Dosimetric effects of prostate calcifications in high-dose rate brachytherapy calculations

Abstract

ABSTRACT

Purpose: At the Nova Scotia Cancer Center (NSCC), high-dose rate (HDR) brachytherapy using 192Ir is often used as a single-fraction dose boost for intermediate and high-risk prostate cancer patients prior to external-beam radiation therapy (EBRT). Recent dosimetric concerns have focused on material heterogeneities that cannot be accounted for in clinical treatment planning systems using the Task Group 43 (TG-43) water-based dose calculation algorithm. This work investigates the impact of prostate calcifications, a heterogeneity with a high atomic number, on dose distributions calculated using the EGSnrc code for Monte Carlo (MC) radiation transport in both water phantoms and heterogeneous virtual patient phantoms created from planning computed tomography (CT) images.

Methods: A model of Nucletron’s microSelectron-v2 192Ir source was created using the EGSnrc C++ class library (egspp) and validated by computing TG-43 dosimetry parameters. In a series of phantom simulations, spherical prostate calcifications (with radii of 1 mm and 3 mm) were embedded near the source at distances of 5 mm and 5 cm and compared to homogeneous water phantom simulations. Finally, patient-specific calculations were performed using several tissue assignment schemes to assign prostate and calcified materials to a virtual patient phantom based upon a patient’s planning CT. MC simulations were then performed using the dwell positions and weights derived from the treatment plan, while clinical metrics were extracted from the cumulative dose volume histograms (cDVHs) extracted from the resultant dose distributions.

Results: When benchmarked against literature results, TG-43 parameter values generally agreed within 2%. Shadowing effects were observed beyond the calcification in the phantom study, resulting in mean dose decreases of 1.48% and 6.80% for the small and large calcifications placed 5 mm away from the source. The maximum observed reductions to the prostate V100, V150 and V200 in patient-specific phantoms were 1.78%, 1.18% and 0.40% respectively, compared to a water phantom, while the urethra D10 was reduced by only 26 cGy.

Conclusions: While the dosimetric effects were mild in the context of the patient’s treatment, larger prostate calcifications simulated in water phantoms resulted in mean dose reductions of approximately 6.80% in the water phantom study. Hence, calcifications should be investigated in future work involving other patients with larger calcifications.