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Background In 2013 the ASCO/CAP Guidelines for HER2 testing were updated and revised to improve the accuracy of Her2 testing and its utility as a predictive marker in breast cancer. In this prospective cross-sectional study, we assessed the proportion of Her2 positive and equivocal cases in a real-world setting in Germany. Patients' characteristics and tumor biological factors were evaluated. Furthermore, we analyzed the effect of the updated recommendations on the clinical management. Methods This IRB approved prospective analysis included patients with primary breast cancer. Patients with primary stage IV and recurrent breast cancer were excluded. Data was collected from 2012-2017 in six certified breast centers using a personal questionnaire and data from the patients' medical records. Results 2705 primary breast cancer cases were analyzed. The groups consisted of 320(11.8%) TNBC, 1956 (72.31%) luminal breast cancer patients and 381 (14.08%)Her2 positive cancer patients. 18 (1%) patients were classified as equivocal. 30 patients did not meet the criteria of any of these groups. No significant difference was seen in positive lymph node status between all groups. Tumor size at diagnosis was larger for TNBC compared to the other groups. 81,9% of all TNBC, 65.1% of the Her2 type, 44.2% of the luminal type and 44.44% (8/18) of the equivocal cases were grade 3 tumors. 3 cases with unresolved Her2 status were diagnosed in 2013 and 6 cases in 2016. 17/18 equivocal cases were hormone receptor positive, 1/18 was hormone receptor negative. In 12/18 cases adjuvant or neoadjuvant chemotherapy was recommended. Trastuzumab was recommended in 9/18 of the cases. 2/18 patients were BRCA mutation carriers. 2/18 received an oncologic treatment for another cancer prior to their breast cancer diagnosis. 1/18 patient received primary endocrine therapy with an aromatase inhibitor. 6/18 patients had a positive family history for breast cancer. Conclusions Our study indicates that the 2013 ASCO/CAP update does not affect the overall Her2 positivity rate in breast cancer. Although the proportion of tests and retests increased, only few unresolved cases remained. Despite the fact that the benefit of anti-Her2 directed therapy in equivocal cases is unproven, oncologists were more likely to recommend Trastuzumab, if chemotherapy was indicated for high risk breast cancer. There are numerous factors that could explain equivocal findings. Interestingly, a significant number of our patients presented endogenous or exogenous risk factors for chromosomal aberrations that might be responsible for unresolved Her2 cases. Citation Format: Kuehnle E, Wulf S, Kristina L, Iris S, Stefanie N, Alexander M, Jutta J, Thomas N, Momme A, Peter H, Thilo D-B, Tjoung-Won P-S. Frequency of equivocal Her2-test results after revision of the ASCO/CAP guidelines in a real-world setting. Results of a prospective cross-sectional study of primary breast cancers in Germany [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P6-08-22.

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