Optimization of Permanent Seed Implant Dosimetry Incorporating Tissue Heterogeneity

Abstract

Permanent seed implant is a brachytherapy technique for adjuvant radiation therapy of early stage prostate and breast cancer patients. In this method low dose rate (LDR) radioactive seeds are permanently implanted across the treatment volume to deliver a prescribed amount of dose. The current standard for calculation of dose surrounding the brachytherapy seeds is based on the recommendations of American Association of Physicist in Medicine Task Group No. 43 (i.e., TG43 formalism) which generates the dose in homogeneous water medium. For low energy sources, however, the dose is highly dependent on the atomic composition of the irradiated volume. In the case of breast, large dose calculation errors was found using the TG43 formalism. Our work focuses on the development of an efficient dose calculation algorithm accounting for tissue heterogeneities in the medium. We used Monte Carlo simulations to evaluate dose distributions in a heterogeneous model of breast. The radial dose distribution away from a single 103Pd seeds in heterogeneous media was compared with that in homogenous water medium to evaluate the error generated by the TG43 formalism. The Monte Carlo results were also compared with a novel analytical dose calculation algorithm in heterogeneous media. The analytical dose calculation method is based on the physics of interaction of radiation with matter and accounts for variation in density as well as the tissue composition. An increase of up to 40% was noticed to the dose to skin in a heterogeneous model of breast. This is mainly due to the high fat content of breast. The results of our analytical dose calculation algorithm compared very well with Monte Carlo simulations in heterogeneous model of breast as well as cases of extreme heterogeneity variations. By introducing heterogeneity corrections, dose to the skin and other critical organs are estimated more accurately which can potentially lead to lower incidence of toxicities. We have developed a new analytical dose calculation method which enables personalized dose calculations in heterogeneous media. The methodology offers several advantages including fast calculation time enabling easy integration into a clinical setting.