Objective: The aim of this study was to determine the rate of Her-2 gene amplification in breast cancer cases with a previous negative Her-2 result as determined by immunohistochemistry (score 0 or 1).

Material and Method: 552 cases of invasive breast carcinoma were assessed with the contribution of 9 centers. Previous immunohistochemistry score was either 0 or 1+ in all cases. These cases were re-tested by Her-2 silver in situ hybridization in the central laboratory. Her-2 gene amplification was defined as Her-2/CEP 17 ratio of more than 2.2. Cases with a ratio between 1.8 and 2.0 were defined as equivocal and cases with a ratio of less than 1.8 were defined as negative.

Results: Re-testing of the 552 cases with silver in situ hybridization showed a total of 22 cases with Her-2 gene amplification, of which 11 (3.2%) were found to be score 0, and 11 were found to be score 1+ (5.3%) by immunohistochemistry previously. Her-2 gene amplification rate of cases (score 0 and 1+) ranged from 0% to 10.48% among the centers. Polysomy was found in 28 (8.1%) of the score 0 cases and 25 (12.1%) among the score 1+ cases. Five (9.4%) of the cases with polysomy were found to be amplified, and 48 (90.6%) were not.

Conclusion: The results of the study show that a group of cases (3.98%) with a potential to benefit from anti-Her-2 therapy may be missed with the immunohistochemical method. This indicates the importance of quality assurance, especially in central laboratories with many breast cancer cases in daily practice.