Small cell malignant melanoma: a variant of naevoid melanoma. Clinicopathological features and histological differential diagnosis

Abstract

Aims—To describe the clinical and histopathological features of a rare variant of naevoid melanoma, small cell melanoma, and discuss the histological differential diagnoses.

Methods—The clinical and histological features of cases of malignant melanoma with the histological features of small (non-Merkel like) melanoma were reviewed and documented. In addition, five cases had available material for immunohistochemistry and this was performed using antibodies to the S100 protein and melan-A, and the HMB-45 antibody.

Results—There were 15 cases of small cell melanoma from 14 (10 female, four male) patients, aged between 30 and 77 (mean, 48.6) years. The trunk was the most common location. In more than half the cases, the provisional diagnosis was melanoma/borderline lesion. All shared similar histological appearances of an intraepidermal component of in situ melanoma and a dermal component of nests of cells with hyperchromatic nuclei and scanty cytoplasm, usually in tightly packed nests. All components (junctional and intradermal) of the lesions investigated by immunohistochemistry were positive both for S100 protein and melan-A. All junctional components were positive with HMB-45, but with variable staining of the dermal components with this antibody.

Conclusions—Small cell malignant melanoma is postulated to be a distinct histopathological entity and a rare variant of naevoid melanoma. Such lesions can be difficult to interpret and easily missed at scanning magnification because the cells of the dermal component mimic benign naevus cells.

Key Words: malignant melanoma • naevoid melanoma • small cell melanoma

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Figure 1 Conventional malignant melanoma. The cells become smaller deep in the lesion, at the base (small melanoma cells), mimicking benign naevus.

Figure 2 Small cell melanoma. Lesion from the calf of a 46 year old woman, with the clinical history of probable melanoma. Cells within the dermis are hyperchromatic and nested.

Figure 3 Small cell melanoma, as in fig 2. Note the destructive junctional component and the malignant epidermal component merging with the dermal nests.

Figure 4 Small cell melanoma. Lesion from the thigh of a 30 year old woman with a clinical history of probable melanoma. Note the atypical junctional component and the merging of the epidermal and dermal components.

Figure 5 Small cell melanoma. Lesion from the thigh of a 56 year old woman. No clinical history was supplied. There is a lentiginous proliferation of melanocytes at the junction. There is a dermal component composed of expansile nests of small cells with hyperchromatic nuclei.

Figure 6 High power of the dermal nests as in fig 5. The nests have flattened peripheral nuclei, suggesting expansile growth. The constituent cells have scanty cytoplasm, mild nuclear pleomorphism, and occasional nucleoli.