Abstract P4-16-09: Dosimetric feasibility and acute toxicity in a prospective trial of ultra-short course accelerated partial breast irradiation (APBI) using a multi-lumen balloon brachytherapy device

Abstract

Abstract Background: Shorter courses of APBI with novel fractionation schedules are being investigated; a large randomized trial from Europe has recently shown the safety and efficacy of a single-fraction adjuvant approach (with limited follow-up). We designed a prospective trial to identify and address the potential radiobiological and logistical limitations of single-fraction, intraoperative APBI. Methods: We designed a single-arm, multi-institutional, prospective phase II trial that sequentially treats three cohorts of women (each n=30) with three 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. Using a reference schedule of 60 Gy delivered in 2 Gy fractions, and assuming tumor parameters: a/b = 4 Gy; a = 0.27 Gy-1, and repopulation parameters of: Teff = 26 days; delay time = 0 days, the reference tumor BED is ∼ 86 Gy4. We began with a schedule of 4 fractions of 7 Gy delivered twice-daily using a Contura MLB multi-lumen device. We defined very conservative dosimetric criteria for acceptability: maximum skin and rib dose to not exceed 100% of prescription dose, and V150 and V200 to not exceed 40 cc and 10 cc, respectively. Subsequent schedules are 3 fractions of 8.25 Gy and 2 fractions of 10.25 Gy, both delivered over 2 days. The primary endpoint is to exclude a local failure rate exceeding 10% with the upper limit of a 95% confidence interval. Results: A total of 30 patients have been enrolled at the 7 Gy × 4 fractions dose-level and followed for a minimum of 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). 96% of the PTV_eval received a median of 95% of PD (range 85–103). The V150 (range 14–48cc) and V200 (range 0–29cc) 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. No acute toxicities greater than CTCAE grade 2 have been observed. 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 three fractions. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P4-16-09.