Dosimetric effects of saline- versus water-filled balloon applicators for IORT using the model S700 electronic brachytherapy source

Purpose: The Xoft Axxent Electronic Brachytherapy System (Xoft, Inc., San Jose, CA) is a viable option for intraoperative radiation therapy (IORT) treatment of early-stage breast cancer. The low-energy (50-kVp) X-ray source simplifies shielding and increases relative biological effectiveness but increases dose distribution sensitivity to medium composition. Treatment planning systems typically assume homogenous water for brachytherapy dose calculations, including precalculated atlas plans for Xoft IORT. However, Xoft recommends saline for balloon applicator filling. This study investigates dosimetric differences due to increased effective atomic number (Z_eff) for saline (Z_eff = 7.56) versus water (Z_eff = 7.42).

Methods: Balloon applicator diameters range from 3 to 6 cm. Monte Carlo N-Particle software is used to calculate dose at the surface (D_s) of and 1 cm away (D_1cm) from the water-/saline-filled balloon applicator using a single dwell at the applicator center as a simple estimation of the dosimetry and multiple dwells simulating the clinical dose distributions for the atlas plans.

Results: Single-dwell plans show a 4.4-6.1% decrease in D_s for the 3- to 6-cm diameter applicators due to the saline. Multidwell plans show similar results: 4.9% and 6.4% D_s decrease, for 4-cm and 6-cm diameter applicators, respectively. For the single-dwell plans, D_1cm decreases 3.6-5.2% for the 3- to 6-cm diameter applicators. For the multidwell plans, D_1cm decreases 3.3% and 5.3% for the 4-cm and 6-cm applicators, respectively.

Conclusions: The dosimetric effect introduced by saline versus water filling for Xoft balloon applicator-based IORT treatments is ∼5%. Users should be aware of this in the context of both treatment planning and patient outcome studies.