Prevalence of ALK gene alterations among the spectrum of plexiform spitzoid lesions

Background ALK receptor tyrosine kinase gene ( ALK ) rearrangements have been described in spitzoid lesions with a plexiform growth pattern. Objective To investigate the prevalence of ALK alterations in a large series of spitzoid lesions. Methods ALK immunohistochemical and fluorescence in situ hybridization analyses of 78 spitzoid plexiform lesions including 41 Spitz nevi, 29 atypical Spitz tumors (ASTs), and 8 spitzoid melanomas. Results ALK immunohistochemical staining was observed in 14.6% of Spitz nevi (6 of 41) and 13.8% of ASTs (4 of 29); the spitzoid melanomas were ALK negative. Fluorescence in situ hybridization confirmed ALK translocation in 9 cases and amplification in 1 case. In 2 of the translocated cases it was possible to determine the fusion partner gene (ie, tropomyosin 3 gene [ TPM3 ] or dynactin 1 gene [ DCTN1 ]). Of the 4 cases of AST examined, 2 carried the B-Raf proto-oncogene, serine/threonine kinase gene ( BRAF ) V600E mutation. The 10 patients had a mean age of 18.7 years (range, 1-39) and a female predominance (female-to-male ratio, 7:3). Seven lesions arose on the extremities; the 2 lesions occurring in infants were located on the face. The lesions' mean diameter was 6.2 mm (range, 3-13), and their mean Breslow thickness was 1.83 mm (range, 0.6-3.6). The results of sentinel node biopsy were negative in 2 ASTs. Limitations BRAF status was tested in only 4 of 10 samples because of the limited amount of material. Conclusion ALK alterations characterize a significant subset of spitzoid lesions.