Abstract BACKGROUND Molecular parameters have been incorporated into the 2016 WHO Classification of CNS Tumors and subsequent cIMPACT-NOW updates to facilitate clinical management of glioma patients. However, there have been few reports of the overall clinical utility of comprehensive genetic testing in adult glioma patients. We report the results of sequencing and chromosomal microarray analysis of adult gliomas seen at the Mayo Clinic and through the Mayo Clinic Laboratories (MCL) reference practice. METHODS A consecutive series of 379 Mayo Clinic adult glioma patients were consented to receive targeted next generation sequencing and chromosomal microarray analysis, regardless of standard clinical ordering practices. Known diagnostic, prognostic, and predictive alterations were annotated for each case. These results were compared to the larger MCL reference practice of 2400 adult glioma samples that received one or both tests clinically. RESULTS Of the consecutive Mayo Clinic cases, 67% had alterations that fell into at least one of the diagnostic, prognostic, and/or predictive categories. Molecular testing generated diagnostic, prognostic, and predictive information in 44%, 34%, and 36% of cases, respectively. Diagnostically, molecular testing mainly aided in arriving at a final integrated diagnosis, and only a small number of cases (n= 5; 1%) had a change in diagnosis. The consecutive cases represent the typical distribution of adult glioma types (47% IDH-wildtype glioblastoma/astrocytoma, 22% IDH-mutant astrocytoma/glioblastoma, 13% IDH-mutant 1p/19q-codeleted oligodendroglioma, and 18% other tumor types). The MCL practice is enriched in tumors with atypical molecular patterns (37% IDH-wildtype glioblastoma/astrocytoma, 17% IDH-mutant astrocytoma/glioblastoma, 18% IDH-mutant 1p/19q-codeleted oligodendroglioma, and 29% other tumor types). CONCLUSIONS Molecular testing provides useful diagnostic, prognostic, and predictive information for the clinical management of adult glioma patients. While the current guidelines for ordering molecular testing are effective for diagnostic purposes, additional prognostic and predictive information can be gleaned from comprehensive molecular testing of adult gliomas.
LGG-36. DESMOPLASTIC INFANTILE GANGLIOGLIOMA (DIG) WITH A PPP1CB-ALK FUSION IN A 6-YEAR-OLD GIRL
Abstract Desmoplastic infantile astrocytoma (DIA) and desmoplastic infantile ganglioglioma (DIG) are benign glioneuronal tumors that typically occur in infants, involve the superficial cerebral cortex, and have an excellent prognosis. DIA/DIG are a distinct molecular entity based on DNA methylation profiling. BRAF600 mutations are frequently reported in DIG/DIA. A recent comprehensive genetic analysis of infantile hemispheric gliomas identified 2 unique groups: group 1 harbored alterations in the receptor tyrosine kinase (RTK) genes ALK, ROS1, NTRK, and MET and group 2 harbored alterations in the RAS/MAPK pathway. We report a case of a 6.5-year-old girl who presented with seizures and right homonymous hemianopia. MRI of her brain demonstrated a large cystic/solid left hemispheric mass with remodelling of the overlying skull, consistent with a long-standing process. She underwent a gross total resection (GTR) and pathology demonstrated a DIG with a PPP1CB-ALK gene fusion (exon 5 to exon 20) identified by RNA sequencing. She remains disease free 12 months following GTR. A literature review identified 4 reported cases of pediatric brain tumors with PPP1CB-ALK gene fusions including: a 3-month-old with a hemispheric high-grade glioma which recurred 4 years later and pathology showed mature ganglioglioma, with both tumors showing the identical PPP1CB-ALK gene fusion; a 10-month-old infant with a hemispheric low-grade glioma; an infant with a “congenital” hemispheric high-grade glioma; and a child with an astrocytoma with no further clinical data. PPP1CB-ALK gene fusion appears to be a rare oncogenic driver in gliomas of infancy, including DIG.
PATH-13. PLEOMORPHIC XANTHOASTROCYTOMA INTEGRATED GENOMIC CHARACTERIZATION - WHAT HAVE WE LEARNED?
Abstract Pleomorphic xanthoastrocytoma (PXA) is a rare astrocytoma occurring predominantly in children and young adults. It is characterized histologically by large pleomorphic, spindled and lipidized cells with frequent eosinophilic granular bodies and pericellular reticulin deposition. BRAF p.V600E mutation and CKDN2A/B deletion are the most common genetic alterations. We report the integrated genomic characterization of a cohort of 67 patients (37 F, 30 M; median age 20.3 years (interquartile 13.4–32.9) with histologically defined PXA (52, 78%) or anaplastic PXA (A-PXA) (15, 22%), using genome-wide cytogenetic (ThermoFisher Oncoscan, n=67), methylation profiling (Illumina EPIC array, n=43), and targeted next generation sequencing (n=32). BRAF p.V600E mutation (n=51, 76.1%) and CDKN2A/B deletion (n=63; 94%) were the most frequent alterations. Of 16 BRAF p.V600E negative cases, 7 showed an alternative BRAF activating mutation (n=2), NF1 (n=3) mutation or ATG7-RAF1 fusion (n=2). Targeted TERT analysis found promoter mutations in 3 (of 58) cases, but TERT amplification was absent. Supervised and unsupervised methylation profiling against a comprehensive reference cohort demonstrated consensus grouping with the PXA class in 36 of 43 cases; while the minority grouped with a ganglioglioma class (n=3), with reactive brain or had no resolvable subgroup (n=4). Follow-up was available in 61 patients (91.0%) (median 63 months). Overall survival was significantly different between PXA and A-PXA (5-year:80.4% vs. 55.1%; p=0.001), but not progression-free survival (5-year:61.7% vs. 39.8%; p=0.128). Our data confirm the high frequency of MAP-K abnormalities and CDKN2A/B deletion in PXA. WHO grade remains a strong predictor of patient overall survival.
CTNI-29. CODEL: PHASE III TRIAL OF RT ALONE, RT PLUS TMZ, OR TMZ ALONE FOR NEWLY-DIAGNOSED, 1p/19q CODELETED ANAPLASTIC OLIGODENDROGLIOMA. ANALYSIS FROM THE INITIAL STUDY DESIGN. (NCCTG N0577, ALLIANCE)
Abstract BACKGROUND The original 3-arm CODEL design included a radiotherapy (RT)-alone control arm, an RT plus temozolomide (TMZ) arm, and an exploratory TMZ-alone arm. We report the analysis involving patients treated per the initial design. METHODS Adults (18+ years) with newly-diagnosed 1p/19q codeleted WHO grade III oligodendroglioma were randomized to RT (5940 cGy) alone (Arm A); RT with concomitant and adjuvant TMZ (Arm B); or TMZ alone (Arm C). Primary endpoint was OS, Arm A vs. B. Secondary comparisons were performed for OS and PFS, comparing pooled RT arms with the TMZ-alone arm. RESULTS 36 patients were randomized equally to the three arms. At median follow-up of 7.5 years, 83.3% (10/12) TMZ-alone patients had progressed, versus 37.5% (9/24) patients on the RT arms. PFS was shorter in TMZ-alone patients compared to RT-treated patients (HR=3.12; 95% CI: 1.26, 7.69; p=0.014). Death from disease progression occurred in 3/12 (25%) of TMZ-alone patients and 4/24 (16.7%) of RT-treated patients. OS did not statistically differ between arms, although this comparison was underpowered. After adjustment for IDH status (mutated vs. wildtype) in a Cox regression model, with IDH status and RT treatment status as co-variables (Arm C vs pooled A and B), PFS remained shorter for patients not receiving RT (HR= 3.33; 95% CI: 1.31, 8.45; p=0.011), and OS differences remained non-significant ((HR = 2.78; 95% CI 0.58, 13.22, p=0.20). Grade 3+ adverse events occurred in 25%, 42% and 33% patients (Arms A, B and C, respectively). Neurocognitive assessments, comparing baseline and 3 month timepoints, showed no significant differences between arms. CONCLUSIONS TMZ-alone treated patients experienced significantly shorter PFS than patients treated on the pooled RT arms, which remained significant when adjusting for IDH status. CODEL has been redesigned to compare the efficacy and toxicity of RT+PCV versus RT+TMZ. Clinicaltrials.gov Identifier: NCT00887146. Support: U10CA180821, U10CA180882, https://acknowledgments.alliancefound.org.
Abstract PO-245: Evaluating educational interventions to increase breast density awareness among Latinas: A randomized clinical trial in a Federally Qualified Health Center
Abstract Purpose: Among a Latina population in a low resource healthcare setting, to test the effectiveness of bilingual (Spanish/English) written and interpersonal educational interventions in increasing breast density (BD) awareness, knowledge, and conversations with healthcare providers, which are goals of BD notification mandates. Procedures: This randomized trial was conducted at a Federally Qualified Health Center (FQHC) in an urban community of the Southwest U.S. Women ages 40- 74 presenting for screening mammography were recruited and randomized 1:1:1 to usual care (mailed letter with mammogram and BD results), enhanced care (letter plus BD educational brochure), or interpersonal care (letter, brochure, and telephonic promotora education). The baseline survey was administered in person at enrollment prior to the mammogram. Follow-up surveys were by phone or mail after the mammography results letter was mailed and promotora delivery was complete (median days from baseline to follow-up survey = 97 days). Differences between groups at baseline or follow-up were assessed with chi-square tests. Logistic regression using generalized estimating equations to account for repeated data was used to assess differences by time (baseline to follow-up) within group, including a time-by-group interaction. Results: Between October 2016 and October 2019, 1377 women were enrolled. Of these, 1085 (79%) self-identified Latinas completed baseline and follow-up surveys. Median age was 47 years. Most (91%) reported speaking mostly Spanish, and 69.1% had less than a high school degree. At baseline, only 20.9% reported having heard of BD, and only 30.5% of those knew their personal BD category. At follow-up, women in the interpersonal group were more likely to report seeing their BD results in the notification letter compared to the usual care and enhanced groups (70.4% vs. 52.9% and 54.3%, p < 0.001) and to report having spoken with a provider about their BD (27.3% vs. 14.2% and 15.7%, p < 0.001). They also reported higher mean understanding of BD (7.7 on a 0-10 scale vs. 6.3 and 6.6, p < 0.001). There were no significant differences on these outcomes between the usual care and enhanced groups. All groups saw significant improvement in correct responses from baseline to follow-up for questions on the ability of BD to mask tumors on a mammogram and BD as a breast cancer risk factor (p < 0.001 all groups), but the degree of improvement in masking knowledge was significantly higher for the interpersonal vs usual care group [OR for a correct response in follow-up vs baseline: 6.4 (interpersonal) vs 3.7 (usual care), p=0.01]. Conclusion: Among a Latina population in a FQHC, telephonic promotora education was more effective than written notification alone or a mailed brochure on key goals of notification mandates. Adding a brochure does not appear to be more effective than sending a letter alone on key outcomes, but the interpersonal nature of promotora education may prompt attention to mailed information and provide an opportunity for questions. Citation Format: Jennifer L. Ridgeway, Sarah M. Jenkins, Deborah J. Rhodes, Bhavika K. Patel, Edna Ramos, Bijan Borah, Karthik Ghosh, Vera J. Suman, Aaron Norman, Davinder Singh, Celine M. Vachon. Evaluating educational interventions to increase breast density awareness among Latinas: A randomized clinical trial in a Federally Qualified Health Center [abstract]. In: Proceedings of the AACR Virtual Conference: Thirteenth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2020 Oct 2-4. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(12 Suppl):Abstract nr PO-245.
Abstract PO-238: Breast density knowledge and awareness among Latinas in a low-resource setting: A comparison nationally-representative sample of Latinas
Abstract Objective: Breast density notification laws aim to increase women’s awareness and knowledge about breast density (BD). This study’s aim was to compare BD knowledge and awareness in two Hispanic populations: a safety-net clinic serving Latina population in Maricopa County, Arizona (AZ) and a national sample of Hispanic women participating in an online research panel. Materials/Methods: Women ages 40-74 were recruited at the AZ clinic from 2016-2019 at a screening mammography appointment as part of a larger clinical trial (AZ cohort). Surveys were completed in person (English or Spanish) at enrollment to assess awareness of BD, understanding of BD risk, and mammography history. The same questions were asked in a nationally representative online panel survey conducted in 2017. All data from the national survey were weighted to adjust for probability of selection into the panel and post-stratified to match known U.S. Hispanic population distributions (NS cohort). Univariate comparisons between the clinic and the national panel were performed using Rao-Scott chi-square tests. Associations with awareness and correct knowledge were examined with multivariable logistic regression. All summaries and analyses are weighted, and were performed with SAS version 9.4 SURVEY procedures (SAS Institute Inc., Cary, NC). Results: The analysis included 1332 Hispanic women from the AZ clinic and 152 Hispanic women who completed the national survey. The AZ cohort was younger (mean age: 48.5 vs 52.8), had less education (less than high school: 68.7% vs 34.8% less than high school), more likely to prefer Spanish (92.3% vs 53.1%) and less likely to have had a prior mammogram as compared NS cohort (82.8% vs 90.4%) (all p≤ 0.03). NS cohort was more likely to have awareness of BD (32.6% vs 20.7%); correct understanding of the masking effect of BD, (67.8% vs 37.0%,) and breast cancer risk (72.2% vs 32.6%,), compared to the AZ cohort (all p < 0.004). When adjusted for differences in education, age, language, and screening mammography history, BD awareness was similar between the two cohorts (adjusted odds ratio [ORadj] 0.95, p=0.83). Awareness was positively associated with more education, previous mammogram history, and English language. However, differences in BD knowledge remained (ORadj=2.8 [p=0.03] and 7.2 [P<0.001] for knowledge of masking and risk, respectively). Conclusions: There were significant differences in BD awareness and knowledge between a nationally-representative sample of Latinas and Latinas in a low-resource setting. The differences in BD awareness, but not knowledge, could be explained by education, screening history and preferred language. These findings suggest that a one-size-fits-all approach to raising BD awareness and knowledge will not work. To ensure equitable dissemination of health care to Latina women undergoing screening mammography, future work should be aimed at developing effective tools and resources for educating Spanish-speaking women about BD and risk-based screening. Citation Format: Jillian Leaver, Bhavika K. Patel, Jennifer L. Ridgeway, Sarah Jenkins, Vera Suman, Deborah J. Rhodes, Aaron Norman, Karthik Ghosh, Bijan Borah, Carrie Hruska, Edna Ramos, Davinder Singh, Matt Jewett, Carmen Radecki-Breitkof, Celine Vachon. Breast density knowledge and awareness among Latinas in a low-resource setting: A comparison nationally-representative sample of Latinas [abstract]. In: Proceedings of the AACR Virtual Conference: Thirteenth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2020 Oct 2-4. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(12 Suppl):Abstract nr PO-238.
Abstract PR09: Postmenopause as a key factor in the composition of the Endometrial Cancer Microbiome (ECbiome): Putative role of Porphyromonas somerae in the disease
Abstract Incidence rates for endometrial cancer (EC) are rising, particularly in postmenopausal and obese women. Our previous work has showed that the uterine and vaginal microbiome distinguishes patients with EC from those without. We now examine the impact of patient factors (such as menopause status, body mass index, and vaginal pH) in the microbiome in the absence of EC and how these might contribute to the microbiome signature in EC. We find that each factor independently alters the microbiome and identified postmenopausal status as the main driver of a polymicrobial network associated with EC (ECbiome). Of the 17 taxa we found enriched in EC patients, 8 were also enriched by postmenopause. Because postmenopausal status is a main risk factor for endometrial cancer, this system can be thought of as an ecological succession towards a disease state. Organisms within the ECbiome have been previously identified as polymicrobial associations in chronic soft tissue infections, ulcers, morbid obesity, and bacterial vaginosis. This suggests that this microbiome network is likely involved in mechanisms of host disease. Within the ECbiome, Porphyromonas somerae stands out because of its very significant and specific association with EC (p = 5.73E-6; Q = 0.009). We had previously identified this species as significantly associated with EC, and hereby validate that finding. Because the association of P. somerae with EC was robust and specific enough to potentially be considered as a biomarker for the disease (AUC 76.7%, CI: 67.9-85.5%), we developed a P. somerae-specific qPCR assay to detect this species in vaginal swabs. This becomes of particular interest because the lower tract and uterine microbiome are significantly correlated in these patients (p = 0.01), indicating that the vaginal microbiome can be used as a proxy for the uterine environment in EC. In agreement with the 16S rRNA gene sequencing data, P. somerae was detected significantly more frequently in vaginal swabs of patients with EC than those without EC (p = 3.02E-7). Overall, the test achieved a sensitivity of 74% and specificity of 63%, with a positive predictive value of 0.86 for obese and postmenopausal patients. In summary, we identify Porphyromas somerae presence as the most predictive microbial marker of EC, and we confirm this using targeted qPCR, which could be of use in detecting EC in high-risk, asymptomatic women. Given the established pathogenic behavior of P.somerae, we have also explored mechanistic routes through which this microbe and associated consortia may be involved in the mechanisms of disease. Citation Format: Marina Walther-Antonio, Dana Walsh, Alexis Hokenstad, Jun Chen, Jayeun Sung, Gregory Jenkins, Nicholas Chia, Heidi Nelson, Andrea Mariani. Postmenopause as a key factor in the composition of the Endometrial Cancer Microbiome (ECbiome): Putative role of Porphyromonas somerae in the disease [abstract]. In: Proceedings of the AACR Special Conference on the Microbiome, Viruses, and Cancer; 2020 Feb 21-24; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2020;80(8 Suppl):Abstract nr PR09.
COVID-19–Associated Nonocclusive Fibrin Microthrombi in the Heart
Background: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its resultant clinical presentation, coronavirus disease 2019 (COVID-19), is an emergent cause of mortality worldwide. Cardiac complications secondary to this infection are common; however, the underlying mechanisms of such remain unclear. A detailed cardiac evaluation of a series of individuals with COVID-19 undergoing postmortem evaluation is provided, with 4 aims: (1) describe the pathological spectrum of the myocardium; (2) compare with an alternate viral illness; (3) investigate angiotensin-converting enzyme 2 expression; and (4) provide the first description of the cardiac findings in patients with cleared infection. Methods: Study cases were identified from institutional files and included COVID-19 (n=15: 12 active, 3 cleared), influenza A/B (n=6), and nonvirally mediated deaths (n=6). Salient information was abstracted from the medical record. Light microscopic findings were recorded. An angiotensin-converting enzyme 2 immunohistochemical H-score was compared across cases. Viral detection encompassed SARS-CoV-2 immunohistochemistry, ultrastructural examination, and droplet digital polymerase chain reaction. Results: Male sex was more common in the COVID-19 group ( P =0.05). Nonocclusive fibrin microthrombi (without ischemic injury) were identified in 16 cases (12 COVID-19, 2 influenza, and 2 controls) and were more common in the active COVID-19 cohort ( P =0.006). Four active COVID-19 cases showed focal myocarditis, whereas 1 case of cleared COVID-19 showed extensive disease. Arteriolar angiotensin-converting enzyme 2 endothelial expression was lower in COVID-19 cases than in controls ( P =0.004). Angiotensin-converting enzyme 2 myocardial expression did not differ by disease category, sex, age, or number of patient comorbidities ( P =0.69, P =1.00, P =0.46, P =0.65, respectively). SARS-CoV-2 immunohistochemistry showed nonspecific staining, whereas ultrastructural examination and droplet digital polymerase chain reaction were negative for viral presence. Four patients (26.7%) with COVID-19 had underlying cardiac amyloidosis. Cases with cleared infection had variable presentations. Conclusions: This detailed histopathologic, immunohistochemical, ultrastructural, and molecular cardiac series showed no definitive evidence of direct myocardial infection. COVID-19 cases frequently have cardiac fibrin microthrombi, without universal acute ischemic injury. Moreover, myocarditis is present in 33.3% of patients with active and cleared COVID-19 but is usually limited in extent. Histological features of resolved infection are variable. Cardiac amyloidosis may be an additional risk factor for severe disease.
Clinical Value of Next Generation Sequencing in the Detection of Recurring Structural Rearrangements and Copy Number Abnormalities in Acute Myeloid Leukemia
Purpose: Acute myeloid leukemia (AML) is the most common acute leukemia in adults, affecting approximately 20,000 patients annually in the United States. AML genetic subtypes, as defined by the World Health Organization (WHO), are identified through fluorescence in situ hybridization (FISH), conventional chromosome analysis, and sequencing techniques. Mate pair sequencing (MPseq) is a next generation sequencing (NGS) technology optimized to detect genome wide structural variants and copy number alterations at high resolution. Our study goal was to investigate the prognostic value of MPseq in comparison to FISH, chromosome, and sequencing studies in the evaluation of AML patients. Methods: We performed a prospective study using blood and bone marrow samples from 105 patients with a diagnosis of AML, using MPseq, along with chromosome, FISH, and NGS or PCR studies to detect small mutations. Cytogenetic and molecular genetic results were correlated with MPseq findings. We also analyzed the MPseq data for chromoplexy, chromothripsis, and progressive complexity. Junction and copy number burden, the incidence of structural variation in the genome and the percent of the genome with aberrant copy number, were evaluated. Overall survival statistics were stratified by AML subtypes and observed anomalies. Results: Although structural variants in AML were characterized at a high resolution using MPseq when compared to conventional cytogenetic methods, risk stratification using current European Leukemia Net (ELN) guidelines was not improved by MPseq. The cohorts involving 5q and/or 7q deletions exhibited high levels of genomic complexity when compared to normal karyotype AML (NK-AML). The incidence of copy number gains, losses and junctions was greatest in 5q and 7q co-deletions (5q/7q) (16.5, 25.0, 69.3) and 5q deletions (5q) (9.8, 16.7, 31.6) subtypes compared to 7q deletions (3.4, 7.0, 6.7) and NK-AML (2.6, 4.3, 3.8) (p<0.001) subtypes. Chromoplexy, chromothripsis, and progressive structural complexity were detected in most samples with 5q deletions and 5q/7q co-deletions, but absent in samples with 7q or NK-AML. Biallelic inactivation of TP53 by sequencing mutation and/or deletion was common in the 5q/7q co-deletion (14/18 cases) and 5q deletion cohorts (7/10 cases), rare in the 7q deletion cohort (1/11 cases), and absent in the NK-AML (n=44) cohort. The median OS was significantly worse for patients with 5q/7q deletions (122.5 days) and 5q deletions (248 days) compared to NK-AML (413.5 days; p<0.001 and p=0.017, respectively) and between 5q/7q deletions and 7q deletions (370.5 days; p<0.001). No significant difference was observed between 5q/7q and 5q deletion subtypes, between NK and 7q deletion subtypes and between 5q and 7q deletion subtypes. The median OS was also significantly shorter for patients with TP53 alterations compared to patients with normal TP53 status. Patients with chromoplexy, chromothripsis and/or progressive structural complexity identified by MPseq had a significantly shorter median OS compared to patients without these features. (p<0.0001) Discussion: Risk stratifications based on current guidelines using cytogenetic and sequencing results were not adjusted due to MPseq results, which is not surprising when primary abnormalities are observed by conventional cytogenetic methods. NK-AML cases did not appear to benefit from a high resolution genomic evaluation. However, MPseq added value when structural variation required additional characterization, detecting novel rearrangements, such as a KAT6A/SORBS3 fusion. Lastly, we recognized common mischaracterizations made by conventional chromosome studies - including missed TP53 deletions in 7 cases, 5q/7q deletions misinterpreted as monosomies, cryptic NUP98 rearrangements, and unappreciated genomic complexity correlating with poor OS. These mischaracterizations challenge the use of conventional chromosome studies as a gold standard without accompanying FISH or MPseq studies. MPseq, similar to other structural methodologies such as optical mapping and long read sequencing, should be considered important complements to standard cytologic techniques given the important additional genomic information obtained. The additional structural variant characterization will be critical in paving the way for genomic discovery with the overall goal of improving prognostication for patient care. Figure Disclosures Vasmatzis: WholeGenome LLC.: Other: Owner; Mayo Clinic: Membership on an entity's Board of Directors or advisory committees, Research Funding.
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.