1103 Acute Toxicity Effect of Hypofractionated Whole-Breast Radiation Therapy With Simultaneous Boost of the Tumor Bed in Patients With Breast-Conserving Treatment J. Luo, Z. Zhou, X. Chen, Z. Yang, J. Ma, X. Mei, L. Zhang, Q.C. Hu, X. Guo, and X. Yu; Fudan University Shanghai Cancer Center, Shanghai, China Purpose/Objective(s): It has been well established the long-term safety and efﬁcacy of hypofractionated whole-breast irradiation (H-WBI) in randomized trials, but little is known about the acute toxic effects expe- rienced by patients treated with H-WBI and simultaneous boost of the tumor bed (TB). We herein assess the acute toxicity in patients treated with H-WBI and simultaneous boost of the TB after breast conserving surgery. (Clinicaltrials.gov : NCT02617043) Materials/Methods: From April 2015, 151 patients enrolled on an IRB- approved prospective study to assess the efﬁcacy and toxicity of hypo- fractionated RT combined with simultaneous boost of TB in patients with breast conserving treatment. All patient received total dose of 40 Gy/15 Fx for whole breast, combined with simultaneous boost of TB, 3.2 Gy/Fx, total dose of 48 Gy/15 Fx. Simple intensity-modulated radiotherapy (sIMRT) treatment planning were applied in the whole set of patients. The acute toxicity effects were assessed according to the National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE) 4.0. Results: From April 2015 to December 2015, 151 patients with T1- 2N0M0 tumors were consecutively and prospectively enrolled into study. The median age was 45 years old (range, 25-70 years). One hundred eleven (73.5%) patients had T1 tumors and 40 (26.5%) patients had T2. Among them, 88 (58.3%) patients received chemotherapy, including anthracycline, paclitaxel and platinum. One hundred thirteen (74.8%) patients were hormone receptor positive and 109 (72.2%) patients received adjuvant hormone therapy. When the acute toxicity effects of radiotherapy were assessed, 8 (5.3%) of them had no obvious acute physician-assessed skin reaction. 120 (79.5%) and 23 (15.2%) patients had Grade 1 and Grade 2 radiation dermatitis, respectively. Most of them showed skin erythema or day desquamation. Nine patients had moist desquamation mainly in the nipple and areola area, and twenty-one patients had mild lymphedema of breast. No grade 3 radiation dermatitis or pneumonitis was observed during treatment. The median treatment time of radiotherapy was 20 days (range, 17-25 days). Conclusion: Hypofractionated whole breast irradiation combined with simultaneous boost of the tumor bed can be well tolerated in patients with early breast cancer, in terms of dermatitis, lymphedema, and pneumonitis. Long term follow up of local control need to be identiﬁed in the future. Author Disclosure: J. Luo: None. Z. Zhou: None. X. Chen: None. Z. Yang: None. J. Ma: None. X. Mei: None. L. Zhang: None. Q. Hu: None. X. Guo: None. X. Yu: None. 1104 Postmastectomy Radiation in Breast Cancer Patients With Pathologically Positive Lymph Nodes After Neoadjuvant Chemotherapy: An Analysis of Treatment and Survival Trends From the National Cancer Data Base N. Ohri, 1 A.Y. Ho, 2 S. Green, 1 R. Rhome, 1 and C.J. Tsai 2 ; 1 Icahn School of Medicine at Mount Sinai, New York, NY, 2 Memorial Sloan Kettering Cancer Center, New York, NY Purpose/Objective(s): To analyze patterns of postmastectomy radiation therapy (PMRT) utilization and the association between PMRTand overall survival (OS) in breast cancer patients with pathologically positive lymph nodes after neoadjuvant chemotherapy (NAC). Materials/Methods: Using the National Cancer Data Base (NCDB), we identiﬁed women with invasive, non-metastatic breast cancer diagnosed between 2004 and 2013 who received NAC and underwent mastectomy with macroscopic pathologically positive lymph nodes. Patients with bilateral or inﬂammatory breast cancer were excluded, as were those who received neoadjuvant hormone therapy, neoadjuvant radiation therapy, or intraoperative chemotherapy. PMRT was deﬁned as receipt of 45-65 Gy of external beam radiation therapy to the breast/chest wall with or without regional nodal irradiation. Multivariate logistic regression models were performed to identify predictors of PMRT utilization. Five year OS was calculated using the Kaplan-Meier method. Cox propor- tional hazards models were used to identify factors associated with mortality. Results: The study cohort included 29,270 patients, of whom 59.1% received PMRT. Fifty-four percent of patients had ypN1 disease, 31.0% had ypN2 disease, and 14.7% had ypN3 disease. The PMRT utilization rates among ypN1, ypN2, and ypN3 patients were 55.0%, 64.7%, and 62.5%, respectively. Overall, the use of PMRT increased over the study period, from 50.0% in 2004 to 61.4% in 2013. PMRT utilization was lower among patients with higher Charlson co-morbidity scores (2 vs. 0-1; odds ratio [OR] Z 0.77, 95% CI Z 0.63 to 0.94), black patients (OR Z 0.92, 95% CI Z 0.86 to 0.99), Hispanic patients (OR Z 0.73, 95% CI Z 0.65 to 0.81), and patients with Medicaid/Medicare compared to private insurance (OR Z 0.82, 95% CI Z 0.77 to 0.87). Patients treated at an academic facility (OR Z 1.08, 95% CI Z 1.02 to 1.14) and those who received hormone therapy (OR Z 2.22, 95% CI Z 2.10 to 2.35) were more likely to receive PMRT. Compared to ypN1 patients, those with ypN2 (OR Z 1.52, 95% CI Z 1.43 to 1.62), and ypN3 (OR Z 1.44, 95% CI Z 1.33 to 1.57) disease were also more likely to receive PMRT. Age (50 vs. >50 years) and T stage (ypT0/Tis vs. ypT1/T2 vs. ypT3/T4) were not signiﬁ- cantly associated with PMRT use. The 5-year OS was similar among ypN1 patients (76.9 vs. 76.9%, P Z 0.487) but signiﬁcantly higher with PMRT among ypN2 (67.0 vs. 60.7%, P < 0.001) and ypN3 (57.4 vs. 46.7%, P < 0.001) patients. On multivariable analysis adjusting for demographic and clinicopathologic covariates, PMRT was not associated with improved survival in the analysis combining ypN1-N3 patients (hazard ratio [HR] Z 1.00, 95% CI Z 0.94 to 1.06). Stratifying by nodal stage, survival was improved with PMRT among ypN3 patients only (HR Z 0.88, 95% CI Z 0.78 to 0.99). Conclusion: PMRT use in breast cancer patients with pathologically pos- itive lymph nodes after NAC has increased over the past decade but remains underutilized, particularly among patients with ypN3 disease who may have an overall survival beneﬁt. Author Disclosure: N. Ohri: None. A.Y. Ho: None. S. Green: None. R. Rhome: None. C. Tsai: None. 1105 Do Young Women Beneﬁt From a Radiation Therapy Boost to the Tumor Bed in the Hormone Therapy Era? L.E. Beaton, 1 , 2 E.K. Chan, 1 S. Tyldesley, 1,2 L. Gondara, 1 C. Speers, 1 and A. Nichol 1,2 ; 1 BC Cancer Agency, Vancouver, BC, Canada, 2 University of British Columbia, Vancouver, BC, Canada Purpose/Objective(s): The EORTC 22881 boost trial showed a substan- tial beneﬁt of delivering a radiotherapy boost to the tumor bed (RTB) in women aged 40 years and younger, with an improvement in 10-year local relapse-free survival (LRFS) of 10%. However, this trial was carried out in an era where pre-menopausal women did not receive adjuvant hormone therapy (HT). We sought to determine how the use of HT and RTB changed in a population-based cancer care program in response to new practice guidelines, and whether this had an impact on LRFS. We also set out to determine whether the anticipated beneﬁt of RTB for young women was observed in the era of routine HT. Materials/Methods: A provincial database was used to identify all women 40 years and younger with breast cancer that met the inclusion criteria of the EORTC 22881 trial: treated with whole breast radiotherapy after breast conserving surgery, margin negative (not at ink), and stage I and II. The percentages of women receiving HT and RTB were compared across three International Journal of Radiation Oncology  Biology  Physics S208