Malignant melanoma with metastasis to a colonic polyp.

Tumor-to-tumor metastases are rare occurrences. A 75-yr-old male presented with an enlarging axillary mass. Further investigation revealed an adenocarcinoma of the colon and a colonic polyp. Metastatic malignant melanoma was present within the colonic polyp and in axillary lymph nodes. In the polypectomy specimen, the distinctly dimorphic histologic appearance was the best clue to the metastatic nature of the malignant component of the polyp. The diagnosis was confirmed by histochemistry and immunocytochemistry. This is the third reported instance of a colonic polyp acting as the host in a case of tumor-to-tumor metastasis.