Rare case of a male breast tumour secondary to poorly differentiated rectal adenocarcinoma

Rectal cancer metastasis to the breast is rare. A case history is presented of a 57-year-old man with breast metastases from rectal carcinoma. However, this patient did not have metastasis in common metastatic sites, such as the liver, lung, and other organs. The patient had undergone chemotherapy for advanced rectal carcinoma 6 months earlier and presented with a mammary mass. An ultrasound-directed core needle biopsy of the breast mass was performed. Cytology indicated an adenocarcinoma with poor to moderate differentiation in the breast mass Immunohistochemistry (IHC) showed cytokeratin (CK) expression with a pattern that is characteristic of colorectal tumours: CK7(-), CK20(+), CDX2(-), Villin(+) TOPOII(-), and a Ki-67 index of 30%. The 3 main breast tumour markers were negative. Based on these histopathological and immunohistochemical findings, the patient was diagnosed with breast metastases from rectal carcinoma. Distant metastasis should be taken into account when a patient has a medical history of rectal adenocarcinoma, even when a rare metastasis site is involved. We should be vigilant when patients have some features that are favorable for metastasis. Histopathological characteristics and immunohistochemical tests are helpful for diagnosis. Regardless of surgical treatment after neoadjuvant chemotherapy, standard chemotherapy regimens for intestinal tumors, and EGFR molecular-targeted drugs, there is no obvious effect and the prognosis is poor. The treatment method needs further study.