is usually found on the head and neck region as a spreading indurated plaque or bulky, firm tumefaction. Atypical presentations occur. The lesions are often non-pigmented, although areas of lentigo maligna may overlie part of the lesion or be found at the periphery. Sometimes the desmoplastic pattern is found only in the recurrence, or in the metastases of a more usual type of melanoma. Desmoplastic melanomas are often stubbornly recurrent; in many instances this is a reflection of inadequate surgical excision of a lesion which is often, locally, quite advanced in its growth. In one series the 5 year disease free survival rate was 68%, better than is seen for other categories of melanoma of comparable thickness. In another, desmoplastic melanomas with neurotropism had a significant decrease in survival. In a large series reported from the Sydney Melanoma Unit, the 5 year survival was 75%, which was similar to that for patients with other cutaneous melanomas of similar thickness.
Desmoplastic/spindle cell melanomas are composed of strands of elongated spindle shaped cells surrounded by mature collagen bundles. The stromal component varies considerably in different tumors. Sometimes there are scattered spindle cells and abundant collagen, whereas in others there is little stroma. This latter group is usually referred to as spindle cell melanomas, although it should be noted that desmoplastic melanoma and spindle cell melanoma form a continuum without a discrete separation. The desmoplastic features are usually more prominent in the local recurrences, in contrast to lymph node and visceral metastases, which resemble conventional melanomas. The cells resemble fibroblasts; there are scattered cells with hyperchromatic and even bizarre nuclei.
Multinucleate cells are often present. Small foci of neural transformation and neurotropism may be seen. The tumor infiltrates deeply. The full extent of the tumor is sometimes difficult to discern with accuracy. The use of S 100 staining is a valuable adjunct in determining the extent of some lesions.
There may be scattered collections of lymphocytes and plasma cells within the tumor. In paucicellular tumors these small foci of inflammatory cells can provide a clue to the diagnosis on scanning magnification. This paucicellular variant is easy to misdiagnose on a small punch biopsy or superficial shave. There may even be a resemblance to dermatofibroma with a storiform appearance.
Another variant is the superficial or early lesion characterized by cytological atypia, stromal myxoid change, aggregates of lymphocytes and poor circumscription. Heterotrophic bone and cartilage and sweat duct proliferation may form. There is often a lentigo maligna epidermal component overlying or towards one edge of the lesion. The tumor cells are nearly always amelanotic, but they are positive for vimentin in all cases and for S 100 protein and neuron specific enolase in approximately 95% of cases. Sometimes only a minority of the malignant cells (10- 20%) express S 100 protein.
HMB-45, which detects premelanosomes, is detected in from 0 to 20% of cases, but only small clusters of cells are stained in the positive cases. If desmoplastic/spindle cell melanomas are separated into two groups, pure desmoplastic melanomas are negative for HMB-45. Nearly 50% of spindle cell melanomas show some staining for HMB-45, and this subset of tumors appears to have a more aggressive potential than negative lesions. Sensitivity is increased if antigen retrieval methods are used. MelanA (MART1) gives mostly negative results with desmoplastic melanoma and is of little use with this morphologic type. NKI/C3 is expressed in approximately one quarter of cases. In one study, microphthalmia transcription factor (MiTF) had a sensitivity for desmoplastic/ spindle cell melanomas that equaled or exceeded that of HMB-45 or melanA. In another series, it stained only one case of desmoplastic melanoma. Overall, this new marker lacks sufficient sensitivity and specificity for wide spread diagnostic use.
Smooth muscle actin is present in a patchy distribution in nearly half the cases. Basic fibroblast growth factor (bFGF), an established growth factor for melanocytes, and other fibrogenic cytokines are expressed in the nuclei of the tumor cells in most desmoplastic melanomas. They presumably play a role in mediating the desmoplastic phenotype. p75 nerve growth factor receptor stains the cells in desmoplastic and neurotropic tumors, but it also stains other tumors of putative neural crest origin. Basement membrane antigens such as laminin and type IV collagen are often expressed in spindle cell melanomas. CD34 expression has been reported in a metastatic desmoplastic melanoma. The cells in a desmoplastic melanoma have abundant rough endoplasmic reticulum, and sometimes intracytoplasmic collagen and macular desmosomes. Nonmembrane bound melanin granules and premelanosomes have been noted ir some cases, although they have been specifically excluded in others. The most widely accepted view is that the desmoplastic component is derived from melanocytes that have undergone adaptive fibroplasia, although contrary views favor a fibroblastic stromal response or neurosarcomatous differentiation.
Although mature scars are easily differentiated from desmoplastic melanoma on light microscopy, immature scars share many features including lymphoid infiltrates, myxoid change, hypercellularity and atypia. Kaneishi and Cockerell found that an epidermal proliferation of melanocytes, neurotropism and S100 and/or HMB 45 positivity were the only distinguishing features. Parallelism of fibrocyte nuclei may be present in scar tissue. Nuclei in desmoplastic melanoma have a haphazard array.
In the neurotropic variant,there are spindle shaped cells with neuroma like patterns (neural transformation) and a tendency to adopt circumferential arrangement around small nerves in the deep dermis and subcutaneous tissue (neurotropism). Interlacing bundles of cells are seen. Desmoplastic and neurotropic patterns often occur together in the same neoplasm. The cells vary in size and nuclear staining. The cells usually lack melanin pigment, although a case with prominent melanization has been reported. The absence of S 100 protein using immunoperoxydase techniques does not exclude the diagnosis, as this antigen may be absent in up to 20% of cases; this figure seems unusually high in the author's experience. There is a significant risk of local recurrence in the presence of neurotropism.
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