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

HER2 Equivocal (Score = 2+) Breast Carcinoma Cases Identified by Immunohistochemistry at a South African Hospital. What is the Impact of Fluorescent In Situ Hybridization Testing?

The American Society of Clinical Oncology and the College of American Pathologists (ASCO/CAP) guidelines are used for human epidermal growth factor receptor 2 (HER2) reporting in breast carcinoma. Cases that demonstrate weak to moderate complete membrane immunohistochemical staining in >10% of the tumor are scored as 2+ (equivocal). This study aimed to determine what proportion of HER2 immunohistochemistry (IHC) score = 2+ breast carcinomas were confirmed to be positive by HER2 fluorescent in situ hybridization (FISH). There were 241 HER2 IHC score = 2+ breast carcinomas included. Most (74.3%) carcinomas were estrogen and progesterone receptor-positive. Invasive breast carcinoma of no special type (89.2%) was the commonest histologic subtype. Most tumors were grade 2 (64.3%). As per the FISH report, at the time of diagnosis, 27 cases (11.2%) were HER2 FISH positive. All HER2 FISH equivocal cases and one FISH positive case assessed using the 2013 ASCO/CAP HER2 criteria were reclassified to HER2 FISH negative when the 2018 criteria were applied. There was a high level of agreement (κ = 0.979) between HER2 FISH results obtained using the 2013 and the 2018 criteria. This study provides insight into the frequency of HER2 FISH positivity (11.2%) among HER2 IHC score = 2+ breast carcinomas and the impact of modifications to the ASCO/CAP HER2 guidelines. Elimination of the HER2 FISH equivocal category by the 2018 guidelines has reduced the need for repeat testing and simplified clinical management. Reclassification of previous HER2 FISH positive to negative has resulted in some patients being ineligible for costly anti-HER therapy.

Breast Cancer With a HER2 FISH Group 2 Result: Should HER2 Tests be Repeated?

BACKGROUND

Breast cancer with fluorescence in situ hybridization (FISH) group 2 pattern (HER2 <4 and HER2/CEP17 ratio ≥2, a subset of monosomy CEP17) was historically considered HER2-positive, but mostly HER2-negative according to updated 2018 American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines unless 3+ by immunohistochemistry (IHC). Therapeutic relevance of this group remained elusive, therefore we assessed if repeat IHC and FISH can assist final HER2 classification.

PATIENT AND METHODS

We retrospectively reviewed HER2 FISH performed at our institution from 2014 to 2018 and identified 23 of 3554 (0.6%) breast cancer cases with at least one-time measurement of HER2 FISH categorized as group 2. Repeat HER2 tests were performed for cases with available alternative tumor samples and compared with initial testing following 2018 ASCO/CAP guidelines.

RESULTS

Only 1 of 23 group 2 cases was HER2-positive, 0/18 in primary and 1/5 in metastatic/recurrent tumors. Of 13 primary tumors with repeat HER2 results; 10 (77%) remained HER2-negative; 3 (23%) changed from HER2-negative (group 2 and IHC 2+) to HER2-positive (group 1 and IHC 2+). Among 8 of these 13 patients receiving neoadjuvant systemic therapy containing anti-HER2 agent, 3 (38%) achieved pathologic complete response (pCR). Two of 3 pCR cases were HER2-positive converters on repeat testing. Three pCR cases were ER-negative or -low positive and Ki67 ≥40%, while 5 partial responders were ER-positive and Ki67 <40% (P < .05).

CONCLUSION

Breast cancer with HER2 FISH group 2 result may represent heterogeneous populations of tumor cells being originated de novo or preferentially selected secondary to therapy. Repeat HER2 tests on alternative samples may be considered to guide anti-HER2 therapy.

Development and validation of an AI-enabled digital breast cancer assay to predict early-stage breast cancer recurrence within 6 years

BACKGROUND

Breast cancer (BC) grading plays a critical role in patient management despite the considerable inter- and intra-observer variability, highlighting the need for decision support tools to improve reproducibility and prognostic accuracy for use in clinical practice. The objective was to evaluate the ability of a digital artificial intelligence (AI) assay (PDxBr) to enrich BC grading and improve risk categorization for predicting recurrence.

METHODS

In our population-based longitudinal clinical development and validation study, we enrolled 2075 patients from Mount Sinai Hospital with infiltrating ductal carcinoma of the breast. With 3:1 balanced training and validation cohorts, patients were retrospectively followed for a median of 6 years. The main outcome was to validate an automated BC phenotyping system combined with clinical features to produce a binomial risk score predicting BC recurrence at diagnosis.

RESULTS

The PDxBr training model (n = 1559 patients) had a C-index of 0.78 (95% CI, 0.76-0.81) versus clinical 0.71 (95% CI, 0.67-0.74) and image feature models 0.72 (95% CI, 0.70-0.74). A risk score of 58 (scale 0-100) stratified patients as low or high risk, hazard ratio (HR) 5.5 (95% CI 4.19-7.2, p < 0.001), with a sensitivity 0.71, specificity 0.77, NPV 0.95, and PPV 0.32 for predicting BC recurrence within 6 years. In the validation cohort (n = 516), the C-index was 0.75 (95% CI, 0.72-0.79) versus clinical 0.71 (95% CI 0.66-0.75) versus image feature models 0.67 (95% CI, 0.63-071). The validation cohort had an HR of 4.4 (95% CI 2.7-7.1, p < 0.001), sensitivity of 0.60, specificity 0.77, NPV 0.94, and PPV 0.24 for predicting BC recurrence within 6 years. PDxBr also improved Oncotype Recurrence Score (RS) performance: RS 31 cutoff, C-index of 0.36 (95% CI 0.26-0.45), sensitivity 37%, specificity 48%, HR 0.48, p = 0.04 versus Oncotype RS plus AI-grade C-index 0.72 (95% CI 0.67-0.79), sensitivity 78%, specificity 49%, HR 4.6, p < 0.001 versus Oncotype RS plus PDxBr, C-index 0.76 (95% CI 0.70-0.82), sensitivity 67%, specificity 80%, HR 6.1, p < 0.001.

CONCLUSIONS

PDxBr is a digital BC test combining automated AI-BC prognostic grade with clinical-pathologic features to predict the risk of early-stage BC recurrence. With future validation studies, we anticipate the PDxBr model will enrich current gene expression assays and enhance treatment decision-making.

The effect of neoadjuvant chemotherapy on tumor-infiltrating lymphocytes in patients with breast cancer

Aim: This study investigated the effect of neoadjuvant chemotherapy (NAC) on stromal tumor-infiltrating lymphocytes (sTILs) and their treatment response. Materials & methods: One hundred fifteen patients with pre-NAC core biopsies and post-NAC surgical resection specimens were reviewed. Results: There was no significant change between pre- and post-treatment sTILs. Both pre- and post-NAC sTILs were significantly lower in patients with luminal A subtype. An increase in sTILs was observed in 21 (25.9%) patients after NAC, a decrease in 29 (35.8%) and no change in 31 (38.3%; p = 0.07). Pretreatment sTIL density was independent predictor of pathological complete response in multivariate analyses (odds ratio: 1.025, 95% CI: 1.003-1.047; p = 0.023). Conclusion: High sTIL density in core biopsies was independently related to pathological complete response. In addition, ER appears to be the most crucial factor determining the rate of sTIL.

Real-world Evidence of Diagnostic Testing and Treatment Patterns in US Patients With Breast Cancer With Implications for Treatment Biomarkers From RNA Sequencing Data

OBJECTIVE/BACKGROUND

We performed a retrospective analysis of longitudinal real-world data (RWD) from patients with breast cancer to replicate results from clinical studies and demonstrate the feasibility of generating real-world evidence. We also assessed the value of transcriptome profiling as a complementary tool for determining molecular subtypes.

METHODS

De-identified, longitudinal data were analyzed after abstraction from records of patients with breast cancer in the United States (US) structured and stored in the Tempus database. Demographics, clinical characteristics, molecular subtype, treatment history, and survival outcomes were assessed according to strict qualitative criteria. RNA sequencing and clinical data were used to predict molecular subtypes and signaling pathway enrichment.

RESULTS

The clinical abstraction cohort (n = 4000) mirrored the demographics and clinical characteristics of patients with breast cancer in the US, indicating feasibility for RWE generation. Among patients who were human epidermal growth factor receptor 2-positive (HER2+), 74.2% received anti-HER2 therapy, with ∼70% starting within 3 months of a positive test result. Most non-treated patients were early stage. In this RWD set, 31.7% of patients with HER2+ immunohistochemistry (IHC) had discordant fluorescence in situ hybridization results recorded. Among patients with multiple HER2 IHC results at diagnosis, 18.6% exhibited intra-test discordance. Through development of a whole-transcriptome model to predict IHC receptor status in the molecular sequenced cohort (n = 400), molecular subtypes were resolved for all patients (n = 36) with equivocal HER2 statuses from abstracted test results. Receptor-related signaling pathways were differentially enriched between clinical molecular subtypes.

CONCLUSIONS

RWD in the Tempus database mirrors the overall population of patients with breast cancer in the US. These results suggest that real-time, RWD analyses are feasible in a large, highly heterogeneous database. Furthermore, molecular data may aid deficiencies and discrepancies observed from breast cancer RWD.

An Evaluation of Clinicopathological Correlation and Outcome of Human Epidermal Growth Factor Receptor 2 Subgroups Reclassified According to the Latest ASCO/CAP Guideline

BACKGROUND

The latest American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guideline has updated the interpretation of uncommon human epidermal growth factor receptor 2 (HER2) in situ hybridization (ISH) patterns (groups 2-4) with concomitant HER2 immunohistochemistry, leading to changes in the diagnosis of these subgroups. We sought to assess the clinicopathological features and outcomes in these subgroups in detail with our local cohort.

PATIENTS AND METHODS

Clinicopathologic features of groups 2 to 4 were compared to the typical amplified group (group 1: HER2/CEP17 ≥ 2, HER2 ≥ 4) and non-amplified group (group 5: HER2/CEP17 < 2, HER2 < 4).

RESULTS

Group 2 (HER2/CEP17 ≥ 2, HER2 < 4) cases showed lower Ki67 expression and grade (P ≤ .002) than group 1 but no differences compared with group 5. Group 4 (HER2/CEP17 < 2, HER2 = 4-6) cases were associated with less necrosis, more estrogen receptor positivity, lower grade, more nodal metastases, and more special histotypes (P ≤ .037) than group 1, but higher grade and more nodal metastases (P ≤ .021) than group 5. Except for presenting as a larger tumor and of special histotypes, group 3 (HER2/CEP17 < 2, HER2 ≥ 6) cases showed no other significant differences from group 1, but were of higher grade and Ki67 level than groups 2, 4, and 5. Group 4, similar to group 5, showed worse survival than group 1 (disease-free survival: log-rank = 5.547, P = .019; overall survival: log-rank = 4.678, P = .031). The rate of relapse was similar in group 4 with and without anti-HER2 therapy, albeit with limited cases.

CONCLUSION

Our findings indicate more similarities among groups 2, 4, and 5 than between groups 1 and 3, supporting the HER2 categorization in the latest guideline. Additional studies may be warranted to assess the outcomes of these patients with different management approaches.

HER2 testing in breast cancers: comparison of assays and interpretation using ASCO/CAP 2013 and 2018 guidelines

PURPOSE

HER2 overexpression and gene amplification are routinely tested by immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH), respectively. In addition, HER2 mRNA expression is also tested by the Oncotype DX assay. Discordance between laboratories among the different assays remains a problem. To improve the routine HER2 reporting, the American Society of Clinical Oncology (ASCO) and the College of American Pathologists (CAP) updated their guidelines in 2018. Our study will compare concordance of HER2 status by IHC and FISH using ASCO/CAP 2013 and 2018 guidelines with Oncotype DX.

METHODS

We retrospectively reviewed 657 estrogen receptor positive primary breast cancer cases with available Oncotype DX tests between 2011 and 2018. Medical records were reviewed for HER2 results by IHC, FISH, and Oncotype DX. The HER2 results by different assays and between 2013 and 2018 guidelines were compared.

RESULTS

Of the 657 cases, 280 were tested by IHC, FISH, and Oncotype DX. HER2-equivocal cases by IHC 2013 guidelines were all negative (67/67, 100%) by FISH 2018 guidelines and by Oncotype DX. HER2-equivocal cases by FISH 2013 guidelines were all negative (16/16, 100%) by FISH 2018 guidelines, while 15/16 (93.8%) negative and 1/16 (6.2%) equivocal by Oncotype DX. The HER2-equivocal and HER2-negative groups were similar in age, gender, histology, grade, and Ki67 score.

CONCLUSIONS

HER2 concordance was highest between Oncotype DX (99.6%) and FISH per 2018 guidelines. This suggests that the ASCO/CAP 2018 guidelines improved the accurate stratification of HER2-equivocal cases.

Clinical significance of quantitative categorization of HER2 fluorescent in situ hybridization results in invasive breast cancer patients treated with HER2-targeted agents

HER2 (ERBB2) gene status serves as a strong predictive marker of response to HER2-targeted agents in invasive breast cancers, albeit with heterogeneous response. Our aim was to determine the distribution and prognosis of HER2 groups by fluorescent in situ hybridization (FISH) using the updated 2018 American Society of Clinical Oncology-College of American Pathologist (ASCO-CAP) guidelines. We identified 226 cases of equivocal or positive HER2 FISH invasive breast cancer (interpreted by ASCO-CAP guidelines at the time of reporting) who received HER2-targeted agents from 2006 to 2017. We subcategorized Group 1 further into three subgroups: low amplified (HER2/CEP17 ratio ≥ 2.0-2.99, mean HER2/cell 4.0-5.9), amplified (HER2/CEP17 ratio ≥ 2.0-2.99, mean HER2/cell ≥ 6), and excessive amplification (HER2/CEP17 ratio ≥ 3.0, mean HER2/cell ≥ 4.0). Outcomes studied were recurrence, metastasis, second breast primary, disease-specific survival (DSS), and overall survival (OS). Univariate analysis showed that the five categories of HER2 FISH were significantly associated with OS (p < 0.01), specifically higher HER2 amplification was associated with fewer deaths. HER2 FISH status also statistically significantly relates to DFS (p < 0.01) and metastasis (p = 0.01) but not with recurrence or second breast primary in our study. Tumor type and HER2 ISH Groups are independent predictors for both OS and DFS in our cohort. The proposed Group 1 subcategories were significantly associated with OS (p < 0.01) and DFS (p < 0.01), excessive HER2 amplification was associated with longer median survival. The Cox regression models showed better survival outcomes for the excessive amplification subgroup than the low amplified subgroup, with OS (hazard ratio = 0.63, 95% CI 0.42-0.93) and DFS (HR = 0.55, 95% CI 0.37-0.83). We demonstrated that in HER2 FISH Group 1 patients, high HER2 amplification was significantly associated with longer OS and DFS; these patients seem to benefit more from HER2-targeted regimens. We recommend reporting these Group 1 subcategories when assessing HER2 FISH.

Quantitative Impact of the 2018 American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) Practice Guideline Update on Human Epidermal Growth Factor Receptor 2 Testing in Breast Cancer: A Systematic Analysis

CONTEXT.—

The global impact of the new 2018 American Society of Clinical Oncology/College of American Pathologists human epidermal growth factor receptor 2 (HER2) practice guideline update on the overall HER2 status designation, compared with the prior 2013 iteration, is unknown.

OBJECTIVES.—

To report the quantitative impact of the new guideline on HER2 status distribution.

DESIGN.—

The analysis comprised a retrospective cohort of patients from the authors' institution, combined with other peer-reviewed publications that assessed the impact of the 2018 guideline in relation to the 2013 guideline.

RESULTS.—

Our study revealed that the new guideline led to an average 9% reclassification rate for the overall HER2 status, with a net gain in overall HER2 negative designation. This is largely due to reclassification of the equivocal (Group 4) groups. Unexpectedly, infrequent but consistent discordance between Group 1/5 and fluorescence in situ hybridization results are observed across studies (1.8%; 73 of 3965 cases where fluorescence in situ hybridization and immunohistochemistry are both reported).

CONCLUSIONS.—

Early clinical recognition of these resultant changes, including emerging issues of tumor heterogeneity, and potential discordance between immunohistochemistry to fluorescence in situ hybridization, is important for accurate clinical assessment of individual HER2 test results.

HER2 Testing for Breast Cancer in the Genomics Laboratory: A Sea Change for Fluorescence In Situ Hybridization

CONTEXT.—

Guidelines for HER2 testing in breast cancer have changed over time, from the US Food and Drug Administration guideline to the American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines published in 2007, 2013, and 2018.

OBJECTIVE.—

To investigate the change in assignment of HER2 status in breast cancers with equivocal (2+) immunohistochemistry (IHC) results by fluorescence in situ hybridization (FISH) following implementation of the ASCO/CAP 2018 guideline.

DESIGN.—

The study included 3556 invasive breast cancers that were HER2 equivocal (2+) by IHC and were submitted to our FISH laboratory after July 2018. Reflex testing (with repeat IHC staining) was performed on certain categories of FISH results known as groups 2, 3, and 4. Concomitant review of IHC and FISH was performed on these reflex cases per 2018 guideline recommendations. The FISH data were analyzed to compare US Food and Drug Administration and ASCO/CAP 2007, 2013, and 2018 interpretations.

RESULTS.—

Of 3548 invasive breast cancers with complete data available, the percentage agreement for FISH according to different guidelines was highest for ASCO/CAP 2018 versus US Food and Drug Administration (96.5%), followed by ASCO/CAP 2018 versus 2007 (93.8%), and lowest with ASCO/CAP 2018 versus 2013 (83.7%). Per the 2018 guideline, reflex IHC testing was performed on 633 breast cancers (17.8%); the majority of reflex testing results were negative (541 of 633; 85.5%). The overall distribution of HER2 FISH results (per the 2018 guideline) was 88.5% negative and 11.5% positive.

CONCLUSIONS.—

By eliminating the equivocal FISH category, the 2018 ASCO/CAP guideline significantly reduced the HER2 FISH-positive rate in tumors with equivocal (2+) IHC results.

Impact of the Updated Guidelines on Human Epidermal Growth Factor Receptor 2 (HER2) Testing in Breast Cancer

Purpose

In 2007, the American Society of Clinical Oncology and the College of American Pathologists had established a human epidermal growth factor receptor 2 (HER2) testing guideline, which was updated in 2013 and subsequently in 2018. We assessed the clinical impact of the recent update by comparing the in situ hybridization (ISH) results based on the 2007, 2013, and 2018 guidelines.

Methods

We assessed 2 cohorts. The first cohort included 1,161 primary invasive breast cancer (IBC) samples including 18 bilateral IBC cases, with both immunohistochemistry (IHC) and silver-enhanced ISH (SISH) results available for the HER2 status. The second cohort included 160 IBC cases with equivocal HER2 IHC, assessed using SISH. We retrospectively evaluated and compared the HER2 SISH results.

Results

There were 22 (1.9%) and 20 (12.5%) cases with altered SISH results according to the 2013 guidelines in cohorts 1 and 2, respectively. As per the 2018 guidelines, final HER2 statuses of 16 (1.4%) and 14 (8.5%) cases changed in cohorts 1 and 2, respectively. The 2013 guidelines increased the positive rate compared to the 2007 guidelines, in both cohorts (0.6% and 6.2%, respectively). Most equivocal cases in cohorts 1 (92.3%) and 2 (100%) as per the 2013 guidelines were reclassified as HER2-negative according to the 2018 guidelines. The 2018 guidelines increased the negative rates (1.3% in cohort 1 and 8.7% in cohort 2) and slightly decreased the positive rates (−0.2% in cohort 1 and −3.1% in cohort 2), compared to the 2013 guidelines. With each update, minor changes in the positive and negative rates were observed in whole breast cancer samples (cohort 1). However, the 2018 guidelines affected previously defined HER2-positive IBC with equivocal IHC results.

Conclusion

Under the 2013 guidelines, the positive and equivocal cases increased. However, the 2018 guidelines eliminated ambiguous cases by reclassifying them as HER2-negative.

The impact of 2018 ASCO-CAP HER2 testing guidelines on breast cancer HER2 results. An audit of 2132 consecutive cases evaluated by immunohistochemistry and in situ hybridization

The 2018 iteration of the ASCO-CAP HER2 testing guidelines proposes significant changes with an emphasis on the integration of concurrent immunohistochemistry (IHC) and in situ hybridization (ISH). We wished to evaluate the impact of these changes on clinical practice. Between Jan 2012 to Feb 2017, 2132 consecutive invasive breast carcinomas were evaluated with IHC and ISH for HER2. The sample tested was the breast primary or axillary nodes in all but 57 (2.7%) distant metastases. For 1824 cases with both dual-probe ISH and IHC results, the ISH subgroup was 1: 299 (16.4%), 2: 19 (1.0%), 1.0%, 3: 6 (0.3%), 4: 48 (2.6%) and 5: 1452 (79.6%). Ultimately 21% of group 2 and 4 cases and 80% of group 4 cases were positive. The change in HER2 status between the 2018 vs 2013 was: amplified in 323 (15.2%) vs 15.5%; not amplified in 1804 (84.6%) vs 82.2%; equivocal in 0 vs 2.3% previously. In 22 of 2127 cases (1.03%) the 2013 and 2018 results were discordant, all in groups 2-4. The discrepant cases included 15 of 331 (4.5%) of 2013 amplified cancers, now negative (all in groups 2 or 3) and 7 of 1796 (0.4%) 2013 nonamplified cases, now positive (all in group 4). Because of routine testing with both IHC and ISH, we found 6 of 1147 (0.52%) IHC negative (0 or 1+) cases were amplified by ISH. Further, 19 of 289 (6.6%) of IHC 3+ cases were nonamplified by ISH, circumstances not covered by these guidelines. In summary at the population level, the 2018 ASCO-CAP guidelines have a 99% agreement with the 2013 results. A major advantage is the abolishment of the clinically problematic equivocal category. Routine performance of both IHC and ISH uncovers a small proportion of cancers whose HER2 status is not addressed by these guidelines.

Association between HER2 and IL-6 genes polymorphisms and clinicopathological characteristics of breast cancer: significant role of genetic variability in specific breast cancer subtype

Clinical implications of single nucleotide polymorphisms (SNPs) in breast cancer have been explored to determine the impact of SNP in modulating the pathogenesis of breast cancer. This study aimed to evaluate the association between HER2 (rs2517956) and (IL-6) (rs1800795 and rs2069837) and clinicopathological characteristics in HER2-positive and HER2-negative breast cancer in Tunisian women. A retrospective cohort study included 273 patients. Genomic DNA was extracted from peripheral blood samples, and genotyping of selected SNP was performed by PCR-RFLP assays. Statistical analysis was then carried out to assess genotypic frequencies and genetic association in relation to breast cancer subtypes. SHEsis software was applied to IL-6 haplotypic structure analysis. The distribution of genotype frequencies of rs2517956, rs1800795 and rs2069837 showed no statistically difference between HER2-positive and HER2-negative breast cancer. HER2 (rs2517956) was associated with tumor size (p = 0.01) and age at diagnosis (p = 0.02) in HER2-negative breast cancers, but no significant association was observed in HER2-positive breast cancer. For IL-6 gene, none of the clinicopathological parameters were associated with rs1800795 and rs2069837 in both breast cancer subtypes (p > 0.05). SHEsis analysis revealed a high linkage disequilibrium between rs1800795 and rs2069837; differences in the distribution of IL-6 two loci haplotypes were statistically negative between HER2-positive and HER2-negative breast cancer (p = 0.20) which confirmed no association with HER2 overexpression. This study demonstrates that rs2517956 is associated with clinicopathological characteristics in HER2-negative breast cancer, which could have a differential prognostic role compared to HER2-positive breast cancer.

The updated 2018 American Society of Clinical Oncology/College of American Pathologists guideline on human epidermal growth factor receptor 2 interpretation in breast cancer: comparison with previous guidelines and clinical significance of the proposed in situ hybridization groups

The aim of the study was to evaluate the impact of the updated 2018 American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guideline on human epidermal growth factor receptor 2 (HER2) interpretation in breast cancer compared with that of the previous guidelines and also the significance of in situ hybridization (ISH) groups proposed by the updated guideline. HER2 ISH reports and immunohistochemistry (IHC) data from 1,348 invasive breast cancers diagnosed at a single institution were included in this study. HER2 IHC was reassessed using the 2018 guideline, and HER2 ISH status was determined by the 2007, 2013, and 2018 guidelines. When applying the updated guideline, most of the HER2 ISH-equivocal cases as per the previous guidelines were reclassified as ISH negative, and 0.8% of HER2 ISH-positive tumors as per the 2007 guideline and 2.5% of those as per the 2013 guideline were changed to ISH negative. Accordingly, the negative HER2 ISH results significantly increased in the 2018 guideline compared with the 2013 guideline. HER2 ISH-positive tumors in ISH group 3 (HER2/chromosome enumeration probe 17 [CEP17] ratio <2.0 and average HER2 copy number ≥6.0 per cell) were characterized by equivocal HER2 protein expression, CEP17 copy number gain, and low HER2 copy numbers compared with classic HER2 ISH-positive tumors in ISH group 1 (HER2/CEP17 ratio ≥2.0 and average HER2 copy number ≥4.0 per cell). HER2 ISH-negative tumors in ISH group 4 (HER2/CEP17 ratio <2.0 with average HER2 copy number ≥4.0 and < 6.0 per cell) revealed more aggressive clinicopathologic features and poorer clinical outcomes than those in ISH group 5 (HER2/CEP17 ratio <2.0 and average HER2 copy number <4.0 per cell), especially in the hormone receptor-positive subgroup. In conclusion, implementation of the updated 2018 ASCO/CAP guideline leads to a significant increase in HER2 ISH-negative results compared with the 2013 guideline, mainly via reclassification of the ISH-equivocal cases to ISH-negative ones. ISH groups proposed by the updated guideline provide additional information on the clinicopathologic characteristics of the tumors.