Fluorescence in Situ Hybridization (FISH) Copy Number Abnormalities at 6p (RREB1), 6q (MYB), and 11q (CCND1) Reliably Distinguish Metastatic Versus Benign Melanocytic Lesions

Objectives: Malignant melanoma can be difficult to distinguish from a benign melanocytic lesion by histology. In this study, we investigated the sensitivity and specificity of FISH to distinguish between benign nevi and metastatic melanomas to lymph nodes. Methods: Multicolour FISH was performed using a commercially available probeset (Abbott Laboratories, Abbott Park, IL), on formalin-fixed, paraffin-embedded tissue samples from 40 tumours: 20 benign melanocytic nevi, and 20 metastatic melanomas within lymph nodes, as determined by histologic assessment. Fluorescent signals for each probe were enumerated by 2 observers in 30 cells each per lesion. An algorithm using signal counts from a combination of 4 probes targeting chromosome 6p25 (containing RREB1 gene), 6 centromere (CEP6), 6q23 (containing MYB gene), and 11q13 (containing CCND1 gene) was used as suggested by the manufacturer. Results: Of the 20 metastatic melanomas assessed, 18 were FISH positive. FISH detected significant abnormal nuclei for RREB1 in 17/20 cases (85%) and significant MYB loss in 12/20 cases (60%). Average signals per nuclei greater than 2.5 for CCND1 and MYB were present in only 7/20 (35%) and 4/20 (20%) cases respectively. All 20 benign nevi were FISH negative. Overall, the FISH test showed a sensitivity of 90% and specificity of 100% in the diagnosis of metastatic melanoma in lymph nodes. Conclusions: These results provide further compelling evidence for the utility of multicolour FISH directed against 6p25 (RREB1), centromere 6, 6q23 (MYB), and 11q13 (CCND1) as an aid in determining malignant behavior in melanocytic lesions.