Monte Carlo and analytic calculation of absorbed dose near ¹³⁷CS intracavitary sources

This paper presents dose-rate tables and treatment planning data needed to accurately implement the Sievert line-source integral, found on most commercial computer-aided treatment planning systems, for two recently introduced ¹³⁷CS intracavitary sources. One source uses a high-density active core designed to reproduce the non-elliptical isodose curves characteristic of the traditional radium tube. The other source consists of two or three discrete ¹³⁷CS seeds encapsulated in stainless steel. Using Monte Carlo dose-rate calculations as the standard of accuracy, we show that the Sievert model, using conventionally defined filtration corrections, overestimates kerma-rate in free space by as much as 20%. In addition, tissue attenuation and scatter build-up factors, derived from an isotropic point source, do not accurately characterize the distribution of scatter dose about heavily filtered sources. By varying the input parameters of the Sievert line-source integral so as to optimize its agreement with the more rigorous Monte Carlo data, accuracy of 3% can be achieved.