Answer: Metastatic melanoma, desmoplastic type

Histology: The parotid parenchyma is diffusely infiltrated and largely replaced by a proliferation of spindle cells. The tumor cells are composed of narrow, elongated cells with wavy tapering nuclei. The cells are very uniform without appreciable pleomorphism, and the mitotic rate is very low. The cells are separated by bundles of collagen. An immunohistochemical stain for S100 (not shown) was strongly and diffusely positive.

Discussion: The parotid gland is unique among the salivary glands in that it harbors, on average, 10 intraparenchymal lymph nodes, and these drain the skin of the ipsilateral face, scalp and ear. One of the more common pitfalls in salivary gland pathology is failure to recognize the cutaneous origin of a poorly differentiated malignant neoplasm. Many of these represent metastatic squamous cell carcinomas and metastatic melanomas, and one should always consider this possibility before assuming that a tumor is of primary salivary origin just because it involves the substance of the gland.
In this particular case, inquiry into the clinical history uncovered a prior desmoplastic melanoma removed from the ipsilateral ear. Accordingly, the parotid tumor represents a metastatic desmoplastic melanoma. In the absence of a supportive history, the diagnosis is difficult to establish on purely immunohistochemical grounds. One must consider that malignant melanoma, malignant peripheral nerve sheath tumor, and myoepithelial carcinoma would all be expected to be S100 positive. Moreover, the majority (80%) of desmoplastic melanomas are not immunoreactive for specific markers of melanocytic differentiation, HMB45 and Melan A.