Abstract 1093P
Background
RET (REarranged during Transfection) gene fusions occur in 1-2% of non-small-cell lung cancer (NSCLC). Rapid and accurate detection screening methods to identify patients likely to benefit from RET-targeted therapy is mandatory. Fluorescence In Situ Hybridization (FISH) is commonly used for RET gene rearrangements screening, since RET immunohistochemistry shows low sensitivity and specificity. In the present study, we performed an up-to-date comprehensive review of publications in which RET rearrangement testing was performed by FISH and compared the methods used by the various laboratories with our own data.
Methods
An electronic search for all publications to date using RET FISH analysis in lung cancer was performed and the findings were compared with the molecular results obtained from 2013 to 2021 at the Grenoble University Hospital. 784 EGFR, KRAS, ALK and ROS1-negative NSCLC samples were tested by RET breakapart FISH using a commercial probe. Samples with ≥15% of tumor cells showing separated 5’ and 3’ signals and/or isolated 3’ signals were considered positive and a confirmatory RNA-sequencing analysis was performed.
Results
Out of the >80 publications found using RET FISH analysis, 48 referred to lung cancer patients and 38 used commercial probes, with various thresholds and signal patterns for positivity. Regarding our own data, out of the 784 analyzed samples, 32 (4.0%) were positive by RET FISH, 728 were negative and 3% were not interpretable. The mean percentage of RET FISH-positive cells in the negative samples was of 4.4% [0-14.3], and of 55.3% [17.7-94] in the positive samples. A RET fusion transcript was detected in 22/32 (68.8%) FISH–positive samples. The main FISH pattern in RET FISH positive / RNA-seq negative samples was represented by isolated 3’ signals.
Conclusions
Overall, RET FISH is a sensitive technique but the specificity is lowered by false-positive results, requiring a confirmatory technique for FISH-positive samples. Based on the mean percentages of positive cells in negative and positive samples in our cohort, the ≥15% positivity threshold, the most largely used in the literature, seemed adequate for the use of FISH as a RET rearrangement pre-screening tool with the commercial probe we used.
Clinical trial identification
Editorial acknowledgement
Legal entity responsible for the study
Grenoble Alpes University Hospital.
Funding
Institut National du Cancer.
Disclosure
A. Mc Leer: Financial Interests, Personal, Advisory Role: JFR Access, Pfizer, Lilly, The Marketech Group, Takeda, Roche; Financial Interests, Personal, Invited Speaker: Boehringer, AstraZeneca, Pfizer; Financial Interests, Institutional, Research Grant: Novartis, Pfizer, Takeda. D. Benito: Financial Interests, Personal, Full or part-time Employment: Eli Lilly. All other authors have declared no conflicts of interest.