On the relevance of a testing algorithm for the detection of ROS1-rearranged lung adenocarcinomas.

Objectives: ROS1 proto-oncogene translocations define a new molecular subgroup in non-small cell lung cancers (NSCLC) and are associated with a response to the MET/ALK inhibitor, crizotinib. These rearrangements are described in 0.9-1.7% NSCLC, in wild-type EGFR, KRAS and ALK ("triple negative") lung adenocarcinomas. Rapid and efficient identification of these alterations is thus becoming increasingly important.

Materials And Methods: In this study, 121 triple negative lung adenocarcinomas were screened by both IHC with the ROS1 D4D6 antibody, and FISH using two commercially available ROS1 break-apart probes. To address a possible cross-reactivity of the ROS1 antibody with other protein kinase receptors, we screened 80 additional cases with known EGFR, KRAS, PI3KCA, BRAF, HER2 mutations or ALK-rearrangement.

Results: We diagnosed 9 ROS1-rearranged adenocarcinomas, with both a positive FISH result (51-87% rearranged nuclei) and a positive IHC staining (2+/3+ cytoplasmic staining). Only one of the ROS1-positive FISH cases was characterized by a classical split pattern, the others showed a variant pattern, most commonly involving a loss of the 5' telomeric probe. Considering a positivity threshold of 2+ stained cells, the sensitivity of the ROS1 D4D6 antibody compared to FISH was 100% and the specificity 96.9%, as two HER2-mutated tumors were positive with D4D6 antibody, without any translocation in FISH. All the ROS1-positive cases were at an advanced stage, arising in never or light smokers. They were mainly solid cribriform and acinar adenocarcinomas, with signet ring cells noted in 5 cases, and calcifications in 3 cases. One positive case was an invasive mucinous carcinoma.

Conclusion: Our results show that a screening algorithm based on an IHC detection of ROS1 fusion proteins, confirmed if positive or doubtful by a ROS1 break-apart FISH assay, is pertinent in advanced "triple negative" lung adenocarcinomas, since the prevalence of ROS1-positive cases in this selected population reaches 7.4% in our series.