Use of fluorescence in situ hybridization (FISH) to distinguish intranodal nevus from metastatic melanoma.

With the increase in sentinel lymph node biopsies in melanoma patients, pathologists are frequently confronted with small deposits of morphologically bland melanocytes in the node, which occasionally cannot be readily classified as benign nodal nevi or melanoma. As most melanomas harbor characteristic chromosomal aberrations which can be used to distinguish them from benign nevi, we used fluorescence in-situ hybridization (FISH) with markers for 3 regions on chromosome 6 and 1 on chromosome 11 to determine the presence of chromosomal aberrations in sentinel lymph node specimens with small foci of melanocytes that had been diagnosed as metastatic melanoma or nodal nevi by histopathology. Fifty-nine tissue samples from 41 patients (24 lymph node metastases, 17 with nodal nevi, and 18 of the available corresponding primary melanomas) were analyzed by FISH. Twenty of 24 (83%) cases diagnosed as metastatic melanoma showed aberrations by FISH. Of the 4 negative cases, 3 were unequivocal melanoma metastases, whereas 1 on re-review was histopathologically equivocal. Of the 17 nodal nevi, 1 (6%) also showed aberrations by FISH, whereas the remainder was negative. Multiple aberrations were present in the positive case, some of which were also found in the corresponding primary tumor, suggesting that this case represents a deceptively bland melanoma metastasis that had been misclassified by histomorphology. Our data indicate that FISH is a useful adjunct tool to traditional methods in the diagnostic workup of deposits of melanocytes in lymph nodes that are histopathologically ambiguous.

Fluorescence in situ hybridization (FISH) as an ancillary diagnostic tool in the diagnosis of melanoma.

Although the clinical and pathologic diagnosis of some melanomas is clear-cut, there are many histopathologic simulators of melanoma that pose problems. Over-diagnosis of melanoma can lead to inappropriate therapy and psychologic burdens, whereas under-diagnosis can lead to inadequate treatment of a deadly cancer. We used existing data on DNA copy number alterations in melanoma to assemble panels of fluorescence in situ hybridization (FISH) probes suitable for the analysis of paraffin-embedded tissue. Using FISH data from a training set of 301 tumors, we established a discriminatory algorithm and validated it on an independent set of 169 unequivocal nevi and melanomas as well as 27 cases with ambiguous pathology, for which we had long-term follow-up data. An algorithm-using signal counts from a combination of 4 probes targeting chromosome 6p25, 6 centromere, 6q23, and 11q13 provided the highest diagnostic discrimination. This algorithm correctly classified melanoma with 86.7% sensitivity and 95.4% specificity in the validation cohort. The test also correctly identified as melanoma all 6 of 6 cases with ambiguous pathology that later metastasized. There was a significant difference in the metastasis free survival between test-positive and negative cases with ambiguous pathology (P=0.003). Sufficient chromosomal alterations are present in melanoma that a limited panel of FISH probes can distinguish most melanomas from most nevi, providing useful diagnostic information in cases that cannot be classified reliably by current methods. As a diagnostic aid to traditional histologic evaluation, this assay can have significant clinical impact and improve classification of melanocytic neoplasms with conflicting morphologic criteria.