Validation of ROS1 by immunohistochemistry against fluorescent in situ hybridisation on cytology and small biopsy samples in a large teaching hospital

Abstract

Objective: This is a prospective evaluation of ROS1 analysis by IHC against FISH on cytology and small biopsy samples in a routine diagnostic setting and demonstrates both technical and clinical validation. Methods: From 1st March to 31st December 2019, cytology cell blocks and small biopsy samples from a selected cohort of NSCLC patients were concurrently tested for ROS1 gene rearrangement by Vysis 6q22 Break Apart FISH probe and IHC using Cell Signalling D4D6 antibody. Mismatch cases were tested by an RNA fusion NGS panel. Results: In a prospective population of 95 cases, 91 were negative and 2 were positive by both FISH and IHC. Both dual positive cases were female never smokers and benefited from TKI treatment. Another 2 cases were positive by FISH but negative by IHC and repeat by NGS showed one to be negative but one failed. Turnaround time for IHC was 0 to 8 days from request to authorisation whilst that of FISH was 9 to 42 days at a cost of £51 and £159 respectively. Conclusion: IHC to assess for the protein product of ROS1 gene rearrangement on cytology cell blocks and small biopsy samples in a routine setting is a promising screening method to assess eligibility for TKI treatment with positive and indeterminate cases confirmed by FISH or NGS as it has good NPV, faster turnaround time and is cost effective with proven technical and clinical validation