Omission of chemotherapy for the treatment of mucinous breast cancer: A nationwide study from the Korean breast cancer society

Purpose: Mucinous breast carcinoma (MBC) is a rare type of breast cancer. Although patients with MBC may have a better prognosis than that of patients with invasive ductal carcinoma, many clinicians administer adjuvant chemotherapy regimens similar to those for other breast tumors. Using data from a nationwide clinical database, this study evaluated the significance of adjuvant systemic chemotherapy and whether it can be omitted in MBC patients. Methods: We included 3,076 patients with a diagnosis of MBC recorded in the Korean Breast Cancer Registry between January 1990 and August 2016. We used the Kaplan-Meier method to analyze breast cancer-specific survival (BCCS) and overall survival (OS). Multivariate analysis was performed using a Cox proportional hazard ratio (HR) model to estimate the adjusted HR for each prognostic factor. Results: A total of 2,988 MBC patients were enrolled and followed-up for a median of 100 months (range, 2–324 months). Multivariate analysis revealed that axillary lymph node (ALN) metastasis and estrogen receptor (ER) negativity were significant prognostic factors for BCSS. Meanwhile, old age, pathologic tumor stage, and ALN metastasis were significant prognostic factors for OS. Subgroup analysis of ER-positive MBC showed that ALN metastasis was a significant prognostic factor for BCSS. Additionally, old age, pathologic tumor stage, and ALN metastasis were prognostic factors for OS. Ultimately, ALN metastasis was the most statistically significant prognostic factor for MBC. However, chemotherapy had no significant effect on BCSS and OS. The Kaplan-Meier curves of BCSS and OS based on pathologic tumor and nodal stages and age revealed that chemotherapy did not statistically significantly improve prognosis, except for the N3 stage. Conclusion: Our large retrospective analysis revealed that adjuvant chemotherapy provided little benefit to improve the prognosis of most ER-positive MBC patients. Therefore, chemotherapy can be omitted in the treatment of most ER-positive MBC.