1022 Background: Trastuzumab deruxtecan is approved in HER2-low (IHC 1+ or 2+/ISH negative) or -ultralow (IHC 0 with membrane staining in ≤10% of tumor cells) metastatic BC per DESTINY-Breast04 and -06 trials. Manual HER2 IHC scoring can be time-consuming and subjective. This retrospective, real-world study assessed manual scoring (ground truth) vs standalone AI-computational pathology-assisted (CPa) tools for scoring. Methods: Whole slide images (WSIs; N = 600, scanned with Aperio AT2) of BC samples stained with PATHWAY HER2 (4B5) assay, originally scored as HER2 IHC 0 (n = 400) or 1+ (n = 200), were rescored by 3 pathologists and using 4 CPa/AI tools in development as either IHC 0 absent membrane staining, 0 with membrane staining in ≤10% of cells, 1+, or 2+. Using 2023 ASCO/CAP guidelines, each pathologist performed 2 blinded readings per WSI; a reconciled score was used if the 2 readings differed. Manual consensus required ≥2/3 agreement. Concordance between pathologist manual consensus vs standalone CPa scoring was measured by positive and negative percentage agreement (PPA; NPA), overall percentage agreement (OPA), and Cohen κ, with review time recorded. Results: Of 600 WSIs, 586 (97.7%) had manual consensus. CPa tools were faster (Table) than manual scoring (manual median review time: 7.0 min; range, 3.0-11.5). PPA was high (≥92.5%) between manual consensus scoring and CPa tools; NPA across the tools was 79.1%, 68.9%, 67.4%, and 23.1%. Overall concordance varied across CPa tools; OPA ranged from 48.5% to 75.4% and Cohen κ from 0.26 to 0.61. Conclusions: Integrating CPa/AI tools as decision support aids for pathologists may reduce pathologist review time and augment pathologist inter-observer reproducibility, especially in HER2-ultralow identification. Refinement of CPa/AI algorithms in development may improve scoring to an even greater extent. DP tool (N = 586 a ) Review time, median (range), min PPA, b,c % (95% CI) NPA, b,d % (95% CI) OPA, b,e % (95% CI) Cohen κ e (95% CI) RV73X 2.7 (0.3-56.8) 94.9 (92.5-96.7) 79.1 (70.6-85.7) 75.4 (71.7-78.9) 0.61 (0.56-0.66) MQ52G 0.7 (0.1-8.5) 93.4 (90.9-95.4) 68.9 (60.5-76.2) 73.0 (69.3-76.6) 0.57 (0.52-0.63) KL84Q 3.1 (0.5-84.4) 92.5 (89.8-94.6) 67.4 (59.1-74.8) 69.5 (65.6-73.2) 0.54 (0.48-0.59) ZX19P 2.5 f (Not available) 99.1 (97.6-99.6) 23.1 (16.4-31.5) 48.5 (44.4-52.6) 0.26 (0.22-0.31) a Primary and metastatic samples from biopsies, effusions, fine needle aspirations, and surgical resection; due to the inbuilt pre-QC module, a few WSI outputs were not processed by RV73X and ZX19P. b Rounded to 1 decimal. c Agreement for consensus-positive cases (IHC 0 with membrane staining, 1+, or 2+). d Agreement for consensus-negative cases (IHC 0 absent membrane staining). e Agreement across all IHC scores. f Mean review time; median not available.

This study aims to describe the pathologic features and biomarker profiles of a large microinvasive breast carcinoma (MiBC) cohort diagnosed on biopsy (CNB) and compare these findings with corresponding tumors on excision. Out of 263 MiBC, approximately half of the DCIS cases were classified as high-grade. On CNB, ER, PR, and HER2 were positive in 166/226 (73%), 124/225 (55%), and 48/174 (28%) of the tested cases, respectively. Excision specimens from 132 cases revealed invasive carcinoma in 52/132 (39%), MiBC in 31/132 (23%), DCIS only in 35/132 (27%), LCIS only in 4/132 (3%), and benign findings in 10/132 (8%). The concordance rates between CNB and excision were initially 100% for ER (40/40), 95% for PR (38/40), and 90% for HER2 (26/29). While routine retesting of ER/PR may not be necessary in cases of MiBC on CNB, selective repeat HER2 testing should be considered when larger tumors are present on excision.

The 6th edition of the WHO Classification of Breast Tumours introduces both major and minor changes based on recent advances in our understanding of breast biology, developments in diagnostic modalities, identification of specific molecular targets and new treatment regimens necessitating modifications to pathology reporting and tumour biomarker categorisation. This review summarises the main changes that strive towards a classification of global relevance. In invasive carcinoma, predictive factors increasingly inform modern breast cancer treatment. The 6th edition provides an update on HER2 reporting categories following the DESTINY-Breast 04 and 06 trials. Terminologies used to classify invasive tumours are clarified, with the term 'variant' now reserved for molecular/genetic alterations. Invasive lobular carcinoma (ILC) with extracellular mucin is recognised as a new diagnostic entity with prognostic implications. The diagnosis of mucinous carcinoma (MC) is reserved for mucin secreting carcinomas with grade 1 or 2 morphology and a favourable biomarker profile. The diagnosis of malignant phyllodes tumours requires only four of the original five adverse histological criteria. Classification of neuroendocrine tumours (NETs) is revised, recognising that the unified model, promoted in the 5th edition, is difficult to apply to the breast. New approaches to the classification of adenomyoepithelioma are discussed but the 5th edition system is broadly retained. A new section on 'Small Diagnostic Samples' outlines the merits of non-operative biopsy diagnosis, the B coding system and the importance of multidisciplinary review. Changes to diagnostic practice and the emerging role of artificial intelligence, with advantages and challenges, are discussed in a new section on 'Digital Pathology'.

BACKGROUND:The Destiny B04 trial led to the recognition of HER2 low as a new entity defined as HER2 immunohistochemistry (IHC) score of 1 +/2 + and negative in situ hybridization (ISH) requiring detailed HER2 IHC scoring (negative = 0 & 1 +, equivocal = 2 +, and positive = 3 +). As per ASCO-CAP guidelines, biomarkers need not be repeated on excisions when done on core biopsy with some exceptions. The goal of our study was to compare the concordance of HER2 low between core biopsies and excisions and assess the need to repeat on excision. METHODS:At the study institution biomarkers are first performed on core biopsies and then repeated on all excisions in negative cases. We identified 301 cases of primary breast carcinomas with matched HER2 IHC on core biopsies and excisions. We reviewed and scored all HER2 IHC slides as per 2018 ASCO/CAP guidelines. RESULTS:The incidence of HER2 low on core biopsies decreased from 64% to 46% on excisions. The best concordance was seen in HER2 + (82%) and negative cases (84%), with most variability in predominantly 1 + and some 2 + cases in both directions. There was a greater loss (38%) than gain (16%) in HER2 low values from core biopsy to excision. CONCLUSIONS:HER2 low discordance in our series was predominantly due to loss from core biopsies to excisions, which maybe attributed to better meeting the pre analytic criteria on core biopsy. In contrast, the gain maybe due to intratumoral heterogeneity and or interobserver variability and despite low, worth repeating HER2 IHC on excisions in negative cases.

The extent of residual disease after neoadjuvant chemotherapy (NAC) in patients with breast cancer (BC) holds prognostic value. However, current practices for reporting post-NAC BC specimens according to the ypTNM classification vary. This study aimed to map these practices and provide recommendations for standardization. A survey was developed and globally circulated to pathologists with a special interest in BC through personal networks and working group mailing lists. The survey included general questions about tumor diameter assessment, as well as graphical scenarios presenting different distributions of tumor cells. We did not provide definitions mentioned in reporting guidelines to capture unbiased current real-world practices. A total of 208 pathologists from 35 countries completed the survey. Almost all responding pathologists (97.1%) reported the ypTNM in daily practice. Despite self-reported strict adherence to the eighth edition of the international ypTNM classification, we found substantial variation in practice concerning the application of this staging system, particularly in cases with an uneven distribution of scattered residual disease. Notably, 57.2% of respondents reported measuring the largest "continuous cluster of tumor cells," but the interpretation of this definition varied widely. This international survey identifies the challenges and practice heterogeneity in the current application of the ypTNM staging system, which hampers the value of ypTNM reporting in daily practice. To enhance reproducibility and to provide more reliable post-NAC risk stratification, we recommend adopting standardized reporting with clearer pattern-based definitions of the ypTNM guidelines, supplemented with the elements of the residual cancer burden system.

Fibroepithelial lesions (FELs) of the breast represent a diverse group of biphasic tumors with varying morphologies and clinical behavior. The classification of FELs is mainly based on a constellation of diagnostic criteria, and intralesional heterogeneity is not uncommon. Therefore, reporting FELs in a core needle biopsy (CNB) with limited tissue material can be challenging as not all the features may be represented for assessment. Differentiating a classic fibroadenoma from a well-sampled phyllodes tumor (PT) is generally straightforward. However, cellular fibroadenoma, morphologically heterogeneous benign PT, and myoid hamartoma can overlap histologically. Accurate grading of PT is also challenging on CNB and carries significant management implications. In this article, we provide an overview and propose a pragmatic approach to reporting FELs on CNB, particularly for lesions with overlapping features. Guidance using the UK/European "B" classification of FELs alongside descriptive reporting of the various lesions, is also presented to aid in management decisions.

Phyllodes tumours (PTs) of the breast present diagnostic challenges due to their complex histological features and potential for malignant behaviour. The World Health Organisation (WHO) classification requires the presence of five adverse histological criteria to categorise PTs as malignant, aiming to avoid overdiagnosis and improve diagnostic consistency. However, emerging evidence suggests that these strict criteria may underdiagnose tumours with metastatic potential and histological features that would otherwise be considered malignant in soft tissue tumours, leading to significant implications for prognosis and treatment. Recent studies have highlighted cases where tumours classified as borderline PT by WHO criteria exhibited metastatic behaviour, emphasising the need to refine the diagnostic framework. Microscopic criteria used to classify PT also vary among reporting pathologists, resulting in suboptimal reproducibility. This review examines the histological parameters utilised in the classification of malignant PT, highlights existing evidence gaps and analyses international breast pathologist survey data to propose a pragmatic diagnostic approach. We recommend redefining malignant PTs to include cases meeting four of the five WHO criteria, supplemented by comprehensive sampling and clinical context. This approach balances the risk of underdiagnosis with the need for standardised, reproducible diagnostic practices. Future collaborative efforts should focus upon developing evidence-based, biologically relevant classification systems and leveraging technological advancements to enhance diagnostic precision. These efforts aim to refine classification, improve prognostic accuracy and optimise patient management strategies.

The concept of "HER2-negative" breast cancer is evolving, with the recognition of HER2-low and HER2-ultralow subsets. These subsets are clinically relevant regarding treatment with the antibody-drug conjugate trastuzumab deruxtecan (T-DXd), which has shown survival benefit in patients with metastatic carcinoma with minimal HER2 protein expression that lack HER2 gene amplification by in situ hybridization. In clinical trials using T-DXd, HER2-low was defined as an immunohistochemistry (IHC) score 1+ or an IHC score 2+ without HER2 gene amplification. HER2-ultralow was defined as faint or barely perceptible, incomplete membrane staining in >0% to ≤10% of tumor cells (IHC score 0+/with membrane staining) and HER2-null as the complete absence of staining (IHC score 0/absent membrane staining). These results now necessitate more detailed evaluation and reporting of traditional "HER2-negative" results to identify patients with metastatic breast cancer who may benefit from T-DXd therapy. Both the US Food and Drug Administration and the European Medicines Agency have extended the regulatory approval of T-DXd to patients with metastatic breast cancer showing HER2-low or HER2-ultralow expressions. Updated clinical management guidelines now, therefore, incorporate the spectrum of HER2 results into treatment selection algorithms in the metastatic setting. To align histopathologic practice with these developments, the College of American Pathologists has issued a new biomarker-reporting template that recommends explicit distinction between IHC 0/absent membrane staining and IHC 0+/with membrane staining. Key concerns among pathologists include assay variability, scoring reproducibility, and quality assurance standards for accurately detecting such low levels of HER2 expression. This manuscript provides expert consensus, evidence-based practical recommendations for identifying and reporting tumors with HER2-low and HER2-ultralow expression. We emphasize standardized testing protocols, validated assays, robust internal and external controls, and focused training for pathologists. A universal structured pathology report is proposed to highlight the accurate distinction between IHC 0 (null), IHC 0+ (ultralow), and HER2-low expressions.

Background In the DESTINY-B04 trial, patients with pretreated HER2 low metastatic breast cancer (defined as immunohistochemistry score of 1+ or 2+ and negative in situ hybridization) had significant survival improvement with Trastuzumab therapy. Methods The goal of our study was to compare the HER2 immunohistochemistry scores of paired primary and metastatic breast cancer, with emphasis on HER2 low criteria and its implications for detailed immunohistochemistry interpretation. Using the pathology database from 2011, we identified 272 cases of primary breast cancers with paired metastases. We reviewed and performed immunohistochemistry concordance between the primary and metastases and calculated the HER2 low incidence. Results Compared to the primary, HER2 immunohistochemistry score in the metastases remained the same in 156/272 cases (57%) and by immunohistochemistry was: 0 (67/114=59%), 1+ (22/52=42%), 2+ (34/67=51%), 3+ (33/39=85%) and HER2 low (85/119=71%). The HER2 score changed from 0 to HER2 low in 46 cases (17%) and vice versa in 30 (11%). Conclusions The concordance rate of HER2 immunohistochemistry scores was 57%, highest in 3+ cases, followed by HER2 low and then HER2 negative. The incidence of HER2 low was higher in the metastases by 6% compared to the primary. HER2 should be tested in all metastases and compared with the primary, due to differences in scores as seen in 28% of our cases, which may have clinical implications in the new HER2 low era.

AIMS:Atypical ductal hyperplasia (ADH) in male breast tissue is a rare condition with limited understanding. We aimed to elucidate the clinicopathological characteristics of ADH in male patients, focusing on its prevalence, presentation, and associated factors. METHODS AND RESULTS:We analysed 40 cases of ADH from 1626 male breast cases encountered between 2013 and 2023. Clinicopathological data were reviewed to identify key features and trends. The mean age of the patients in our cohort was 43 years. ADH was mainly discovered incidentally during the workup for gynecomastia in 85% (34/40) of cases. Only two cases, 5% (2/40), initially presented as a palpable mass; one was pure ADH and the other one an ADH with intraductal papilloma (IDP). Nipple discharge was the initial presentation in 7.5% (3/40) of cases, all of which were associated with IDP. Additionally, 5% (2/40) of cases were identified due to calcifications on imaging. Excision was the initial diagnostic procedure in 77.5% (31/40) of cases, and core needle biopsy (CNB) in 22.5% (9/40). In most patients 70% (28/40) had unilateral disease, while 84.4% (27/32) exhibited multifocal lesions, and 90.6% (29/32) showed cribriform architectural patterns. Notably, 77.3% (17/22) of patients had a history of medications linked to gynecomastia. During follow-up (9 months to 26 years), two patients developed ductal carcinoma in situ (DCIS). CONCLUSION:ADH in male patients primarily presents incidentally alongside gynecomastia. Multifocality and cribriform patterns are common histological features. The association with medication-induced gynecomastia and the potential progression to DCIS highlight the clinical significance of ADH in males.

PDxBr is a digital test, which generates an artificial -intelligent tumor grade and phenotype utilizing morphometric features derived from H and E images of invasive breast cancer to predict outcome. Analytical validation of the image analysis platform including robust accuracy of cell type identification and tissue architecture composition, combined with test reproducibility and reliability, is a critical requirement for approval and clinical adoption. Background: PreciseDx Breast (PDxBr) is a digital test that predicts early -stage breast cancer recurrence within 6years of diagnosis. Materials and Methods: Using hematoxylin and eosin -stained whole slide images of invasive breast cancer (IBC) and artificial intelligence -enabled morphology feature array, microanatomic features are generated. Morphometr ic attr ibutes in combination with patient's age, tumor size, stage, and lymph node status predict disease free survival using a proprietary algorithm. Here, analytical validation of the automated annotation process and extracted histologic digital features of the PDxBr test, including impact of methodologic variability on the composite risk score is presented. Studies of precision, repeatability, reproducibility and interference were performed on morphology feature array -derived features. The final risk score was assessed over 20 -days with 2 -operators, 2-runs/day, and 2 -replicates across 8 -patients, allowing for calculation of within -run repeatability, between -run and within -laboratory reproducibility. Results: Analytical validation of features derived from whole slide images demonstrated a high degree of precision for tumor segmentation (0.98, 0.98), lymphocyte detection (0.91, 0.93), and mitotic figures (0.85, 0.84). Correlation of variation of the assay risk score for both reproducibility and repeatability were less than 2%, and interference from variation in hematoxylin and eosin staining or tumor thickness was not observed demonstrating assay robustness across standard histopathology preparations. Conclusion: In summary, the analytical validation of the digital IBC risk assessment test demonstrated a strong performance across all features in the model and complimented the clinical validation of the assay previously shown to accurately predict recurrence within 6 -years in early -stage invasive breast cancer patients.

Context.— The Nottingham Grading System (NGS) developed by Elston and Ellis is used to grade invasive breast cancer (IBC). Glandular (acinar)/tubule formation is a component of NGS. Objective.— To investigate the ability of pathologists to identify individual structures that should be classified as glandular (acinar)/tubule formation. Design.— A total of 58 hematoxylin-eosin photographic images of IBC with 1 structure circled were classified as tubules (41 cases) or nontubules (17 cases) by Professor Ellis. Images were sent as a PowerPoint (Microsoft) file to breast pathologists, who were provided with the World Health Organization definition of a tubule and asked to determine if a circled structure represented a tubule. Results.— Among 35 pathologists, the κ statistic for assessing agreement in evaluating the 58 images was 0.324 (95% CI, 0.314–0.335). The median concordance rate between a participating pathologist and Professor Ellis was 94.1% for evaluating 17 nontubule cases and 53.7% for 41 tubule cases. A total of 41% of the tubule cases were classified correctly by less than 50% of pathologists. Structures classified as tubules by Professor Ellis but often not recognized as tubules by pathologists included glands with complex architecture, mucinous carcinoma, and the “inverted tubule” pattern of micropapillary carcinoma. A total of 80% of participants reported that they did not have clarity on what represented a tubule. Conclusions.— We identified structures that should be included as tubules but that were not readily identified by pathologists. Greater concordance for identification of tubules might be obtained by providing more detailed images and descriptions of the types of structures included as tubules.

AIMS:Human epidermal growth factor receptor 2 (HER2) expression is an important biomarker in breast cancer (BC). Most BC cases categorised as HER2-negative (HER2-) express low levels of HER2 [immunohistochemistry (IHC) 1+ or IHC 2+/in-situ hybridisation not amplified (ISH-)] and represent a clinically relevant therapeutic category that is amenable to targeted therapy using a recently approved HER2-directed antibody-drug conjugate. A group of practising pathologists, with expertise in breast pathology and BC biomarker testing, outline best practices and guidance for achieving consensus in HER2 IHC scoring for BC. METHODS AND RESULTS:The authors describe current knowledge and challenges of IHC testing and scoring of HER2-low expressing BC and provide best practices and guidance for accurate identification of BCs expressing low levels of HER2. These expert pathologists propose an algorithm for assessing HER2 expression with validated IHC assays and incorporate the 2023 American Society of Clinical Oncology and College of American Pathologist guideline update. The authors also provide guidance on when to seek consensus for HER2 IHC scoring, how to incorporate HER2-low into IHC reporting and present examples of HER2 IHC staining, including challenging cases. CONCLUSIONS:Awareness of BC cases that are negative for HER protein overexpression/gene amplification and the related clinical relevance for targeted therapy highlight the importance of accurate HER2 IHC scoring for optimal treatment selection.

Context.-Human epidermal growth factor receptor 2 (HER2) status in breast cancer is currently classified as negative or positive for selecting patients for anti-HER2 targeted therapy. The evolution of the HER2 status has included a new HER2-low category defined as an HER2 immunohistochemistry score of 1+ or 2+ without gene amplification. This new category opens the door to a targetable HER2-low breast cancer population for which new treatments may be effective. Objective.-To review the current literature on the emerging category of breast cancers with low HER2 protein expression, including the clinical, histopathologic, and molecular features, and outline the clinical trials and best practice recommendations for identifying HER2-low- expressing breast cancers by immunohistochemistry. Data Sources.-We conducted a literature review based on peer -reviewed original articles, review articles, regulatory communications, ongoing and past clinical trials identified through ClinicalTrials.gov, and the authors' practice experience. Conclusions.-The availability of new targeted therapy potentially effective for patients with breast cancers with low HER2 protein expression requires multidisciplinary recognition. In particular, pathologists need to recognize and identify this category to allow the optimal selection of patients for targeted therapy.