An audit of Her2/neu FISH signal patterns in immunohistochemically equivocal breast carcinomas from a tertiary cancer centre in India

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Introduction Breast cancer (BC) is the second most common cancer in Indian women. Overexpression /amplification of Her2/neu is detected in approximately 20% to 30% cases. Apart from the classical amplified and non-amplified signal patterns, ‘nonclassical’ FISH (fluorescence in situ hybridization) patterns are recognized where in the ratio and signals/cell data would independently yield different results or results that are close to a threshold, the relevance of which is unknown and require further study. Aims and objectives To delineate the spectrum of Her-2 Neu FISH signal patterns in immunohistochemically equivocal cases of BC. Materials and methods Retrospective data of 393 cases of Her2 equivocal (Score 2 + by immunohistochemistry) cases were collected from the electronic medical records of the oncopathology department over a 3-year period (2020–2022). Demographic data, hormone receptor status were recorded and subjected to FISH by the ERBB2/CEN 17 Dual Color Probe and analyzed according to ASCO-CAP 2018 guidelines. Results Females and males comprised (96.43%, n = 379) and (3.56%, n = 14) respectively. The age range was 26–85 years (mean 52.36 year). The sites included breast primary (95.6%, n = 374) and metastatic sites (4.83%, n = 19). FISH analysis revealed about 18.32% amplified cases [ (classic amplified (10.94%) low amplified (6.8%) and co-amplified (0.5%)]. Other signal patterns observed were classic non amplified (70.99%), equivocal (6.1%), monosomy (1.52%) and heterogenous (2.54%). Polysomy (mean of > = 3 CEP17 signals per nucleus) was observed in 8.9% cases. Conclusion This is one of the few studies delineating the spectrum of FISH signals. It would be better to consider all the clinicopathologic features on a case-by-case basis when determining whether to offer HER2-targeted therapies in non classic amplified patterns. Clinical trial registration Not applicable.