The Value of Routine Reflex In Situ Hybridization Testing in Low-grade Tubule-forming Invasive Breast Cancers With Equivocal HER2 Immunohistochemistry: A Reappraisal of the ASCO/CAP Guidelines.

Guidelines from the American Society of Clinical Oncology and College of American Pathologists (ASCO/CAP) recommend an equivocal score (2+) and reflex in situ hybridization (ISH) testing for invasive breast cancer (IBC) with moderate to strong lateral or basolateral staining by HER2 immunohistochemistry (IHC). While this recommendation mainly addresses aggressive tumor types such as micropapillary carcinoma, it is applied to low-grade, tubule-forming IBCs unlikely to show HER2 amplification. A total of 62 cases of IBC with equivocal HER2 IHC, low histologic grade (1/3), and tubule formation scores of 1 or 2 according to Nottingham criteria were retrospectively identified from 2020 to 2023 to determine the frequency of HER2 amplification using fluorescent ISH (FISH). Following slide review, 3 cases were reclassified as grade 2, leaving 59 cases for analysis. Demographic and clinicopathological data were collected from medical records and analyzed. A total of 98.3% of cases were not amplified by HER2 FISH, with only 1 case showing amplification. All cases were ER positive. Oncotype DX scores, available for select cases, showed low recurrence scores (below 25). The single HER2-amplified case contained a ductal carcinoma in situ component with HER2 3+ staining, potentially leading to a false positive result. In conclusion, HER2 amplification is rare in low-grade, tubule-forming IBCs. These results suggest reconsidering current guidelines to reduce unnecessary FISH testing, potentially improving cost-effectiveness and clinical efficiency without compromising diagnostic accuracy.