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 ¹⁹²Ir-, ⁷⁵Se-, and ¹⁶⁹Yb-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 ¹⁹²Ir-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_w,w); (b) D_w,w taking the applicator material into consideration (D_w,wApp); (c) dose to water in medium (D_w,m); (d and e) dose to medium in medium with mass densities assigned either nominally per structure (D_{m,m (Nom)}) or voxel-by-voxel (D_m,m). Results: Ignoring the plastic Venezia applicator (D_w,wApp) overestimates D_m,m by up to 1% (average) with high energy source (¹⁹²Ir and ⁷⁵Se) and up to 2% with ¹⁶⁹Yb. Scoring dose to water (D_w,wApp or D_w,m) generally overestimates dose and this effect increases with decreasing photon energy. Reporting dose other than D_m,m (or D_{m,m Nom}) for ¹⁶⁹Yb-based conventional and intensity-modulated brachytherapy leads to a simultaneous overestimation (up to 4%) of CTV_HR D₉₀ and underestimation (up to 2%) of bladder D_2cc 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_m,m by assigning nominal mass densities to structures. Tissue and applicator heterogeneities do not significantly impact dosimetry for ¹⁹²Ir and ⁷⁵Se, but do for ¹⁶⁹Yb; dose reporting must be explicitly defined since D_w,m and D_w,w may overstate the dosimetric benefits.