ROS1 pattern of immunostaining in 11 cases of spitzoid tumour: comparison with histopathological, fluorescence in-situ hybridisation and next-generation sequencing analysis.

Immunohistochemistry showed 100% sensitivity and specificity as compared with the FISH results, corresponding to ROS1 rearrangement in 31% of cases studied. These observations shed new light on the value of immunohistochemical evaluation of ROS1 in spitzoid tumours. ROS1 patterns of immunostaining probably reflect different subcellular localisations of ROS1 fusions, although no specific correlations were found in the cases studied. Immunohistochemistry and FISH were the most sensitive techniques for detecting ROS1 rearrangement in this subset of neoplasms.