Evidence for Unified Assessment Criteria of HER2 Immunohistochemistry in Colorectal Carcinoma

Human epidermal growth factor receptor-2 (HER2) expression is an important biomarker for the management of RAS wild-type metastatic colorectal carcinoma (CRC). Immunohistochemistry (IHC) with reflex in situ hybridization (ISH) is accepted as a standard method of assessment, yet there are currently the following 2 sets of criteria used to interpret results: the HER2 Amplification for Colorectal Cancer Enhanced Stratification (HERACLES) criteria and the MyPathway criteria. The HER2 Amplification for Colorectal Cancer Enhanced Stratification criteria require ISH confirmation when IHC staining is 3+ in 10% to 49% of cells, whereas the MyPathway criteria mirror those for gastric HER2 assessment and do not recommend ISH confirmation in the previously referenced scenario. We aimed to assess the prevalence of HER2 3+ heterogeneity and its association with ERBB2 copy number amplification to evaluate the necessity of ISH testing when IHC staining is 3+ in <50% of cells. Next-generation sequencing of DNA (592-gene panel or whole exome sequencing) was performed for 13,208 CRC tumors submitted to Caris Life Sciences. HER2 (4B5) expression was tested using IHC. A subset of tumors was tested for ERBB2 amplification via chromogenic ISH and/or via next-generation sequencing (copy number amplification). χ² tests or Fisher exact tests were applied where appropriate, with P values adjusted for multiple comparisons (P < .05). Of 13,208 CRCs with HER2 IHC, 87.4% (11,541/13,208) were negative for HER2 expression (≤3+ intensity and <10% tumor-cell staining) and 11.2% (1473/13,208) demonstrated at least low HER2 expression (1 to 2+ and ≥10%). Only 1.5% (194/13,208) of all tested tumors were either positive or heterogeneously positive for HER2 overexpression (3+ and ≥10%). Of these, 14% (28/194) had heterogenous HER2 overexpression (3+ staining of 10%-49% of cells). Among 22 HER2-positive/heterogenous cases with successful ISH testing, 100% (22/22) demonstrated amplification via ISH. Because the classification of tumors as HER2-positive/heterogenous using IHC correlated very closely with ISH positivity, our results suggest that ISH is likely unnecessary for CRCs with 3+ HER2 overexpression in 10% to 49% of neoplastic cells.