Osteocartilaginous differentiation of mucosal melanoma in the sinonasal cavity.

We present a very rare case of mucosal melanoma with osteocartilaginous differentiation (MMOD). This is the first report of MMOD originating in the sinonasal cavity. Preoperative diagnosis of this tumor from specimens resected for biopsy alone is very difficult, because it has 2 histopathologic components: a primary melanoma site and the part of the melanoma with osteocartilaginous differentiation. The immunohistochemical technique is useful in histopathologic diagnosis of this tumor. Malignant melanoma usually shows reactivity with MelanA (melanoma tumor antigen) and S-100 proteins. We diagnosed this case as MMOD because both regions in this tumor were stained with MelanA and S-100 proteins. Magnetic resonance imaging provides the best clinical ability to detect malignant melanoma in the sinonasal region. In this case, the melanotic melanoma site in the posterior ethmoid sinus appeared hyperintense to gray matter on T1-weighted image magnetic resonance imaging. Thus, it would have been possible to diagnose this case as MMOD before operation, even though we could not obtain a biopsy specimen from the posterior ethmoid sinus lesion.