Sections derived from a tissue microarray, each consisting of 3 cores from 10 NSCLC cases, were independently tested for ALK protein expression by IHC and genomic ALK-breaks by FISH involving 8 institutes of pathology. Based on a pre-screening, 5 cases were identified to be clearly ALK-break negative, whereas the remaining 5 cases were ALK-break positive including one case with low percentage (20 % ) of positive cells. The latter had been additionally tested by RT-PCR.
The 5 unequivocal ALK-break negative NSCLC were almost consistently scored negative by means of FISH and IHC by all 8 experts. Interestingly, 4 of the 5 cases with pre-defined ALK-breaks revealed homogenous FISH results whereas IHC for the detection of ALK protein expression showed heterogeneous results. The remaining case (low number of ALK-break positive cells) was scored negative by 3 experts and positive by the other 5. RT-PCR revealed the expression of an EML4-ALK fusion gene variant 1.
ALK-break negative NSCLC cases revealed concordant homogeneous results by means of FISH and IHC (score 0-1) by all 8 experts. Discordant FISH results were raised in one ALK-break positive case with a low number of affected tumor cells. The remaining 4 ALK-break positive cases revealed concordant FISH data whereas the ALK-IHC revealed very diverse results. The cases with concordant FISH results provide an excellent basis for round robin ALK-FISH testing. As long as standardized ALK-IHC protocols are missing, ALK protein expression cannot by regarded as the method of choice for identification of patients eligible for treatment with ALK inhibitors.
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