Breast metastasis from rectal cancer with BRAF V600E mutation: a case report with a review of the literature.

Metastasis of rectal cancer to the breast is an extremely rare clinical event. We report the case of a 67-year-old woman with a metastatic breast tumor derived from a BRAF V600E mutant rectal carcinoma that was diagnosed and resected curatively 1 year previously. Computed tomography showed a left breast mass and multiple lung nodules suspected to be indicative of recurrent rectal cancer. The ultrasonography examination demonstrated a 10 × 10-mm hypoechoic solid lesion in the left breast with an elevation in the serum carcinoembryonic antigen level and serum carbohydrate antigen 19-9 level. Core needle biopsy was performed, and histopathologic examination showed Cytokeratin 20 and CDX-2 positivity, compatible with rectal cancer. To our knowledge, this is the first case of a metastatic breast tumor arising from rectal carcinoma with BRAF mutation. Although breast metastasis is very rare event, the possibility of breast metastasis from extra mammary sites should be considered when the breast tumor is found in cancer treatment.