Recent studies have indicated that polysomy 17 is a rare event in breast cancer, and polysomy is usually mimicked in FISH analysis by gain or amplification of the centromere covered by the chromosome 17 centromere probe. To estimate the impact of chromosome 17 centromere assessment on routine practice, we conducted a retrospective re-classification study. Four hundred and five consecutive cases were selected. The original molecular pathology reports were available. Centromere 17 copy counts were ignored in the reassessment. Altogether, nineteen (4.69%) discrepant cases were found, from which five (1.23%) were considered originally non-amplified but had an HER2 copy number >6. Therefore, we reclassified them as HER2-amplified, while fourteen (3.46%) cases were originally considered amplified with 6 or fewer HER2 signals/cell. The discrepant cases found in our reassessment study would require further high-resolution genetic analysis to resolve the disagreement. On the other hand, our result also highlights that for the vast majority of breast cancer cases traditional FISH examination is still adequate to reach the correct diagnosis. This diagnostic gap must be filled by more sophisticated genetic examinations. Moreover, upcoming HER2 guidelines should consider the aid that high-resolution karyotyping can give to the diagnostic algorithm.
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