Monte Carlo model for a prototype CT-compatible, anatomically adaptive, shielded intracavitary brachytherapy applicator for the treatment of cervical cancer

Purpose: Current, clinically applicable intracavitary brachytherapy applicators that utilize shielded ovoids contain a pair of tungsten-alloy shields which serve to reduce dose delivered to the rectum and bladder during source afterloading. After applicator insertion, these fixed shields are not necessarily positioned to provide optimal shielding of these critical structures due to variations in patient anatomies. The authors present a dosimetric evaluation of a novel prototype intracavitary brachytherapy ovoid [anatomically adaptive applicator (A³)], featuring a single shield whose position can be adjusted with two degrees of freedom: Rotation about and translation along the long axis of the ovoid. Methods: The dosimetry of the device for a HDR Ir192 was characterized using radiochromic film measurements for various shield orientations. A MCNPX Monte Carlo model was developed of the prototype ovoid and integrated with a previously validated model of a v2 mHDR Ir192 source (Nucletron Co.). The model was validated for three distinct shield orientations using film measurements. Results: For the most complex case, 91% of the absolute simulated and measured dose points agreed within 2% or 2 mm and 96% agreed within 10% or 2 mm. Conclusions: Validation of the Monte Carlo model facilitates future investigations into any dosimetric advantages the use of the A ³ may have over the current state of art with respect to optimization and customization of dose delivery as a function of patient anatomical geometries.