Desmoplastic melanoma--challenges in the diagnosis and management of a rare cutaneous tumor.

Desmoplastic melanoma (DM) represents a distinctive rare variant of spindle cell melanoma with a predilection for chronically sun-exposed skin of the elderly. This neoplasm is notoriously difficult to diagnose, both clinically and histopathologically. Therefore, DM is deeply infiltrative at the time of presentation. Histologically, the tumor presents as a proliferation consisting of non-pigmented spindle cells arranged in poorly formed fascicles. The neoplastic cells have a deceptively bland appearance with slightly pleomorphic and hyperchromatic nuclei, inconspicuous nucleoli and low mitotic activity. DM can mimic a whole range of benign and malignant neoplasms with spindle cell and fibrous appearance. Even though S100 remains the first-choice marker for DM, currently, there is no reliable marker with both high sensitivity and specificity for its detection. However, emerging melanoma markers, such as SOX10, have shown promising results in the diagnosis of DM. An accurate diagnosis of DM should always be based on the integration of all the clinical, histological and immunohistochemical features. Once diagnosed, DM should be aggressively excised with at least 2 cm lateral margins and down to the fascia. We present a case of DM that appeared on a non sun-exposed site. The tumor recurred multiple times in spite of repeated surgery involving wide local excisions and histologically reported negative margins. Recurrences are almost always associated with the presence of neurotropism. In our case, the neurotropism was obvious only in the second recurrence. We highlight the difficulties encountered in the diagnosis and management of both the initial tumor and its recurrence.