Desmoplastic Melanoma

Clinical Findings

DM accounts for less than 4% of all melanomas. It typically develops in chronically sun-damaged skin of the head and neck region of elderly Caucasians ( Figs. 16.1 and 16.2 ). However, it can occur at any age and affect any anatomic site, including the trunk ( Figs. 16.3 and 16.4 ) subungual or submucosal stroma; it does not spare any ethnic group. If DM is found in children or adolescents, it is usually in the context of DNA repair deficiency (e.g., xeroderma pigmentosum).

Approximately two-thirds of invasive DMs are found in association with a precursor lesion, most often a lentigo maligna melanoma in situ. However, the invasive component of superficial spreading or acral melanoma may also display desmoplastic features. On rare occasion DM may be associated with a melanocytic nevus.

DM can be suspected clinically if a firm papule or indurated area develops within an irregular brown patch, especially if there was already a prior diagnosis of lentigo maligna. The clinical recognition of DM without a surrounding visible pigment is more challenging. Amelanotic papules or nodules of DM may simulate the appearance of a hypertrophic scar or keloid (see Fig. 16.4 ), dermatofibroma, neurofibroma, basal cell carcinoma, nevus, or cyst. Dermoscopically, one may see white scar-like areas or polymorphous vessels, including dot, serpentine, and corkscrew vessels and milky red areas. By the time a biopsy is performed, tumors are usually deeply invasive.

In its clinical course, DM differs from conventional melanoma by a higher propensity for local cutaneous or subcutaneous recurrence and less frequent metastases to regional lymph nodes. Local recurrence is primarily related to incomplete surgical resection and/or perineural invasion. Regional lymph node involvement is less common than with conventional melanoma if the primary tumor is a pure DM (see further on). Among patients with deeply infiltrative, thick (Clark V, >4 mm) melanomas, those with pure desmoplastic features (see further on) have longer disease-specific survival rates than patients with conventional or mixed DM. This is likely at least in part related to a lower total number of tumor cells (due to low cell density) compared with mixed or nondesmoplastic melanomas of similar thickness. However, DM, even pure variants, can be lethal. They typically spread hematogenously. Metastases may first be detected in the lung or bone. Pure DM may be thought of as a low-grade sarcomatoid melanoma. Mixed DM and solid spindle cell melanomas are high-grade tumors.

Histopathologic Findings

DM is histopathologically characterized by invasive melanoma with stromal fibrosis ( Fig. 16.5 ). In its pure form DM manifests as a paucicellular infiltrative amelanotic neoplasm of usually fusiform melanocytes dispersed predominantly as solitary units in an abundant fibrous matrix (see Figs. 16.5 and 16.6 ). When reviewed at low magnification using hematoxylin and eosin (H&E)–stained sections, the first impression is that of a diffusely pink lesion suggesting a possible scar or fibroma ( Box 16.1 ).

The overlying epidermis may display features of melanoma in situ (see Fig. 16.5 ), appear normal, or contain a subtle proliferation of junctional melanocytes. Features of melanoma in situ may be present only focally and require multiple sections. In some lesions, melanoma in situ is only or predominantly seen within hair follicles.

Invasive DM is usually amelanotic and fusiform, but in some (mixed) tumors there may be a significant proportion of epithelioid melanocytes ( Figs. 16.7 and 16.8 ). Melanin pigment (melanocytes and/or melanophages) may be seen on occasion, especially in the papillary dermis ( Fig. 16.9 ). Diffuse pigmentation is very rare. In most cases of DM, the tumor extends deeply (Clark level IV or V) by the time it is diagnosed. Only rarely is a DM removed when its invasive component is still superficial (Clark level II or III). Such focal superficial DM is usually detected unexpectedly in an excision specimen of a previously biopsied lentigo maligna. Perineural invasion is commonly present at least focally in DM of the head and neck region ( Fig. 16.10 ). In some neurotropic variants of DM, multiple small nerve twigs are involved in a traumatic neuroma-like pattern at the periphery of the tumor, sometimes at a distance from the main tumor mass ( Fig. 16.11 ). On occasion, angiotropism and/or frank vascular invasion are seen. Although DM is often associated with a lentigo maligna melanoma in situ, the invasive tumor component of acral lentiginous or superficial spreading melanoma may also manifest desmoplasia ( Fig. 16.12 ).