Metastatic seeding of colon adenocarcinoma manifesting as synchronous breast and chest wall localization: report of a case.

Colon carcinoma rarely metastasizes to the breast and it is usually associated with a poor prognosis. Even rarer is metastatic seeding of colon cancer cells in an intramammary location after surgery. Including a primary breast malignancy in the differential diagnosis of such cases is mandatory. We report a rare case of double seeding implantation of colon adenocarcinoma inside the breast parenchyma and intercostal muscles 6 years after resection of a pulmonary metastasis from colon adenocarcinoma. The metastasis was revealed by the presence of bone metaplasia in the intercostal muscles. We discuss how negative immunostaining for estrogen receptors, progesterone receptors, and HER-2, along with the immunohistochemical pattern of cytokeratin (CK) 20+/7-/5- and CDX2-positive immunostaining, excludes a primary breast malignancy, namely, a "matrix-producing" carcinoma, from the differential diagnosis. We also present the hypothesis of a paracrine pathogenetic mechanism to explain the bone metaplasia.