distance from Ir-192. The dose was also determined for Ir-192 centered in 30 × 30 × 30 cm³ and 80 × 80 × 80 cm³ water phantoms. The MC results of g_L(r) and F(r,θ) agreed with the consensus data to within 2% and 3%, respectively. The MC and the measured dose distributions agreed well with a maximum difference of 8.2% at the periphery of the film. The dose at 10 cm from the Ir-192 source with a full scattering medium (80 × 80 × 80 cm³) was 7% higher compared to the dose in (30 × 30 × 30 cm³) water phantom. The dose to water in the presence of a 3, 5, and 7 mm diameter air cavity increased by an average of 3%, 6%, and 9%, respectively, compared to the dose with no air cavity. Ignoring scatter effects and the heterogeneity correction in the presence of an air cavity can lead to significant errors in dose delivered to patients.