Pigmented spindle cell nevus: clues for differentiating it from spindle cell malignant melanoma. A comprehensive survey including clinicopathologic, immunohistochemical, and FISH studies.

Pigmented spindle cell nevus (PSCN), also known as Reed nevus, is a distinctive melanocytic tumor that can show worrisome clinical and histologic features mimicking a malignant melanoma. From a series of 46 pigmented spindle cell melanocytic lesions, including 22 PSCN and 24 spindle cell malignant melanomas (SCMMs), we collected clinical and histopathologic characteristics and evaluated cell cycle and apoptosis regulators by immunohistochemistry. Moreover, fluorescence in situ hybridization (FISH) using probes targeting 6p25 (RREB1), 11q13 (CCND1), 6q23 (MYB), and centromere 6 was performed. PSCN presented in younger people, frequently in women, and were small lesions under 7 mm in diameter affecting the lower limbs, whereas SCMMs arose more frequently in the trunk, upper limbs, and head and neck region. Histologically, symmetry, good lateral demarcation, and uniformity of cellular nests were significantly differential features of PSCN, whereas pagetoid and adnexal spread were frequently seen in both tumors. Immunohistochemical markers that significantly differed from melanomas were Ki-67, cyclin D1, and survivin. FISH was positive in 1 of 15 PSCN and was negative in 4 of 15 SCMMs. These results correlated to a sensitivity of 73% and a specificity of 93%. In conclusion, in the evaluation of pigmented spindle cell melanocytic tumors, the integration of clinical and histologic assessment is essential. However, ancillary techniques such as proliferation antigen Ki-67, cyclin D1, survivin, and FISH can be useful as adjunctive tools.