Background: Multi-beam IMRT enhances the therapeutic index by increasing the dosimetric coverage of the targeted tumor tissues while minimizing volumes of adjacent organs receiving high doses of RT. The tradeoff is that a greater volume of lung is exposed to low doses of RT, raising concern about the risk of radiation pneumonitis (RP).Methods: Between 7/2010-1/2013, patients with node-positive breast cancer received inverseplanned, multibeam IMRT to the breast/chest wall and regional nodes including the internal mammary nodes (IMN). The primary endpoint was feasibility, predefined by dosimetric treatment planning criteria. Secondary endpoints included the incidence of ≥ grade 3 RP and changes in pulmonary function measured by CTCAE v3.0 scales, pulmonary function tests (PFTs) and

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old). 107 (51%) patients were hormone receptor positive and 72 (34.4%) of them were Her2 positive. All patients received NAC and 188 (89.5%) of whom received NAC based on paclitaxel and carboplatin, the median course was 4. 192 (91.4%) patients received postoperative chemotherapy and 151 (76.6%) patients received radiation therapy. Almost all hormone receptor positive patients (104/107) received hormone therapy and only 40.3% (29/72) Her2 positive patients received Herceptin therapy. Of all patients, 42.9% (90/210) reached pCR in primary tumor, who had favorable DFS compared with non-pCR ones (Hazard ratio: 0.4024, 95% confidence interval [CI] 0.1699-0.9530, P Z 0.0658). OS didn't show significant difference between two groups (HR: 0.2723, 95% [CI] 0.05391-1.3749, P Z 0.2029). Postoperative radiation therapy significantly improved DFS in patients with residual tumor (HR: 0.2782, 95% [CI] 0.0777-0.9959, P Z 0.0077), but didn't improve OS (HR: 0.7971 95% [CI] 0.0719-8.8427, P Z 0.8436). Because of relative low sample size in primary tumor pCR patients, radiation therapy improved neither DFS nor OS. Conclusion: Primary tumor pCR was a favorable factor for prognosis in patients who reached axillary pCR after NAC. Patients with residual tumor tend to have lower DFS compared with primary tumor pCR ones, but radiation therapy could improve DFS for them.

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