Objective To investigate the clinical characteristics and treatment options for simultaneous multiple primary cancers (MPC). Methods Using a case report of simultaneous MPC of the breast and rectum as an entry point, we reviewed the literature to summarize and analyze its clinical features and treatment programs. Results Although the patient was diagnosed with a missed diagnosis of rectal cancer and underwent two surgical procedures during the course of her diagnosis, she was treated with tamoxifen (TAM) as adjuvant therapy after surgery for breast cancer. Currently, none of the tumors have metastasized and the patient has a stable disease (SD). Conclusion For simultaneous MPC of breast-rectal cancer, the efficacy of TAM for breast cancer is remarkable, although in this case the disease was stable and progression-free after postoperative TAM, but the efficacy for rectal cancer needs to be further elucidated. In addition, early diagnosis and treatment of MPC are crucial, and individualized treatment plans need to be developed through a multidisciplinary diagnostic and treatment model (MDT).