On the impact of absorbed dose specification, tissue heterogeneities, and applicator heterogeneities on Monte Carlo-based dosimetry of Ir-192, Se-75, and Yb-169 in conventional and intensity-modulated brachytherapy for the treatment of cervical cancer.

Purpose: The purpose of this study was to evaluate the impact of dose reporting schemes and tissue/applicator heterogeneities for Ir-, Se-, and Yb-based MRI-guided conventional and intensity-modulated brachytherapy.

Methods And Materials: Treatment plans using a variety of dose reporting and tissue/applicator segmentation schemes were generated for a cohort (n = 10) of cervical cancer patients treated with Ir-based Venezia brachytherapy. Dose calculations were performed using RapidBrachyMCTPS, a Geant4-based research Monte Carlo treatment planning system. Ultimately, five dose calculation scenarios were evaluated: (a) dose to water in water (D ); (b) D taking the applicator material into consideration (D ); (c) dose to water in medium (D ); (d and e) dose to medium in medium with mass densities assigned either nominally per structure (D ) or voxel-by-voxel (D ).

Results: Ignoring the plastic Venezia applicator (D ) overestimates D by up to 1% (average) with high energy source ( Ir and Se) and up to 2% with Yb. Scoring dose to water (D or D ) generally overestimates dose and this effect increases with decreasing photon energy. Reporting dose other than D (or D ) for Yb-based conventional and intensity-modulated brachytherapy leads to a simultaneous overestimation (up to 4%) of CTV D and underestimation (up to 2%) of bladder D due to a significant dip in the mass-energy absorption ratios at the depths of nearby targets and OARs. Using a nominal mass-density assignment per structure, rather than a CT-derived voxel-by-voxel assignment for MRI-guided brachytherapy, amounts to a dose error up to 1% for all radionuclides considered.

Conclusions: The effects of the considered dose reporting schemes trend correspondingly between conventional and intensity-modulated brachytherapy. In the absence of CT-derived mass densities, MRI-only-based dosimetry can adequately approximate D by assigning nominal mass densities to structures. Tissue and applicator heterogeneities do not significantly impact dosimetry for Ir and Se, but do for Yb; dose reporting must be explicitly defined since D and D may overstate the dosimetric benefits.