Background: In patients with breast cancer, prone radiation therapy (RT) has been shown to reduce heart and lung dose. Though prone positioning is routinely used for whole breast RT, its use when treating the regional lymph nodes (RLNs) is not widespread.
Methods: In this phase I-II trial for stage IB-IIA breast cancer treated with lumpectomy or mastectomy, patients received 40.5Gy in 15 fractions to the breast or chest wall and RLNs with an integrated tumor bed boost for lumpectomy patients. Primary endpoints were grade >2 acute toxicity and dosimetric feasibility. Secondary endpoints were incidence of re-simulation to improve dosimetry and late toxicity. Exploratory endpoints were local recurrence (LR), disease free survival (DFS), distant recurrence free survival (DRFS) and overall survival (OS).
Results: From 2009-2016, 97 patients were enrolled (68% lumpectomy and 32% mastectomy) among which 92 were treated in the prone position. 5 patients were re-simulated and treated supine. Among the prone treated patients, there were no acute toxicities greater than grade 2. 92%, 98%, and 89% met PTV tumor V48Gy ≥ 98%, breast V40.5Gy ≥ 95% and nodal V38.5Gy ≥ 95% respectively. All met the heart V5Gy < 5%, contralateral lung V5Gy < 15%, spinal cord dose maximum (Dmax) ≤ 37.5Gy, esophagus V30Gy < 50% and Dmax ≤ 40.5Gy. 98% met the ipsilateral lung V10Gy. Brachial plexus Dmax < 42Gy was met in 74% with a mean increase of 1.61 Gy (SD 1.96 Gy) over target. At median follow up of 8 years, grade 2-3 late toxicity was 23% for prone patients. There were 2 LRs (2%) and no chest wall or nodal recurrences. 8-year DRFS, DFS and OS were 88% (95% CI 81%, 95%), 86% (95% CI 78%, 95%) and 91% (95% CI 84%, 98%), respectively.
Conclusion: Toxicity was low and outcomes were excellent in this prospective trial of prone hypofractionated nodal RT.
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