Abstract PS15-14: Comparison between a breast specific radiosurgery device and intensity modulated proton therapy for accelerated partial breast irradiation
Authors: Samanta Santanu , Nichols Elizabeth , Zakhary Mark , Guerrero Mariana , Zhang Baoshe , Pollock Ariel , Lamichhane Naru , Becker Stewart 
Published: February 2021

Abstract
Purpose: A breast specific radiosurgery device (BSRD) has recently been FDA cleared for treatment of accelerated partial breast irradiation. Using this device requires a vacuum assisted immobilization system to treat patients in the prone position. Alternatively, APBI delivery using intensity modulated proton therapy (IMPT) treats patients in the supine position at University of Maryland. This study aimed to compare dose distribution to targets and organs at risk (OARS), using BSRD plans versus IMPT plans for APBI. We hypothesized that despite the physical characteristics of IMPT the BSRD plans would be superior in conformality. Methods:An IRB approved retrospective review was performed for 13 patients who previously received a lumpectomy boost using the BSRD in prone position followed by whole breast irradiation in the supine position. APBI plans were created using dose a fractionation of 30Gy in 5 fractions. Previously contoured LPC volumes with a 1cm CTV expansion were created (modified at anatomic boundaries and within 5 mm of skin) followed by a 3 mm PTV expansion for BSRD plans, our standard for all BSRD plans, or CTV with beam specific PTV for IMPT plans. A single physicist created plans for BSRD with a different physicist for IMPT. Physicists were blinded to the results of the comparative plans. Plans were compared with respect to heart max, heart mean, ipsilateral lung V5, V20, normal breast D5, D20, D50 and D80, skin max, CTV coverage, conformality index (CI) and homogeneity index (HI). Paired t-test was used for statistical analysis. Results:Six out of thirteen patients had left sided breast cancer. The CTV volumes were comparable, with no difference in coverage of 95% of the CTV receiving the prescription dose (98% vs 97%). BSRD plans had superior CI (0.82 vs 0.63, p&lt;0.0004), and HI (0.91 vs 0.65, p&lt;0.0001) and improved Ipsilateral Lung V5 (2.3% vs 0.7%, p&lt;0.055). IMPT plans had superior max heart dose (1.17Gy vs 4.13Gy, p&lt;0.001), heart mean dose (0.05Gy vs 0.8Gy, p&lt;0.0006), normal breast D5 (29 vs 31Gy p&lt;0.003), D20 (13 vs 18Gy, p&lt;0.04), D50 (0.79 vs 6.79, p&lt;0.0001), D80 (0.07 vs 2.85, p&lt;0.0001). There was no difference in max skin dose or ipsilateral lung V20. Conclusions: In conclusion, BSRD plans were associated with better dose conformity to the target and lower ipsilateral lung V5. When comparing dose to the heart and ipsilateral normal breast, both treatments plans delivered low doses below the acceptable limits, however plans were comparatively better with IMPT possibly due to target location. Additional patient evaluation is needed to further assess the impact of the breast immobilization device and target location on doses to OARs and the normal breast tissue. Overall, this comparison supports further evaluation of the BSRD technology
Citation Format: Santanu Samanta, Elizabeth Nichols, Mark Zakhary, Mariana Guerrero, Baoshe Zhang, Ariel Pollock, Naru Lamichhane, Stewart Becker. Comparison between a breast specific radiosurgery device and intensity modulated proton therapy for accelerated partial breast irradiation [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS15-14.