Fluorescence in situ hybridisation as an ancillary tool in the diagnosis of acral melanoma: a review of 44 cases.

Acral melanoma is associated with outcomes which are more unfavourable than those of other melanoma subtypes, and acral melanoma has higher mortality. However, histological distinction of acral melanoma from acral naevi may be difficult. Fluorescence in situ hybridisation (FISH) targeting specific genes has been used as an ancillary method for differential diagnosis of melanocytic tumours, but most previous studies have focused on non-acral lesions which may have genetic alterations different from acral lesions. We evaluated use of multi-site FISH in the diagnosis of acral melanoma in a series of 82 acral melanocytic tumours. Two probe groups were applied. Probe set 1 involved a 4-probe FISH targeting 6p25 (RREB1), CEP6 (centromere 6), 6q23 (MYB) and 11q13 (CCND1). Probe set 2 involved a 3-probe FISH targeting 8q24 (MYC), 9p21 (CDKN2A) and CEP9 (centromere 9). In 44 primary acral melanomas, sensitivity was 70.5% (31/44) using probe set 1 alone, and 59.1% (26/44) using probe set 2 alone. When both probe sets were combined, sensitivity increased to 88.6% (39/44). The frequency of each gene alteration was as follows: MYC gain in 54.5% cases (24/44), RREB1 gain in 52.3% cases (23/44), CCND1 gain in 45.4% cases (20/44), MYB loss relative to CEP6 in 25.0% cases (11/44), and CDKN2A homozygous deletion in 20.5% cases (9/44). For lesions with both in situ and invasive disease, FISH findings in these two components were similar. No gene alterations were detected in any of 36 benign acral naevi. In this study FISH exhibited sensitivity and specificity in diagnosis of acral melanoma which allows its application as an auxiliary diagnostic test in acral melanocytic tumours.