Abstract Objectives: To evaluate the clinical outcomes according to the application of an elective nodal irradiation (ENI) and molecular subtypes in high-risk pathologic N1 (pN1) breast cancer. Methods: We performed a retrospective comparison study with high-risk pN1 patients who received curative resection followed by chemotherapy at Samsung Medical Center from January 2009 to June 2011. High risk was defined as having more than two of the following risk factors: ≥2 axillary lymph nodes (ALNs) or level II ALN metastasis, lymphovascular invasion and extracapsular extension. We compared clinical outcomes according to the application of ENI. Results: Among 278 patients, 159 patients received ENI while 119 did not and their characteristics were displayed. Table 1. Characteristics of 278 pathologic N1 breast cancer patients according to the usage of elective nodal irradiation (ENI).Variables no ENI (n=119)ENI (n=159)P-valueAge (years)Median48480.50 Range27-8027-71 Surgery (%)MRM90 (75.6)2 (1.3)<0.001 BCS29 (24.4)157 (98.7) Pathology (%)IDC116 (97.5)153 (96.2)0.74 Other3 (2.5)6 (3.8) T stage (%)T156 (47.1)81 (50.9)0.55 T263 (53.9)78 (49.1) Histologic grade (%)1-284 (70.6)102 (64.2)0.30 335 (29.4)57 (35.8) Nuclear grade (%)1-279 (66.4)90 (56.6)0.11 340 (33.6)69 (43.4) Dissected ALN numberMedian19170.17 Range2-615-48 Estrogen receptor (%)Positive92 (77.3)124 (78.0)1.00 Negative27 (22.7)35 (22.0) Progesterone receptor (%)Positive87 (73.1)123 (77.4)0.48 Negative32 (26.9)36 (22.6) HER-2 amplification (%)Positive33 (27.7)24 (15.1)0.01 Negative86 (72.3)135 (84.9) Hormonal therapy (%)No29 (24.4)31 (21.9)0.38 Yes90 (75.6)128 (78.1) Chemotherapy (%)No13 (10.9)5 (3.1)0.01 Yes106 (89.1)154 (96.9) Trastuzumab (%)No5 (15.2)1 (4.2)0.19 Yes28 (84.8)23 (95.8) During follow-up, 21 patients (6 in ENI and 15 in no ENI) had recurrence, and loco-regional recurrence developed in 8 patients, 6 of whom had not received ENI. Table 2. Patterns of recurrence according to ENI and molecular subtype No ENIENI SitenRecurrence (%)nRecurrence (%)P-valueLoco-regional-n1196 (5.0)1592 (1.3)0.08Luminal A360 (0.0)410 (0.0) Luminal B574 (7.0)860 (0.0) HER-2 enriched120 (0.0)91 (11.1) Triple negative142 (14.3)231 (4.3) Distnat-n11910 (6.5)1595 (3.1)0.06Luminal A362 (5.6)410 (0.0) Luminal B576 (10.5)862 (2.3) HER-2 enriched120 (0.0)90 (0.0) Triple negative142 (14.3)233 (13.0) Total-n11915 (12.6)1596 (3.8)0.01Luminal A362 (5.6)410 (0.0) Luminal B579 (15.8)862 (2.3) HER-2 enriched120 (0.0)91 (11.1) Triple negative144 (28.6)233 (13.0) In both univariate and multivariate analysis, ENI, adjuvant chemotherapy and endocrine therapy were the significant prognostic factors in recurrence-free survival (RFS). ENI showed higher RFS than no ENI when they were analyzed according to molecular subtypes, except HER-2 enriched. RFS was 96.9% at 3-years in luminal A patients with no ENI. Conclusions: The application of ENI may improve RFS in patients with high-risk pN1 breast cancer, so ENI should be considered in those patients. Close observation without ENI might be an option in patients with luminal A who had received optimal systemic managements. Citation Format: Jeong Il Yu, Won Park, Doo Ho Choi, Seung Jae Huh, Seok Jin Nam, Seok Won Kim, Jeong Eon Lee, Won Ho Kil, Young-Hyuck Im, Jin Seok Ahn, Yeon Hee Park, Eun Yoon Cho. Clinical outcomes according to elective nodal irradiation and molecular subtypes in high risk N1 breast cancer patients [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 P1-15-23.