Desmoplastic melanoma, a common missed diagnosis.

Introduction:

Although desmoplastic melanoma represents less than 2 percent of all melanomas, it's frequently misdiagnosed, due to a lack of distinctive clinical presentation features. Histologic diagnosis is rarely straightforward either.

Patients are often middle age-to-elderly and present with the tumor most often on the head and neck region. The lesion may resemble a scar as it is often a hard nodule or plaque.
Pigmentation is variable but often absent. The tumor has ill-defined margins and is very infiltrative, making local control difficult. Sentinel lymph node excision is routinely performed but rarely positive.

Histology :
The histology can also masquerade as a scar . The epidermis is often atrophic and may or may not have a precursor (in situ) lesion. Characteristically, the tumor is in the dermis as spindled melanocytes resembling fibroblasts.Often, there is an edematous or desmoplastic stroma with
scattered lymphoid aggregates. Perineural invasion is common.In about one third of the lesions, there are foci of epithelioid or conventional melanoma.

S-100 and HMB45 immunohistochemical stains can help differentiate tumor from scar.
At low power, there is a fibrotic lesion in the dermis with scattered lymphoid aggregates.

The lesion is paucicellular, but there is cellular atypia .

Reporting of the histologic subtype of melanoma is common practice, but it is unclear what impact, if any, it has on management decisions. One possible exception is desmoplastic melanoma, a distinct subtype with a unique biologic behavior. It is now recognized that desmoplastic melanomas present with greater tumor thickness (Breslow level) than their conventional counterparts but fail to demonstrate a corresponding higher sentinel lymph node involvement or higher mortality.

Some authors have further subdivided desmoplastic melanomas into "pure" and "mixed" forms. Pure (primarily fibrotic) and mixed varieties, which include features common to conventional melanoma and desmoplastic areas. As per these recent studies only 1% of pure desmoplastic melanomas metastasized to regional lymph nodes compared to 10% with mixed histology.

References:
1. Attis MG, Burchette JL, Selim MA, et al. Differential expression of N-cadherin distinguishes a subset metastasizing desmoplastic melanoma.
2. Davison JM, Rosenbaum E, Barrett TL, et al. Absence of V599E BRAF mutations in desmoplastic melanomas. Cancer. 2005 103:788.
3. Hawkins WG, Busam KJ, Ben-Porat L, et al. Desmoplastic melanoma:a pathologically and clinically distinct form of melanoma. Ann Surg Oncol. 2005 12:207.