Abstract PO3-22-04: PMRT Decision with low prediction of genomic test score in pT3N0M0 luminal breast cancer: Protocol MF22-02: International multicenter real-world data

Abstract Background: In current guidelines, there is no definite recommendation regarding postmastectomy radiation therapy (PMRT) in patients with luminal pT3N0M0 breast cancer (BC). The goal of this study is to determine whether PMRT could be safely omitted for a specific subgroup of those patients. Methods and Materials: There were 202 women from 16 centers with pT3N0M0 hormone receptor (HR) positive, Her2 neu (-) BC who underwent mastectomy, were analyzed retrospectively. None of the patients received neoadjuvant chemotherapy. Three patients were excluded because of positive surgical margins. The patients were divided into two groups, PMRT (+) (n=130) and PMRT (-) (n=69). Groups were compared in terms of overall survival, loco-regional recurrence rate, and distant metastases regarding Magee score (MS) ( 18 are considered low risk) (https://path.upmc.edu/onlineTools/mageeequations.html), menopausal status, axillary surgery, pathology, lymphovascular invasion (LVI), adjuvant chemotherapy, and adjuvant endocrine therapy. Results: The majority of the patients had invasive ductal carcinoma (49%, n=98). There was no significant difference regarding tumor size, axillary surgery, and adjuvant endocrine therapy between the two groups (p=0.82, p=0.28, p=0.12, respectively). LVI was 49% (n=98) and it was greater in PMRT (+) group (25% vs. 10%; p=0.01). PMRT (+) patients received more chemotherap(66% vs. 30%; p 0.001), had more grade 3 tumors (28% vs. 9%, p=0.005), and more premenopausal (49% vs. 22%; p=0.0001). At a median follow-up of 51.3 months for the PMRT (-) group and 65.9 months for the PMRT (+) group (p=0.041), 9% (n=6) of patients from the PMRT (-) group and 2% (n=3) from the PMRT (+) group developed locoregional recurrence (LRR) (p=0.047). There was no difference in local recurrence (1% in PMRT (-) group vs. 2% in PMRT (+); p=0.7) and distant recurrence (7% in PMRT (-) group vs. 3% in PMRT (+); p=0.16) between patients who received PMRT and not had PMRT. Further comparison of the LRR in the PMRT (-) and PMRT (+) groups in patients with an MS 18 did not show a significant difference (3% vs. 4%; p=0.64). However, among patients with a Magee score ≥18, the PMRT (-) group had a higher LRR rate compared to the PMRT (+) group (11% vs. 2%; p=0.01). In patients with an MS≥18, the administration of PMRT correlates with statistically significantly better LRR-free survival (HR 0.19; 95%CI 0.05 – 0.79; p=0.02). Conclusions: Our findings imply that when considering PMRT for BC with pT3N0M0, HR (+), and Her2 neu (-), clinicians can benefit from a combination of pathological risk factors and recurrence prediction models. Patients with MS 18 receiving PMRT or not appear to experience a comparable rate of recurrence. Citation Format: Atilla Soran, Caleb King, Parul N. Barry, Rohit Bhargava, Hasan Karanlik, Melis Gultekin, Ferah Yiıdız, Aykut Soyder, Berk Goktepe, Kazim Senol, Caglar Guzel, Ebru Sen Oran, Levent Yeniay, Ahmet Dağ, Selman Emiroglu, Didem Can Trabulus, Alper Coskun, Neslihan Cabioglu, Hagigat Veliyeva, N. Zafer Utkan, Berkay Demirors, Efe Sezgin, John A. Vargo. PMRT Decision with low prediction of genomic test score in pT3N0M0 luminal breast cancer: Protocol MF22-02: International multicenter real-world data abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-22-04.