Puay Hoon Tan

Background: Breast phyllodes tumours (PTs) are rare biphasic fibroepithelial neoplasms, classified into benign, borderline and malignant categories. Stromal mitotic activity is a key criterion in grading PT, but the methodology for mitotic assessment has not been clearly articulated. Materials and methods: We discuss the approach to determining stromal mitotic activity in PTs. Whether the most mitotically active area (hotspot) or an average count across several tumour regions should be adopted is reviewed. We also outline the operational protocol for mitotic hotspot evaluation, accounting for different microscope models with varying field diameters. Results: A standardized method of determining stromal mitoses in PTs is recommended in order to improve interobserver reproducibility and accuracy in grading PTs. The application of digital tools in stromal mitotic counting is also addressed. Conclusion: We advocate using the hotspot method of obtaining the highest stromal mitotic count, together with other histological parameters, to classify PTs.

Aims: Standardised diagnostic criteria in breast pathology primarily guide classification rather than real-world decision-making in borderline or context-dependent scenarios. This study evaluated question-and-answer data from international masterclass cohorts to characterise real-world diagnostic uncertainties, quantify cognitive domain distributions and detail expert decision-making frameworks. Methods: We analysed a structured dataset of 430 attendee questions collected across five international online Breast Pathology Masterclasses (2022–2026), each attended by over 500 participants. Questions were categorised into thematic and cognitive domains by the lead investigator, followed by a collaborative co-author audit to establish classification consensus. Faculty responses were evaluated to assess reasoning patterns, consensus points and areas of diagnostic variation. Results: Diagnostic uncertainty clustered within specific recurring clinical challenges, with borderline and grey-zone lesions constituting a primary focus. Threshold-based queries regarding diagnostic boundaries formed the single largest cognitive domain (40%), rather than queries seeking basic entity definitions. Other major clinical domains included biomarker interpretation, radiology–pathology discordance management and core biopsy sampling limitations. Analysis of faculty responses identified a recurring morphology-first, context-integrated and risk-informed approach to diagnostic decision-making. While consensus on foundational diagnostic principles was high, minor variations occurred in setting specific quantitative thresholds and framing diagnostic uncertainty in pathology reports. Conclusions: These exploratory findings indicate that practical challenges in diagnostic breast pathology frequently arise from boundary determination and contextual ambiguity rather than gaps in established disease classification schemes. Although expert judgement and confidence in managing diagnostic uncertainty develop substantially through experience, educational initiatives may complement experiential learning by making threshold calibration, risk communication and context-dependent reasoning more explicit.

Background: Triple-negative breast cancer (TNBC) exhibits aggressive biology and heterogeneous immune infiltration. Lysine-specific demethylase 1 (LSD1/KDM1A) is an epigenetic regulator implicated in oncogenic transcriptional programs, but its role in immune exclusion remains incompletely defined. Methods: LSD1 expression was evaluated by immunohistochemistry in TNBC specimens and correlated with survival. Functional effects of LSD1 depletion or pharmacological inhibition were assessed in TNBC cell lines and patient-derived xenograft organoids (PDXOs). Transcriptomic, ChIP-seq, multiplex immunofluorescence, and GeoMX digital spatial profiling analyses were integrated to characterize LSD1-associated transcriptional, chromatin, and immune programs. PBMC-PDXO co-cultures evaluated the effects of LSD1 inhibition on tumor growth and immune cell infiltration. Results: High LSD1 expression was associated with inferior overall survival, hypoxia-associated programs and suppressed immune pathways. LSD1-high tumors exhibited reduced infiltration of T cells, B cells, NKT-like cells, and macrophages. Spatial profiling revealed suppressed innate immune responses and type I interferon signaling within B-cell and T-cell compartments, respectively. ChIP-seq demonstrated preferential LSD1 occupancy at AP-1 motif-enriched regions, while co-immunoprecipitation supported association with AP-1 complexes. Integration of LSD1 chromatin occupancy sites with transcriptomics data identified SPPL2A and NCOA3 as candidate LSD1-regulated genes associated with interferon-γ response pathways. LSD1 depletion increased SPPL2A, NCOA3, MHC-I and PD-L1 expression. LSD1 inhibition reduced PDXO growth and increased immune cell infiltration. Conclusions: LSD1 is associated with a poor clinical outcome and immune exclusion in TNBC. These findings support a role for LSD1 in AP-1-associated chromatin regulation and interferon-γ-related transcriptional response; they warrant further investigation into LSD1 inhibition as a therapeutic strategy for immune-cold TNBCs.

Synovial sarcoma is a rare malignant mesenchymal neoplasm that typically occurs in soft tissue sites, and the kidney is an uncommon primary location. Previous studies of primary renal synovial sarcoma are limited by small sample sizes, and our understanding remains incomplete. Here, we present the largest multi-institutional case series to date of primary renal synovial sarcoma, with a focus on novel and molecular findings. A total of 70 cases were contributed by multiple institutions. Comprehensive clinical and histopathologic data were collected and analyzed. The mean patient age was 40 years, with a male-to-female ratio of 1.9:1. The most common presenting signs and symptoms were pain and hematuria. The mean tumor size was 11.6 cm (range: 2.3 to 26 cm), with frequent cystic change and necrosis. The mean follow-up was 29 months (range: 2 to 129 mo), and the rates of metastasis, recurrence, and death due to disease were 62.7%, 33.3%, and 50.0%, respectively. Histologically, 56.1% were monophasic synovial sarcoma, 15.1% were biphasic, and 28.8% were poorly differentiated or showed round cell features. Molecularly, SS18::SSX2 fusion was detected in the majority of cases assessed (n=19), followed by SS18::SSX1 fusion (n=6) and a rare SS18::NEDD4 fusion (n=1). Given the morphologic and immunohistochemical overlaps with many other neoplasms, accurate diagnosis with preferably molecular techniques is crucial for appropriate prognostication and treatment of this aggressive tumor.

Background: Malignant adenomyoepithelioma (M‐AME) is an extremely rare and morphologically diverse breast tumour, posing significant diagnostic and management challenges. We present seven cases of M‐AME with different histological compositions to contribute additional data to the existing literature. Through these cases, we review and discuss potential pitfalls and classification challenges in diagnosing M‐AME. Methods and results: We performed a retrospective clinicopathological review of seven cases of M‐AME from women aged 45–74 years (median 53 years). Each case exhibited distinct architectural patterns and compositions of classic and malignant components: (1) Adenomyoepithelioma (AME) with metaplastic spindle and squamous cell carcinoma; (2) Invasive AME with an aberrant immunohistochemical profile; (3) Invasive AME with an adenomyoepithelial adenosis pattern; (4) Metaplastic spindle cell carcinoma arising from AME with suspicion of M‐AME in situ; (5) Atypical AME with late lung metastasis harbouring HRAS p.G13R and in‐frame PIK3R1 deletion; (6) and (7) M‐AME in situ. Conclusion: M‐AME encompasses heterogeneous patterns that overlap with the infiltrative‐like pattern of classic AME (adenomyoepithelial adenosis form of tubular AME) and metaplastic breast carcinoma. Recognition of the underlying AME architecture, coupled with immunohistochemical and molecular correlation, supports pragmatic classification into in situ or invasive M‐AME or conventional breast carcinoma arising in the context of AME. Together with clinical, radiological, and molecular correlations, this approach can help guide appropriate management.

Background: Although histological criteria underpin current classification, accurate risk stratification, particularly for borderline and malignant tumours, remains challenging. Methods: We analysed a 20-year institutional series of phyllodes tumours (PTs), integrating clinicopathological data with long-term clinical outcomes. Histological features were extracted from original pathology reports, with slide re-review where archival material was available. Results: Among 111 PTs, 80% were benign, 11% borderline and 9% malignant. The follow-up ranged from 0 to 20 years, with 11.3, 10.3 and 7.6 years for benign, borderline and malignant PT, respectively. Most patients underwent local excision; mastectomy was performed in 22% of malignant PT. Final positive margins were identified in 18% of benign, 8% of borderline and none of the malignant tumours. Adjuvant radiotherapy was administered in 18% of borderline and 67% of malignant PT. Local recurrence occurred in 3%, 11% and 22% of benign, borderline and malignant tumours, respectively. Distant metastases were observed exclusively in malignant PTs (33%, 3/9 cases), two of which were preceded by local recurrence. Notably, two of the three metastatic malignant PTs showed either ≤10 mitoses/10 high-power field or th edition recommendation. The low event rate precluded robust multivariable analysis. Conclusion: Our results highlight the biological heterogeneity of malignant PTs and the limitations of rigid histological thresholds. These findings underscore the need for collaborative, multi-institutional prospective studies with standardised tumour sampling and reporting and molecular correlation to refine clinically meaningful risk stratification in PT.

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’.

Loss of the myoepithelial cell (MEC) layer at the epithelial–stromal interface is a key histological marker of invasion in malignant breast lesions. However, certain benign and non‐invasive entities may exhibit diminished or absent MECs, creating significant diagnostic challenges and occasionally leading to erroneous interpretation as invasive carcinoma. Despite their bland cytology, some of these lesions, such as microglandular adenosis (MGA) and pleomorphic adenoma, display infiltrative‐like growth patterns. Loss of MECs around proliferating epithelium‐lined clefts accompanied by spindled stroma in malignant phyllodes tumours may mimic metaplastic carcinoma. Atypical MGA lacking MECs may resemble acinic cell carcinoma. Diagnostic clues include the benign or hyperplastic appearance of the lesional cells, their immunoprofile and the absence of a sudden transition between areas with and without peripheral MECs. Awareness of such entities is key to preventing overdiagnosis and inappropriate clinical management. This review discusses these lesions, highlights their differential diagnoses and outlines key diagnostic pitfalls. Practical guidance is also provided on the optimal use and interpretation of myoepithelial markers to improve diagnostic accuracy and reduce the risk of misclassification.

Aims To objectively evaluate the prevalence of histological border irregularities in fibroadenomas (FAs) and characterise their association with specimen type, histological subtypes, prior procedures and associated pathologies. Methods A retrospective review of 912 FA specimens (549 core needle biopsies (CNBs), 45 vacuum-assisted biopsies (VABs), 242 excisions and 76 mastectomies) from 816 lesions was conducted. An irregular border was defined as an interface between the FA and adjacent parenchyma that was not smooth or circumscribed. Statistical analyses were performed to identify factors associated with irregularities in the FA contours. Results Border irregularity was present in 20.6% (188/912) of specimens. Prevalence was significantly lower in CNB (8.4%) compared with VAB (31.1%), mastectomy (34.2%) and excision (42.1%). By subtype, irregularity was most frequent in myxoid (41.7%), cellular (35.8%) and hyalinised (28.9%) FAs, compared with complex (20.4%), usual (16.2%) and juvenile (13.3%) forms. Calcifications, carcinoma in situ and invasive carcinoma, occurring within FAs, were associated with increased frequency of irregular borders. In multivariable analysis, a history of prior procedure was the strongest independent predictor of irregularity (OR 3.69, pConclusions While FAs are conventionally described as possessing circumscribed contours, our study found irregular borders in approximately one in five specimens. This feature is significantly associated with prior clinical procedures, specific histological subtypes and certain concurrent pathologies. It is essential to recognise that an irregular border in fibroepithelial lesions can be encountered in FAs and is not exclusive to phyllodes tumours.