Abstract P3-19-27: Prognostic impact of postmastectomy radiation therapy in breast cancer patients with T1, 2 and 1-3 lymph nodes from Japan Breast Cancer Registry

Background: The effectiveness of postmastectomy radiation therapy (PMRT) in patients with pT1-2 and 1-3 lymph node metastases remains controversial in the current clinical practice. Patients and Methods: Using data from the Japanese National Clinical Database (NCD) between 2004 and 2012, we evaluated the association of PMRT with recurrence and breast cancer mortality. Patients who underwent mastectomy and axillary node dissection and were diagnosed as pT1-2 with 1-3 node metastases were enrolled. Patients who received presurgical treatment were excluded. We compared clinicopathological factors and prognosis between patients with PMRT (PMRT group) and without PMRT (No-PMRT group). The primary endpoint was the locoregional recurrence (LRR) rate. We also assessed the impact of PNRT according to the number of node metastasis. We considered death as a competing event.Results: Among 8,914 enrolled patients, PMRT group included 492 patients (5.5%), and No-PMRT group did 8,422 patients (94.5%). Patients of PMRT group were younger, had a larger tumor and more node metastases than patients of No-PMRT group. There is no difference in breast cancer subtype between two groups. A median observation time was 6.3 years (arnge 5.0- 9.7 years). There was no significant difference between PMRT group and No-PMRT group in LRR rate (4.0% v.s. 5.0%, P=0.61), any recurrence rate (13.8% v.s. 11.8%, P=0.23), and breast cancer mortality rate (6.0% v.s. 4.3%, P=0.08) at 5 years. Multivariate analysis revealed no significant association between PMRT and LRR while LRR is significantly associated with tumor size larger than 2cm (hazard ratio [HR] 1.48, 95%confidence interval [CI] 1.21-1.82 in 2.1-3.5cm, HR 1.97, 95%CI 1.53-2.53 in 3.6-5.0cm) 2 (HR1.25 95%CI 1.02-1.52) or 3 node metastases (HR 1.40. 95%CI 1.10-1.79), triple-negative subtype (HR 1.64, 95%CI 1.21-2.23). According to the number of node metastasis, LRR in PMRT group was significantly lower than that of No-PMRT group among patients with 3 node metastases (2.6% v.s. 7.0%, P=0.03) while there was no significant difference between two groups among patients with 1 or 2 node metastases. In multivariate analysis, HR was relatively lower in patients with 3 node metastases (HR 0.37, 95%CI 0.11 -1.19) comparing to the patients with 1 (HR 0.97, 95%CI 0.53- 1.77) or 2 node metastases (HR 1.06, 95%CI 0.53-2.09). Tumor size was significantly associated with LRR in patients with 1 (HR 1.51, 95%CI 1.13- 2.02) or 2 node metastases (HR 1.45, 95%CI 1.02-2.07). Chemotherapy was significantly associated with LRR among the patients with 2 (HR 0.57, 95%CI 0.40-0.82) or 3 node metastases (HR 0.4, 95%CI 0.25-0.65).Conclusions: Among the patients with T1-2 and 1-3 node metastases, PMRT was not associated with a reduced risk of LRR in the latest Japanese cohort. Advances in systemic therapy might the main reason to reduce the LRR rate rather than PMRT in this population. The administration of PMRT should be tailored considering the individual risks of LRR, such as 3 node metastases. Citation Format: Akimitsu Yamada, Naoki Hayashi, Hiraku Kumamaru, Masayuki Nagahashi, Shiori Usune, Hiroaki Miyata, Takashi Ishikawa, Kazutaka Narui, Itaru Endo, Shigeru Imoto, Shinji Ohno, Hiromitsu Jinno. Prognostic impact of postmastectomy radiation therapy in breast cancer patients with T1, 2 and 1-3 lymph nodes from Japan Breast Cancer Registry [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P3-19-27.