Implementation of the 2018 American Society of Clinical Oncology/College of American Pathologists Guidelines on HER2/neu Assessment by FISH in breast cancers: predicted impact in a single institutional cohort.

The 2018 American Society of Clinical Oncology (ASCO) and College of American Pathologists (CAP) update modified the interpretation guidelines for human epidermal growth factor receptor 2 (HER2) testing by incorporating immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) results in a subset of cases. Importantly, the new guidelines eliminate "equivocal" results, as well as the use of alternative chromosome 17 probes as the primary strategy for resolving the indeterminate FISH results. Herein, we investigate the predicted impact of implementing the 2018 ASCO/CAP guidelines on HER2 assessment by FISH in breast cancers, using data from a single institution. We compared the HER2 status of 1542 consecutive cases of breast carcinoma, interpreted by 2013 and 2018 ASCO/CAP guidelines. In total, 10.7% (165/1542) of the cases had a different final interpretation by 2018 guidelines compared with 2013 guidelines, including 70 previously HER2-positive cases reclassified as negative, four previously negative cases reclassified as positive, and 91 previously equivocal cases reclassified as negative. Overall, the number of HER2-positive cancers was reduced by 66 cases (4.3% reduction in the HER2 positivity rate). The newly HER2-negative cases were mostly estrogen receptor positive (90%), progesterone receptor positive (80%), stage 1 (60.9%), and grade 1-2 (59.4%) cancers; 70% of them had been designated as HER2 positive only after the use of an alternative chromosome 17 FISH probe after an intially equivocal result from the standard CEP17 probe. Overall, implementing the revised 2018 HER2 guidelines is predicted to change the HER2 results of 10.7% of breast cancers, mainly by reclassifying previously equivocal to negative results.

Breast cancers with a HER2/CEP17 ratio of 2.0 or greater and an average HER2 copy number of less than 4.0 per cell: frequency, immunohistochemical correlation, and clinicopathological features.

The 2013 American Society of Clinical Oncology and College of American Pathologists (ASCO/CAP) guidelines classified breast cancers with a fluorescence in situ hybridization dual-probe HER2/CEP17 ratio of 2 or greater as "amplified," inclusive of cases with a HER2 copy number less than 4. The 2018 ASCO/CAP update assigns HER2/neu status for the latter group in a fashion that is highly dependent on the associated immunohistochemical findings. Herein, the authors define the frequency, immunohistochemical correlates, and other clinicopathological features of breast cancers with HER2/CEP17 ratio of 2 or greater and HER2/neu copy number less than 4 (group A), based on an analysis of an institutional cohort assessed for HER2/neu status by both florescence in situ hybridization and immunohistochemistry and scored using 2013 ASCO/CAP criteria. Group A cases were compared with a group B of HER2/neu-amplified breast cancers with a HER2/neu copy number of 4 or greater regarding a variety of clinicopathological features. One hundred sixty-nine (14%) of 1201 cases were HER2/neu amplified, 18 (10.7%) in group A and 151 (89.3%) in group B. By immunohistochemistry, 61.1% of group A cases were HER2/neu negative, 7 (38.9%) were equivocal, and none were positive. In contrast, 66.9% of group B cases were HER2 positive (3+). We could not demonstrate statistically significant differences between the 2 groups regarding standard clinicopathological variables. In summary, our group A cases account for 1.5% of breast cancers, and 10.7% of all HER2/neu-amplified cancers classified as such based on 2013 ASCO/CAP criteria. They are predominantly HER2/neu negative by immunohistochemistry, which suggests that they are biologically different from classically HER2/neu-amplified cases and which validates the 2018 ASCO/CAP guideline against automatically classifying such cases as HER2/neu amplified.

Comparative Pathologic Analysis of Breast Cancers Classified as HER2/neu-Amplified by FISH Using a Standard HER2/CEP17 Dual Probe and an Alternative Chromosome 17 Control Probe.

At our institution, breast cancer cases that generate an equivocal HER2/neu (HER2) result by fluorescence in situ hybridization (FISH) using the dual HER2/chromosome enumeration probe (CEP17) are reflexed to an assay that utilizes an alternative control probe (lissencephaly gene1 [LIS1] [17p13.3]/retinoic acid receptor α [RARA] [17q21.2]). This study examines whether cancers that are classified as HER2-amplified with an alternate probe are clinicopathologically similar to those that are classified as such using the HER2/CEP17 probe. Reports for 1201 breast cancers were reviewed, and clinicopathologic findings were compared between HER2/CEP17-equivocal cases that became HER2-amplified using the alternate probe (group A: n=48), HER2-amplified cases using the HER2/CEP17 probe (group B: n=169), and HER2-nonamplified cases using the HER2/CEP17 probe (group C: n=910). Of 1201 cases tested using the HER2/CEP17 probe, 169 (14%) were HER2-amplified, 122 (10%) were equivocal, and 910 (76%) were nonamplified. Additional testing with the alternative probe on the 122 equivocal cases reclassified 48 (39%) of them to HER2-amplified, and such cases comprised 22% of all HER2-amplified tumors. A higher proportion of tumors with HER2 copy number between 5.0 and 5.9 became positive upon additional testing when compared with those with a priori HER2 copy numbers between 4.0 and 4.9 (P=0.0362). Group A cases, compared with group B cases, were more frequently positive for estrogen receptor (97.91% vs. 72.18%, P<0.0001) and progesterone receptor (85.41% vs. 59.17%, P=0.0009). Most group A cases (71%) were HER2 equivocal (score 2+) by immunohistochemistry, whereas most group B cases (60%) were positive (score 3+). Groups A and B showed no significant differences regarding patient age, lymph node status, tumor grade, histotype, and stage distribution. In summary, among our HER2-amplified cohort of breast cancers, alternative probe-detected cases were more frequently estrogen receptor and progesterone receptor positive than HER2/CEP17-detected cases, and were more frequently discordant with HER2 immunohistochemistry results. These findings raise the possibility of underlying biologic differences between these 2 groups, which warrants further study. However, the tumors were largely comparable regarding all other clinicopathologic variables. As it is unknown whether HER2-targeted therapy is truly beneficial in this subgroup of patients, future clinical trials should specifically evaluate this subset.