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Desmoplastic melanoma - Submitted on: Monday May, 01, 2000
Contributed by:Martin C. Mihm, Jr., MD
Desmoplastic melanoma (DM) is a malignant melanoma first described by Conley and associates in 1971 that can pose great difficulties in the differential diagnosis both clinically and histologically. It is generally considered to be rare, although recently an increase of its incidence has been noted in referral centers--perhaps reflecting an improvement of the physician's diagnostic skills in the field.

Definition:
DM is a spindle cell vertical growth phase melanoma with various degrees of desmoplasia that can develop de novo or in a pre-existing radial growth phase lesion. In the latter case, it has clinical and histologic features of lentigo maligna, acral-lentiginous melanoma, or mucosal-lentiginous melanoma.

Clinical Features:
DM presents a variable and often indolent clinical picture, which is obviously related to the delayed diagnosis that is reported in a significant percentage of cases. In the majority of cases it affects middle-aged individuals (in their 5th or 6th decade). When de novo, it presents as an indurated, non-ulcerated, and slightly elevated plaque or nodule. It may be flesh-colored rather than melanotic. Pigmentation, when present, is either sparse or irregularly distributed. When DM arises in association with a radial growth phase lesion, it exhibits the characteristics of that lesion, with an indurated, usually heavily pigmented nodule. A persisting and progressively enlarging indurated erythematous lesion around the scar of a previous biopsied lentigo maligna type melanocytic proliferation must always be considered as highly suspicious of desmoplastic melanoma and must be biopsied.
DM has a predilection for the sun-exposed areas of the skin of individuals with phototype I characteristics. Head and neck are the sites of development in the majority of these reported cases and lentigo maligna is a usual concomitant. In cases of acral or mucosal distribution, there is no phototypic predilection.

DM is characterized by local aggressive biologic behavior and high incidence of local recurrences, probably related to the commonly described nerve invasion (neurotropism). Repeated recurrences can lead to a disseminated disease, with intracranial metastases being common in primary tumors of the head and neck region. However, the overall survival rate of patients with DM seems to be better than that of patients with nondesmoplastic melanomas of equal Breslow's depth of invasion.

Epiluminescence Microscopy
The essential epiluminescence microscopic features of melanoma can be found in DM, especially when it is accompanied by a radial growth phase melanoma. Dermoscopic analysis, however, should be viewed as an adjunctive technique in the assessment of the lesion and an excisional biopsy should be advised in all questionable cases.


Histology:
Figures
(Click on an image for a larger view)
Figure 1. Desmoplastic melanoma on low power exhibits intraepidermal melanocytic hyperplasia with hyperpigmentation overlying fibrotic dermis (area of close-up in Figure 2).
Figure 2. Closeup in area of fibrotic dermis in Figure 1 shows epithelioid cells superficially with elongate hyperchromatic spindle cells below dermis.
Figure 3. Note multinucleate cells, some of which appear pyknotic.
Figure 4. In this dense fibrous matrix, highly irregularly shaped hyperchromatic cells are evident and are characteristic of desmoplastic melanoma cells.
Figure 5. The classic features of lentigo maligna shown here include epidermal atrophy, single dispersed atypical melanocytes along the basilar of the epidermis and the external root sheath of the hair follicle. There is also solar elastosis and chronic i
Figure 6. Neurotropism is evidenced by thickening of the stroma by nevoid cells. Note the multinucleate desmoplastic melanoma cells within the stroma in the subcutaneous fat.
Figure 7. Desmoplastic melanoma cells react positively with antibodies directed against S-100 proteins.
DM is characterized histologically as a mainly intradermal ill-defined lesion (Figure 1), composed of elongate hyperchromatic spindle cells distributed singly or in bundles, fascicles, or nests, between variably increased collagen fibers of the papillary and the reticular dermis (Figure 2).

Epithelioid cells are recognized in superficial parts of the lesion, while in deeper areas, the cells retain fibroblastic-like features. Fascicles of hyperchromatic spindle cells disposed in different angles in relation to the long axis of the epidermis are characteristic.

Multinucleated cells and individual necrotic cells may be present (Figure 3). Mitotic figures are rare. Focal lymphocytic aggregates (nodules) are a constant and diagnostically helpful feature. Macrophages may be scattered throughout the lesion.

The degree of cellularity and of collagen production varies greatly from case to case (Figure 4). Areas with myxoid or storiform appearance can be seen.

Whenever an overlying radial growth phase melanoma is coexisting, it has the features of one of the common types of intraepidermal melanomas and contributes greatly to the recognition of the exact nature of the lesion underneath (Figure 5).

Infiltration of the adventitia of blood vessels and/or the perineurium and endoneurium of local nerves is found in a significant percentage of cases and is considered to be helpful in the final diagnosis (Figure 6). In present day thinking, the hypothesis that desmoplasia is due to metaplasia of the melanocytes has supervened the previous belief that increase of collagen represents a host reaction to the neoplastic invasion.

DM's neoplastic cells usually show immunoreactivity with S-100 protein (Figure 7) and NKI/C3. Nevertheless, negativity does not exclude the diagnosis when the clinical picture and/or histology are characteristic. HMB-45, NKI/beteb, and Leu-7 (CD57), and NSE usually do not stain DMs (with rare exceptional cases of usually focal reactivity in an epithelioid neoplastic subpopulation). CD68 has been demonstrated in a minority of cases. The reported SMA positivity has been shown to be related to myofibroblastic differentiation of a spindle cell component in some cases with S-100 and desmin immunonegativity.


Treatment:
Wide excision of the primary lesion with clear margins and close follow-up are necessary. It is extremely important that the lesion be extirpated in the initial surgical excision. Recurrences are usually an ominous sign. However, recurrences must also be widely excised.

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