The prognostic value of HER2 status and efficacy of anti-HER2 therapy in patients with HR-positive mucinous breast cancer: a nationwide study from the Korean Breast Cancer Society

Purpose: The effect of human epidermal growth factor receptor 2 (HER2) status on mucinous carcinoma (MC) of the breast is unknown due to the rarity of HER2-positive cases. We evaluated the prognostic value of HER2 status and the efficacy of anti-HER2 therapy in patients with hormone receptor (HR)-positive MC. Methods: From the data of 154,661 patients recorded in the Korean Breast Cancer Registry between January 1990 and August 2016, 3076 (2.0%) were diagnosed with MC. Overall survival (OS) according to HER2 status and anti-HER2 therapy was analyzed using Kaplan–Meier estimates. Multivariate analysis was performed using the Cox proportional hazards model to estimate the adjusted hazards ratio (HR) for clinicopathologic factors. Results: A total of 2716 HR-positive MC patients were enrolled and followed up for a median 100.1 months. Of these, 2094 (77.1%) were HER2-negative and 228 (8.4%) were HER2-positive. HR-positive, HER2-positive MC patients had more advanced pathologic tumor stages (T3 or T4) (p = 0.001), more axillary lymph node involvement (p < 0.001), higher nuclear grade (p < 0.001), and more lymphovascular invasion (p = 0.012) than HER2-negative patients. Subgroup analysis of HR-positive, node-positive MC showed that HER2-positive MC was an independent prognostic factor for OS (HR = 2.657; 95% CI, 1.665–4.241; p < 0.001). HR-positive, node-positive, and HER2-negative MC had significantly longer OS than HER2-positive MBC (p = 0.017). The node-positive subgroup that received anti-HER2 therapy had increased OS, although not significantly (p = 0.224). Conclusion: Our nationwide database study revealed that HER2-positive status was associated with worse prognosis in HR-positive and node-positive MC. Anti-HER2 therapy might be beneficial in HR-positive, node-positive, and HER2-positive MC.