Abstract P1-15-25: Phase II trial of hypofractionated post mastectomy radiation

Purpose: The efficacy of irradiating the chest wall and draining lymph nodes after mastectomy in improving local regional control has been firmly established by multiple trials comparing mastectomy alone to mastectomy with postoperative radiation. The currently accepted standard of care involves 25 fractions of chest wall and nodal radiation plus an optional 5 fraction scar boost. Given recent publications with intact breast radiation demonstrating equivalence with shorter course radiation treatment, we sought to establish a short course of radiation for women post mastectomy that would be equivalent to conventional radiation in terms of local control and complications. Methods: Starting in 2011, Stage II and III, unilateral breast cancer patients were prospectively consented to participate in our study at Rutgers Cancer Institute of New Jersey and the University of Utah. We designed a phase II non-inferiority trial, with plans to enroll 65 patients, allowing for a 18% loco-regional recurrence rate if the true rate is 10%. Patients were allowed to receive chemotherapy before or after adjuvant radiation with radiation starting 14-63 days after surgery or adjuvant chemotherapy. Patients underwent 3D simulation and were treated with ≥ 6MV photon radiation using 3D conformal tangents or intensity modulation. Regional nodal radiation was included when clinically appropriate with inclusion of internal mammary nodes at the treating provider’s discretion. The prescription dose was prescribed at 333cGy per day to the chest wall and regional nodes for 11 daily fractions followed by a recommend scar boost of 333cGy per day for 4 fractions. Chest wall PTV was keep within 90-115% of the prescribed dose, the maximum brachial plexus dose was keep below 3919cGy (107%) and no portion of the heart could receive >2Gy/day. The maximum dose of the boost field was kept to 120% of the boost prescription. Any breast reconstruction technique was allowed, per institutional standard. Results: 48 patients have thus far been enrolled with a median follow up interval of 17 months. Looking at the 43 patients with > 4 months of follow-up, we have only seen a 4.7% grade 3 or greater acute or late toxicity rate, with one grade 3 wound complications, and one grade 3 fibrosis. There has been one chest wall recurrence, for a 2.3% local recurrence rate. Conclusion: Early results would appear to show that hypofractionated post mastectomy radiation appears to be safe and feasible. Acute toxicity is less than anticipated, however it is too early make any conclusions on efficacy or late effects, with only 48 out of 65 patients enrolled. We hope to use the results of this phase II trial to design a randomized prospective trial comparing hypofractionated post mastectomy radiation to standard fraction. Citation Format: Matthew M Poppe, Bruce G Haffty, Atif J Khan. Phase II trial of hypofractionated post mastectomy radiation [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P1-15-25.