Multiple Primary Malignant Neoplasms: An Unusual Case of Metachronous Breast Ductal and Squamous Cell Carcinomas

Multiple primary malignant neoplasms (MPMN) are generally defined as the co-occurrence of multiple primary malignant neoplasms of distinct histology in the same individual. Second and higher-order primary malignancies now comprise about 18% of all incidence of cancer in the USA. The incidence ratio of developing multiple primary cancers (MPCs) in female cancer survivors is 1.17 to 1.6. In women with breast cancer, the incidence ratio is even higher, according to age at diagnosis of breast cancer. However, the concurrence of breast cancer and squamous cell carcinoma is not described in the literature. Primary squamous cell carcinoma of bone is also rare in the skeletal system other than in the skull, with only three such cases reported in the English literature. We present a case of a 59-year-old woman with high-grade primary invasive ductal carcinoma of the breast and second distinct squamous cell carcinoma metastasis to the bone of unknown primary site. A search for a primary squamous cell carcinoma, including CT head and neck, CT chest, colposcopy, esophagogastroduodenoscopy (EGD) and colonoscopy, did not show any evidence of a primary site.