Physical Dosimetry in an Anthropomorphic Phantom Reveals Superior Organ Sparing with Multicatheter Interstitial Brachytherapy Over MammoSite for APBI

Abstract

For breast cancer patients, accelerated partial breast irradiation (APBI) using brachytherapy provides a highly conformal treatment option. Although planning studies frequently compare brachytherapy techniques, direct measurements of doses to organs at risk (OARs) are scarce. This study hypothesized that multicatheter interstitial brachytherapy (MIB) would demonstrate superior OAR sparing compared to MammoSite balloon brachytherapy (MBS). This research utilized physical dosimetry within an anthropomorphic phantom to quantitatively evaluate and compare the OAR doses from MIB and MBS using an anthropomorphic Rando female phantom; treatment plans for MIB and MBS were designed according to the American Brachytherapy Society/Radiation Therapy Oncology Group formalism to deliver 3.4 Gy per fraction. High-sensitivity thermoluminescent dosimeters (TLD-100H) were placed at key OAR sites including the heart, lungs, ribs, spinal cord, sternum, and skin. Ir-192 high-dose rate irradiation was performed and measured doses were compared against treatment planning system (TPS) calculations and protocol limits. Both techniques successfully met the prescribed target coverage. However, MIB demonstrated superior OAR sparing compared to MBS. TLD measurements revealed that MBS delivered significantly higher doses to the heart (30.9% vs. 19.1% prescribed dose), and the ipsilateral lung (23.5% vs. 21.6%) relative to MIB. Notably, the TPS systematically overestimated superficial doses to areas like the skin, highlighting limitations of algorithmic models in heterogeneous tissues. While MIB provided more conformal target coverage (higher V150 and V200 values) with greater dose homogeneity (dose nonuniformity ratio: 0.218 vs. 0.264). Although both MIB and MBS are safe and viable APBI techniques, but under the specific conditions of this phantom, MIB offered dosimetric advantages for heart and lung sparing. Under the specific conditions of this anthropomorphic phantom study, MIB demonstrated dosimetric advantages for heart and lung sparing compared to MBS. These findings suggest MIB may be the preferred option for left-sided breast cancers when OAR sparing is critical, but this should be confirmed with patient-specific studies.