Lung and cardiac tissue doses in left breast cancer patients treated with single-source breast brachytherapy compared to external beam tangent fields

Purpose: We performed a retrospective analysis of doses to normal critical structures in 26 women with left-sided breast tumors treated with a high-dose-rate single source brachytherapy (SSB) system, versus whole breast external beam tangent fields (EBTF) using a computer model. Methods and materials: Each patient's EBTF radiation plan consisted of ipsilateral tangent fields normalized to 46 Gy. A complementary SSB radiation plan was designed to deliver 34 Gy in 10 fractions at 1 cm into tissue enclosing a sphere of diameter 4.6 cm at tumor bed, simulating our institution's experience with the MammoSite™ applicator. Critical structure volumes and doses analyzed included the volume of whole heart and left ventricle receiving >25 Gy, and left lung (LL) receiving 20 and 13 Gy for EBTF plans. For SSB we determined the linear quadratic 2-Gy-equivalent high-dose-rate doses of 23.4, 20, and 14 Gy to be equivalent to 25, 20, and 13 Gy, respectively. We then compared the two doses for each critical structure. Results: The percent of whole heart receiving >25 Gy equivalent using EBTF and SSB was 2% and 0% (mean dose 2.52 vs. 1.65 Gy), left ventricle receiving >25 Gy using EBTF and SSB was 3% and 0% (mean dose 3.78 vs. 2.68 Gy), LL receiving >20 Gy using EBTF and SSB was 11.5% and 0.5%, and LL receiving >13 Gy using EBTF and SSB was 12.6% and 1.85% (mean dose to LL was 6.06 Gy for EBTF vs. 2.25 Gy for SSB), respectively. Conclusions: When compared to EBTF, breast brachytherapy with SSB is associated with a significant decrease in mean dose and percentage volume receiving a given dose for each organ.