Case 1: The majority are amelanotic as in this example.
Spindle cells with elongated and wavy cellular processes, spindle-shaped nuclei, inconspicuous nucleoli, scant to moderate cytoplasm, and low mitotic index make this tumor deceptively bland.
Case 2: This case is less cellular with a vague storiform spindle cell proliferation.
Desmoplasia and some bluish stroma can be seen in other areas.
Case 3: Neoplastic spindle melanocytes are separated by a collagenized stroma.
This is a rare variant of melanoma, representing less than 4% of all melanomas (Busam).
The neoplastic cells are spindled and uniform, separated by a prominent desmoplastic stroma. The spindled proliferation can be fascicular, storiform or haphazard. Neutropism of the neoplastic melanocytes is also a common feature. (Busam, de Almeida) The associated in situ component is usually that of a lentinginous melanoma (lentigo maligna) .
This tumor is particularly deceptive as it can mimic benign soft tissue neoplasms such as neurofibroma, scar tissue and desmoplastic nevus (Fletcher). Notably, the tumor cells will stain for S-100, but will rarely stain for HMB45 or MelanA (Granter).
Parenthetically, neurotropic melanoma is another subtype of spindle cell melanoma that demonstrates extensive involvement of the nerves at the periphery of the tumor. Neutrotropic melanoma is often amelanoic and may be desmoplastic ("desmoplastic neurotropic melanoma"), but may not necessarily be desmoplastic. This key differential diagnosis is MPNST. MPNST is only focally S100 positive, whereas neurotropic melanoma should be diffusely S100 positive. Furthermore, neurotropic melanoma should be located in the head and neck in the superifical soft tissue and MPNST is generally located in the deep soft tissue and paraspinal regions (Busam).
Clinically, the lesion presents as a nonpigmented, fleshed colored nodule that is commonly mistaken for a nonmelanocytic proliferation (e.g. keloid scar or a dermatofibroma). These lesions usually develop in sun-damaged skin of elderly patients, usually the head and neck. This deceptive appearance may explain why at presentation time the Clark level is IV or V in more than 95% of the cases of DM, and a thickness greater than 4 mm is seen in half of the patients.
In a study of 113 cases of DM (de Almeida et al 2008), the mean age was 71.1 years with equal gender distribution. The neoplasm was located on the head in majoriy (72%) of the cases. Malignant melanoma was the initial clinical diagnosis in only 27% of the cases. 71% of the cases were histologically amelanotic and neural involvement was identified in 35% of cases, predominantly in the thickest tumors.
Compared to conventional melanoma, DM has a higher tendency for local recurrence, but metastases to regional lymph nodes are less common. This particular subtype of melanoma may have a more favorable prognosis, especially if the tumor is less than 4mm thick (Busam).
→Early detection of desmoplastic melanoma is uncommon, as it looks benign clinically (as well as histologically!).
→Characteristic histologic features include spindled neoplastic melanocytes, prominent desmoplasia, and the frequent neurotropism.
→Usually strongly positive for S-100 , but often negative or only focally positive for melanocyte differentiation antigens
de Almeida LS, et al. Desmoplastic malignant melanoma: a clinicopathologic analysis of 113 cases. Am J Dermatopathol. 2008 Jun;30(3):207-15.
Busam KJ. Dermatopathology: Foundations in Diagnostic Pathology 1st Ed. Philadelphia, PA: Elsevier; 2010: 478-80.
Fletcher CDM, ed. Diagnostic Histopathology of Tumors. 3rd Ed. Philadelphia, PA: Elsevier; 2007: 1483.
Granter SR, et al. Microphthalmia transcription factor: not a sensitive or specific marker for the diagnosis of desmoplastic melanoma and spindle cell (non-desmoplastic) melanoma. Am J Dermatopathol. 2001 Jun;23(3):185-9.