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

Abstract

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.

Figure 1. Case 1, 64 year-old white male with lymph node involvement (a,b) originating from a primary melanoma, 1.0 mm in thickness on the right upper arm (c,d)

a) Pigmented, spindled melanocytes within trabecula of the sentinel lymph node (10× objective). b) Increased copy number of 11q13 (CCND1) by FISH as evidence by more than two green signals per nucleus (40x objective). c) Primary melanoma (20× objective). d) Primary melanoma with similarly increased copy number of 11q13 (40× objective).

Figure 2. Case 8, need to put in description as in 1 and 4

a) Trabecular melanocytes with bland, monomorphic morphology (4× objective) that b) are only one to two times the size of the surrounding lymphocytes (20× objective).

Figure 3. Case 6 need to put in description as in 1 and 4

a) Collections of epithelioid melanocytes in the lymph node capsule, extensively involving the lymph node circumferentially (2× objective). b) Melanocyte nuclei show only mild variation in shape and are approximately three times the size of adjacent lymphocytes (40 × objective). c) Primary tumor with sheets of monomophic melanocytes with moderate cytologic atypia (10× objective), and d) scattered deep mitoses (20× objective).

Figure 4. Case 30: femoral sentinel lymph node (a–c) of a 69 year-old female with a 0.9 mm melanoma on the right dorsal foot (d, e)

a) Sentinel lymph node with capsular nevus (a: 4× objective, b: 20× objective) and c) increased copy number of 11q13 (green signals) and 6p25 (red signals) by FISH (40× objective). d) Histopathology of the primary melanoma (10× objective). FISH shows similarly increased copy number of 11q13 (green signals) and 6p25 (red signals) by FISH as the lymph node deposit (40× objective).