Answer: Metastatic malignant melanoma

Histology: A core biopsy of the parotid mass is shown. The tumor is comprised of sheets of small compact cells. The cells have sparse cytoplasm. Their nuclei are eccentrically-placed and uniformly round with inconspicuous nucleoli and evenly dispersed chromatin. Immunohistochemical studies were performed. The tumor cells are strongly and diffusely immunoreactive for S100 and HMB-45 (shown).

Discussion: The parotid gland is unique among the salivary glands in that it harbors, on average, ten intraparenchymal lymph nodes. These lymph nodes drain the skin of the ipsilateral face, ear and scalp. One of the more common pitfalls in salivary gland pathology is failure to recognize the cutaneous origin of a poorly differentiated malignant neoplasm. The majority of these metastases represent squamous cell carcinomas and melanomas.

The recognition of metastatic malignant melanoma is further confounded by its morphologic plasticity. Melanomas may have a spindled appearance and be easily confused with spindle cell sarcomas or myoepithelial neoplasms. Melanomas may have an epithelioid appearance and be confused with poorly differentiated squamous carcinoma or sinonasal undifferentiated carcinoma. Finally, melanomas may be comprised of small blue cells and be confused with lymphoma, plasmacytoma, esthesioneuroblastoma, and various primitive tumors of childhood (e.g. rhabdomyosarcoma).

In effect, one should always consider the possibility of metastatic melanoma before assuming that a poorly differentiated tumor is of primary salivary origin simply because it involves the substance of the gland. Fortunately, the overwhelming differential diagnosis can by resolved by a strategic immunohistochemical panel that includes markers of melanocytic differentiation.