Pitfalls in the diagnosis of malignant melanoma

The histological appearance of benign melanocytic naevi and malignant melanomas can be variable, causing in a significant number of cases severe differential diagnostic problems. The early, thin (less than 1 mm) melanomas have to be differentiated from naevi containing dominant junctional or lentiginous component or pagetoid melanocytosis and from some epithelial tumours, while in cases of thick lesion the diagnosis of thick melanoma, Spitz naevus, deep penetrating naevus or cellular blue naevus should be considered for example. The morphology of the so-called atypical Spitz naevus and atypical pigmented spindle cell naevus show overlapping with malignant melanoma and sometimes in these cases the biological behaviour cannot be assessed. The variable appearance of malignant melanoma is illustrated by the fact that different superficial soft tissue tumours with epithelioid and/or spindle cells or with pigment can mimic it. The rare balloon cell and signet ring cell melanoma is a mimicker of primary or metastatic carcinoma and the desmoplastic variant is often misdiagnosed as benign mesenchymal lesion. Lymph node metastasis of melanoma, when the primary tumour is not known, may raise the possibility of interdigitating reticulum cell tumour or anaplastic large cell lymphoma.