Desmoplastic Melanoma
Desmoplastic melanoma, an uncommon variant of melanoma, poses a diagnostic challenge to the clinician because the tumors frequently appear as nonspecific flesh-colored or amelanotic plaques or nodules. Their innocuous appearance may lead to a delay in diagnosis and may explain why desmoplastic melanomas often are deeply infiltrative at the time of biopsy. The differential diagnosis for desmoplastic melanoma includes atypical fibroxanthoma, cutaneous angiosarcoma, cutaneous leiomyosarcoma, and spindle cell squamous cell carcinoma.
Spindle cell squamous cell carcinomas often present as ulcerated nodules on sun-exposed skin or on sites of prior ionizing radiation.2,12 Like desmoplastic melanoma, spindle cell squamous cell carcinomas are characterized by spindle cells in the dermis. Helpful diagnostic clues may include evidence of squamous differentiation, including keratin pearls or overlying actinic keratosis (Figure 7). However, actinic keratosis is common on sun-damaged skin and cannot be used to definitively confirm this diagnosis. There also may be areas of the tumor with more typical epithelioid cells that are easily identified as squamous cell carcinoma.2 Spindle cell squamous cell carcinoma stains positive for high–molecular weight cytokeratin antibodies and p63,2 which can help to differentiate it from the other spindle cell tumors in the differential.
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| Figure 6. Spindle cells of leiomyosarcoma with cigar-shaped nuclei and adjacent glycogen vacuoles (H&E, original magnification ×600). | Figure 7. Spindle cell squamous cell carcinoma with overlying epidermal atypia that blends with the underlying dermal spindle cells (H&E, original magnification ×100). |
