Primary combined malignant melanoma and ductal carcinoma of the breast. A report of two cases.

BACKGROUND: Tumors consisting of a combination of malignant melanoma and carcinoma are very rare. The authors report two such cases occurring as primary breast tumors.
METHODS: The breast tumors were analyzed by histologic, immunohistochemical, and ultrastructural techniques.
RESULTS: Histologically, the tumors were composed of a closely related admixture of ductal adenocarcinoma and malignant melanoma with abundant melanin pigment. Ductal carcinoma in situ was identified in both cases, confirming their origin in the breast. In both tumors, double-labeling immunohistochemistry showed that the epithelial component was immunoreactive for cytokeratin, the melanoma component was immunoreactive for HMB45, and both components were immunoreactive for S-100 protein. Immunostains for estrogen and progesterone receptors were negative in both tumors. Electron microscopy demonstrated glandular lumens and junctional complexes in the epithelial component and melanosomes and premelanosomes in the melanoma component. In one of the cases, rare tumor cells contained both premelanosomes and desmosomes.
CONCLUSIONS: Combined malignant melanoma and carcinoma is a rare tumor. Only a handful of cases have been reported. The authors report two such cases occurring as primary tumors of the breast. The histology of the tumors revealed a closely related admixture of pigmented malignant melanoma and ductal carcinoma. Double-labeling immunohistochemistry showed that cytokeratin and HMB45 were expressed in the tumors, but not within the same cells. The authors propose describing this type of lesion as a single tumor of breast origin with bidirectional differentiation.