Background: Shorter courses of APBI, including single-fraction intraoperative therapy, are under active investigation. We designed a prospective trial to identify and address the potential radiobiological and logistical shortcomings of single-fraction APBI. Methods: We designed a single-arm, multi-institutional, prospective phase II trial that sequentially treats 3 cohorts of women (each n = 30) with 3 progressively hypofractionated schedules. Eligible women were age ≥50 years with unifocal invasive or in situ tumors ≤3.0 cm, excised with negative margins, and with negative lymph nodes and positive hormone receptors. We defined strict dosimetric criteria for appropriateness. Results: A total of 30 patients were enrolled at the 7 Gy × 4 fractions dose-level and followed for 6 months. The median skin dose as a percent of prescription dose (PD) was 84 % (40-100), and the median rib dose was 71 % (16-119). Also, 95 % of the PTV-eval received a median of 95 % of PD (range 85-103). The V150 (range 14-48 cc) and V200 (range 0-29 cc) criteria were met in all cases. One breast infection occurred and was treated; 2 cases of symptomatic fat necrosis and 2 cases of symptomatic seromas occurred. Conclusion: Short-course APBI is dosimetrically feasible using the Contura MLB and appears to be tolerable in terms of acute toxicities. Our approach is based on well-defined radiobiological parameters and allows for an abbreviated course of treatment that is guided by full pathological review and the ability to objectively achieve and validate acceptable dosimetric criteria in each case. We have opened enrollment to the next schedule of 8.25 Gy for 3 fractions.
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