the aim of current study was to use competing risk model to assess whether medullary carcinoma of the breast (MCB) has a better prognosis than invasive ductal carcinomas of breast cancer (IDC), and to build a competing risk nomogram for predicting the risk of death of MCB. We involved 3,580 MCB patients and 319,566 IDC patients from Surveillance, Epidemiology, and End Results (SEER) database. IDC was found to have a worse BCSS than MCB (Hazard ratio (HR) > 1, p < 0.001). The 5-year cumulative incidences of death (CID) was higher in IDC than MCB (p < 0.001). Larger tumor size, increasing number of positive lymph nodes and unmarried status were found to worsen the BcSS of McB (HR > 1, p < 0.001). We found no association between ER, PR, radiotherapy or chemotherapy and McB prognosis (p > 0.05). After a penalized variable selection process, the SH model-based nomogram showed moderate accuracy of prediction by internal validation of discrimination and calibration with 1,000 bootstraps. In summary, MCB patients had a better prognosis than IDC patients. Interestingly, unmarried status in addition to expected risk factors such as larger tumor size and increasing number of positive lymph nodes were found to worsen the BcSS of McB. We also established a competing risk nomogram as an easy-to-use tool for prognostic estimation of MCB patients. Medullary carcinoma of the breast (MCB) is a rare histology type of invasive breast cancer, accounting for 3-5% of all breast cancers. The histopathologic features of MCB include lymphoplasmacytic infiltration, noninvasive microscopic circumscription, syncytial growth pattern >75%, and grade 2 or 3 nuclei 1. Immunohistochemical results showed that MCB presented with more absence of estrogen (ER), progesterone (PR), and human epidermal growth factor receptor 2 (HER2) 2. Compared with invasive ductal carcinomas of breast cancer (IDC), MCB also exhibited with more BRCA1/2 mutations, greater T stage and tumor size 3. Despite the aggressive features, the prognosis of MCB was conflicting. Studies found MCB showed a better survival than IDC 1,3-6 , while other researches showed MCB (IDC) had a similar outcomes with IDC 7-9. However, since MCB is a rare type of breast cancer, most of these studies were involved with limited samples. Surveillance, Epidemiology, and End Results (SEER) database of the National Cancer Institute is a national collaboration program of Unite State, covering nearly 26% American population's cancer incidence and survival data, which could provide relatively large sample size for survival study. There were two SEER-based studies associated with MCB prognosis 7,10. One study 7 only focused on short-term survival status. The inadequate follow-up time might lead to skewed results. The other study 10 evaluated the prognostic factors of long-term MCB overall survival (OS) by Cox proportional hazards model. However, OS might unable to accurately describe long-term survival of disease such MCB, as the deaths could be caused by other competing risk events. In this case, the cum...