DESMOPLASTIC MALIGNANT MELANOMA
The invasive component of this type of melanoma can resemble a variety of benign and malignant spindle cell tumors of skin. The diagnosis frequently depends on demonstrating the radial growth phase of a melanoma, and this is usually lentiginous. If the radial growth phase of the melanoma is not demonstrated, as is often the case with small biopsies, the diagnosis may depend on a high degree of suspicion by the pathologist that is confirmed by appropriate immunoperoxidase studies. This type of melanoma is almost always S-100 positive and usually HMB-45 negative. It is a good idea to get a keratin study such as AE1/AE3 to check for spindle cell squamous carcinoma, and a muscle specific actin is warranted when the atypical cells resemble smooth muscle. An S-100 study is often necessary to distinguish between tumor and early scar in margins of resection. This type of melanoma has usually invaded into the deep reticular dermis or into the subcutis by the time the biopsy is taken. Neurotropism is sometimes present. This  type of melanoma truly requires wide margins of resection.


 
Scan of a tumor from a sun exposed area from an elderly patient.
Low power of the dermal component. There is a slight suggestion of a storiform pattern in some areas, but the appearance is rather nondescript..
Low power of a superficial part of the tumor.
Low power of another superficial area.
High power of above showing the lentiginous component that enables one to make the diagnosis.

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