A monte carlo dosimetry study of vaginal 192Ir brachytherapy applications with a shielded cylindrical applicator set.

A durable recommendation for brachytherapy treatment planning systems to account for the effect of tissue, applicator and shielding material heterogeneities exists. As different proposed approaches have not been integrated in clinical treatment planning routine yet, currently utilized systems disregard or, most commonly, do not fully account for the aforementioned effects. Therefore, it is of interest to evaluate the efficacy of current treatment planning in clinical applications susceptible to errors due to heterogeneities. In this work the effect of the internal structure as well as the shielding used with a commercially available cylindrical shielded applicator set (Nucletron part # 084.320) for vaginal and rectum treatments is studied using three-dimensional Monte Carlo simulation for a clinical treatment plan involving seven source dwell positions of the classic microSelectron HDR 192Ir source. Results are compared to calculations of a treatment planning system (Plato BPS v.14.2.7), which assumes homogeneous water medium and applies a constant, multiplicative transmission factor only at points lying in the shadow of the shield. It is found that the internal structure of the applicator (which includes stainless steel, air and plastic materials) with no shield loaded does not affect the dose distribution relative to homogeneous water. In the unshielded side of the applicator with a 90 degrees, 180 degrees, or 270 degrees tungsten alloy shield loaded, an overestimation of treatment planning system calculations relative to Monte Carlo results was observed which is both shield and position dependent. While significant (up to 15%) at increased distances, which are not of major clinical importance, this overestimation does not affect dose prescription distances by more than 3%. The inverse effect of approx. 3% dose increase at dose prescription distances is observed for stainless steel shields. Regarding the shielded side of the applicator, it is shown that the default treatment planning system transmission factors for tungsten alloy result in a consistent dose over-estimation thus constituting a safe approach given the nature of associated clinical applications. Stainless steel is shown to be an ineffective shielding material with transmission factors reaching up to 0.68 at increased distances irrespective of shield geometry.