Purpose: Breast cancer is one of the most common malignancies worldwide and the second most common cancer among Korean women. The prognosis of breast cancer is poor in patients with other primary cancers. However, there have been few clinical studies regarding this issue. Therefore, we analyzed the characteristics and prognosis of patients with breast cancer with multiple primary cancers (MPCs).

Methods: Data from the Korean Breast Cancer Society Registry were analyzed. Data from enrolled patients who underwent surgery for breast cancer were analyzed for differences in prognosis dependent on the presence of MPCs, and which MPC characteristics affected their prognosis.

Results: Among the 41,841 patients analyzed, 913 patients were found to have MPCs, accounting for 950 total MPCs. There was a significant difference in survival rates between the breast cancer only group and the MPC group. The 5-year survival rates were 93.6% and 86.7% and the 10-year survival rates were 87.5% and 70.4%, respectively. Among the 913 patients with MPCs, patients with two or more MPCs had significantly worse prognoses than patients with a single MPC. With respect to the time interval between breast cancer and MPC occurrence, patients with a 5-year or greater interval had significantly better prognoses than patients with less than 1 year between occurrences. Among MPCs, thyroid cancer was the most common primary cancer. However, this type was not related to the prognosis of breast cancer. Gynecologic cancer, colorectal cancer, upper gastrointestinal cancer, and lung cancer were related to breast cancer prognosis.

Conclusion: MPCs were a poor prognostic factor for patients with breast cancer. Two or more MPCs and a shorter time interval between occurrences were worse prognostic factors. Although MPCs were a poor prognostic factor, thyroid cancer did not affect the prognosis of patients with breast cancer.