Double-Equivocal HER2 Invasive Breast Carcinomas: Institutional Experience and Review of Literature

Abstract

Context.— HER2 status is a prognostic factor and therapeutic target in invasive breast carcinomas. Reflex testing using an alternate method is recommended on equivocal cases via immunohistochemistry or fluorescence in situ hybridization (FISH). Therapeutic dilemmas arise when both tests are equivocal. The standard chromosome 17 centromere reference probe (CEP17) is in close proximity to the HER2 locus and may be coamplified, leading to equivocal results. Alternate chromosome 17 reference probes may aid in establishing the true HER2 status. Objective.— To describe our institutional experience using D17S122 probe for reflex FISH testing on double-equivocal invasive breast carcinomas and review the literature on alternate reference probes. Data Sources.— Twenty-two patients with double-equivocal invasive breast carcinomas, defined as HER2 immunohistochemistry score 2+ and FISH equivocal per the 2013 guidelines, were reviewed. Reflex FISH was performed with alternate probe D17S122 and the HER2 status classified for 11 cases by using a revised HER2:D17S122 ratio. Seven of 11 cases (63.6%) were ultimately classified as HER2 positive, while 4 cases (36.4%) remained equivocal. The 7 positive cases showed a HER2:D17S122 greater than 2.0. Conclusions.— Alternate probe D17S122 reclassified more than half of our cases as HER2 positive. Alternate probes may establish true HER2 status and direct proper management, as evidenced by our experience and the literature. Additional investigation is needed to determine which alternate probe(s) is(are) best for reflex testing. Finally, the American Society of Clinical Oncology/College of American Pathologists guidelines may need to be updated to reflect more specific recommendations for the utilization of appropriate probes in double-equivocal HER2 cases.

Clinicopathologic features of breast cancer reclassified as HER2-amplified by fluorescence in situ hybridization with alternative chromosome 17 probes

“…9 Several subsequent studies showed that the new equivocal category and the use of alternative chromosome 17 probes resulted in an increase in the proportion of equivocal and amplified HER2 FISH results, particularly among cases with equivocal (2+) IHC results and CEP17 CNA. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] However, copy number variations also have been reported for the most commonly used alternative chromosome 17 control probes [i.e., RAI1, LIS1 (PAFAH1B1), RARA, and TP53]. 16,17 Therefore, it is not clear which probe provides the most accurate reflection of the chromosome 17 copy number and ultimately the most accurate HER2:control ratio.…”

Prognostic and predictive biomarkers with therapeutic targets in breast cancer: A 2022 update on current developments, evidence, and recommendations

“…Interestingly, additional copies of CEP17 detected by ISH may be caused by amplification of the centromeric region due to the proximity of the CEP17 gene to the HER2 gene, 103 prompting the use of a centromeric probe or an alternate probe to correct the absolute HER2 copy number to help refine the threshold of HER2 positivity. 104,105 HER2 low positivity: About 40-50% of breast cancer patients are HER2 low, i.e. they are IHC 1+ or 2+ but HER2 gene amplification was negative.…”