1149 - Post-Mastectomy Radiotherapy in pT1-2N1 HER2-Enriched Breast Cancer: Reassessing Its Role in the Anti-HER2 Era
Presenter(s)
X. Wang1, L. Zhang1, J. Guo2, X. Zhang1, J. Meng1, W. Shi1, X. Chen1, Z. Yang1, X. Mei1, Z. Zhang1, Z. Shao3, X. M. Guo1, X. Yu1, and J. Ma1; 1Department of Radiation Oncology, Fudan University Shanghai Cancer Center; Department of Oncology, Shanghai Medical College, Fudan University; Shanghai Clinical Research Center for Radiation Oncology; Shanghai Key Laboratory of Radiation Oncology, Shanghai, China, 2Lianyungang municipal oriental hospital, Lianyungang, Jiangsu, China, 3Department of Breast Surgery, Precision Cancer Medicine Center, Fudan University Shanghai Cancer Center; Department of Oncology, Shanghai Medical College, Fudan University, Shanghai, China
Purpose/Objective(s): The benefit of post-mastectomy radiotherapy (PMRT) for pT1-2N1 breast cancer remains controversial. While the HER2-enriched subtype exhibits aggressive biology, survival outcomes have significantly improved owing to the advent of anti-HER2 drugs. This study evaluated survival in pT1-2N1 HER2-enriched patients to explore the feasibility of PMRT omission in the era of contemporary systemic treatment.
Materials/Methods: We retrospectively reviewed 478 patients with pT1–2N1 HER2-enriched breast cancer treated with mastectomy at Fudan University Shanghai Cancer Center (2006-2019). HER2-enriched was defined as ER/PR-negative and HER2-positive (IHC 3+ or FISH positive). Endpoints included locoregional recurrence-free survival (LRRFS), breast cancer recurrence-free survival (BCRFS), and breast cancer-specific survival (BCSS). Survival outcomes were compared between PMRT and non-PMRT groups, with propensity score matching (PSM) used to balance clinical variables.
Results: Adjuvant chemotherapy and anti-HER2 therapy were administered to 99.2% and 78.2% of the cohort, respectively; 67.4% (n=322) received PMRT. At a median follow-up of 74 months, 6-year LRRFS, BCRFS, and BCSS were 96.7%, 91.5%, and 95.4%, respectively. In the entire cohort, PMRT was an independent predictor factor of improved LRRFS (HR=0.282, p=0.014). Both PMRT (HR=0.289, p<0.001) and anti-HER2 therapy (HR=0.416, p=0.007) were identified as independent prognostic factors for BCRFS. In the subset receiving Herceptin-based anti-HER2 therapy (n= 367), the 6-year LRRFS, BCRFS, and BCSS rates were 97.2%, 94.4%, and 97.7%, respectively. The PMRT group initially presented with a higher proportion of histological grade 3 tumor (73.8% vs. 66.7%, p=0.031), 3 positive ALNs (18.8% vs. 6.3%, p=0.005), and dual-targeted therapy (14.8% vs. 6.3%, p=0.030) compared to non-PMRT group. Following PSM (95 pairs), all baseline characteristics were well-balanced between the PMRT and non-PMRT groups. The difference in LRRFS between PMRT and non-PMRT groups was not statistically significant, either before (p=0.111) or after PSM (p=0.434). BCRFS benefit with PMRT was observed pre-PSM (p=0.002) but lost significance post-PSM (p=0.057) in the multivariate analysis.
Conclusion: This study demonstrates that LRR rates are low in pT1-2N1 HER2-enriched breast cancer patients treated with mastectomy and contemporary systemic treatment. The addition of PMRT may not provide a significant survival benefit, suggesting a potential for treatment de-escalation in this specific patient population.