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Aims: Recently, SOX17 has emerged as a promising biomarker for non‐mucinous Müllerian (ovarian and endometrial) carcinomas, demonstrating increased specificity in comparison to PAX8 while maintaining similar sensitivity. However, expression of SOX17 in mesonephric‐like adenocarcinoma (MLA), a carcinoma of the female genital tract with uncertain, but probably Müllerian histogenesis, remains unexplored. This study aims to address this gap. Methods and results: SOX17 immunohistochemistry was performed on whole tissue sections from 68 MLAs originating from the endometrium or ovary and seven cervical mesonephric carcinomas, as well as six mesonephric remnants/hyperplasias. Using a four‐tiered scoring system based on distribution and intensity of staining, 68% of MLA displayed a negative/low (< 10%) SOX17 expression pattern, which contrasts with the high expression observed in most Müllerian carcinomas. However, 22% of MLA demonstrated high SOX17 expression, similar to other endometrial and ovarian carcinomas. Similarly, five of seven (72%) mesonephric carcinomas of the cervix were SOX17‐negative, but two cases (28%) were positive. All mesonephric remnants/hyperplasias were SOX17 negative. Conclusions: The majority of MLA are negative or exhibit low SOX17 expression, in contrast to the diffuse and strong expression commonly seen in other types of Müllerian carcinoma. However, a subset of MLAs demonstrate high SOX17 expression. Therefore, absence of SOX17 staining is supportive for MLA when the differential includes another non‐mucinous Müllerian carcinoma. SOX17 may also be useful for differentiating mesonephric remnants/hyperplasias from Müllerian malignancies and benign Müllerian glandular lesions. [ABSTRACT FROM AUTHOR]

Aims: Ulcerative colitis‐associated neoplasia (UCAN) is characterised by multifocal tumourigenesis. A wide range of metachronous lesions have been reported to occur after endoscopic treatment of UCAN, which suggests the development of sporadic tumours in lesions treated as UCAN. Therefore, we aimed to evaluate differences of immunohistochemistry (IHC) in features and clinicopathological characteristics of intramucosal lesions in patients with ulcerative colitis (UC). Methods and results: We examined 35 intramucosal lesions resected for carcinoma or dysplasia by total colectomy from patients with UC and 71 sporadic adenomas (SAs) endoscopically resected from patients without UC. UC lesions were divided into the conventional UCAN group, defined as p53 mutant pattern and normal expression of β‐catenin, and the non‐conventional UCAN group, defined as the rest. Ki‐67 distribution, α‐methylacyl‐CoA racemase (AMACR) expression and mucin phenotypes were compared using IHC, and clinicopathological characteristics were investigated. Conventional and non‐conventional UCAN lesions were located in the left colon and rectum. Relative to the SA lesions, UCAN lesions occurred in much younger patients and exhibited more frequent basal distribution of Ki‐67 in tumour crypts. Conventional UCAN lesions tended to be non‐polyploid and exhibited a higher frequency of normal AMACR expression than SA lesions. UC lesions were heterogeneous—only two of the eight patients with multiple lesions had lesions (both non‐conventional UCAN lesions) exhibiting concordant IHC staining features. Conclusions: The basal pattern of Ki‐67 distribution, normal expression of AMACR and a non‐intestinal mucin phenotype were determined as characteristic features suggestive of UCAN. Non‐polypoid growth was another a key feature of UCAN. [ABSTRACT FROM AUTHOR]

Aims: Decreased oestrogen receptor (ER) expression is a marker of poor prognosis in endometrial carcinomas (EC) of no specific molecular profile (NSMP), but the optimal cut‐off to separate high‐risk 'low ER' versus low‐risk 'high ER' expression has not been defined. Here we characterised the distribution of ER staining in a cohort of ECs. Methods and results: Biopsy specimens from 120 cases of NSMP EC were stained for ER and assigned an Allred score. In 66 additional cases ER staining of matched biopsy and hysterectomy were compared. Twelve of 120 tumours had an Allred score of 0–3, including three endometrioid carcinomas (EEA) (one G1, two G3), four clear cell carcinomas (CCC), two mesonephric‐like adenocarcinoma (MLA) and one each of: gastric‐type adenocarcinoma, carcinosarcoma and endometrial carcinoma NOS. Three had Allred scores of 4–5: two MLA and one high‐grade carcinoma with yolk sac differentiation. Five had Allred scores of 6: four EEA (one G1, one G2, two G3) and one mixed clear cell and endometrioid carcinoma. The remaining 100 tumours with Allred scores ≥ 7 were all EEA (66 G1, 28 G2, five G3 and one grade unknown). Comparing the biopsy versus hysterectomy ER staining (n = 66), the results were within a single Allred score point, except two cases with strong diffuse expression in the biopsy (Allred 8) and moderate expression in the hysterectomy (Allred 5). Conclusions: Most NSMP ECs (> 80%) show high ER expression (Allred score ≥ 7). All non‐endometrioid carcinomas and a few endometrioid carcinomas had lower ER expression (Allred score ≤ 6) or were completely negative. [ABSTRACT FROM AUTHOR]

Aims: The majority of dermatofibrosarcoma protuberans (DFSP) harbour PDGFB or PDGFD rearrangements. We encountered ALK expression/rearrangement in a PDGFB/D‐negative CD34‐positive spindle cell neoplasm with features similar to DFSP, prompting evaluation of ALK‐rearrangements in DFSP and plaque‐like CD34‐positive dermal fibroma (P‐LDF). Methods and Results: We searched the archives of academic institutions for cases previously coded as DFSP and P‐LDF. NGS‐naïve or PDGFB‐negative DFSP were screened for ALK (clone D5F3) expression by immunohistochemistry. NGS or ALK FISH was performed on ALK‐positive cases. Methylome profiling studies were performed and compared with conventional DFSP. One case of "DFSP" and two "P‐LDF" with ALK expression were identified from the archives, while four cases were detected prospectively. These seven cases (6F:1M; 8 months to 76 years) arose in the dermis of the arm (two), scalp, eyelid, thigh, abdomen, and shoulder and ranged from 0.4 to 4.2 cm. Tumours were composed of spindled cells and displayed a storiform growth pattern. Cytologic atypia was absent, and mitotic figures were scarce (0–2/10 HPFs, high power fields). The lesional cells were diffusely positive for CD34 and ALK and negative for S100 protein. By NGS (n = 5), ALK fusion partners included DCTN1 (2), PLEKHH2, and CLIP2 in DFSP‐like cases and FLNA in P‐LDF‐like lesions. ALK FISH was positive in one (of two) cases previously labelled P‐LDF. Methylome profiling of two (of three) ALK‐rearranged DFSP‐like tumours showed clustering with conventional DFSP in the UMAP dimension reduction plot. To date, no tumour has recurred (n = 2; 26, 27 months). Conclusion: We describe a cohort of novel ALK‐rearranged tumours with morphologic features similar to DFSP. [ABSTRACT FROM AUTHOR]

Aims: Areas of gland crowding that do not fulfil diagnostic criteria of endometrioid intra‐epithelial neoplasia (EIN) are often encountered in endometrial biopsies. In this study, we document the prevalence of neoplastic outcome in patients with these subdiagnostic lesions (SL) and assess the utility of morphological features and a three‐marker immunohistochemistry panel (PAX2, PTEN, beta‐catenin) to predict outcome. Methods and results: Of 430 women with SL on endometrial sampling at Brigham and Women's Hospital between 2001 and 2021 with available follow‐up biopsy, 72 (17%) had a neoplastic outcome (EIN or endometrioid carcinoma). Multilayered epithelium and mitoses in SL were statistically associated with a neoplastic outcome. Abnormal three‐marker staining was observed in 93% (53 of 57) of SL with neoplastic outcome and 60% (37 of 62) of a control group with benign outcome. Among the 72 patients with neoplastic outcome, EIN/carcinoma tissue was available in 33; of these, 30 (91%) showed abnormal staining for one or more markers. Remarkably, in 84% of these cases the EIN/carcinoma had the aberrant expression seen in the preceding SL. Based on a prevalence of 17%, the positive and negative predictive values of abnormal staining in one or more markers were 24 and 97%, respectively. Conclusions: The presence of SL warrants clinical surveillance and repeat sampling because it is followed by endometrioid neoplasia in a significant subset of patients. Normal three‐marker staining identifies women with a very low risk of neoplastic outcome. Conversely, abnormal staining is frequent in SL with benign outcome leading to poor specificity and positive predictive value. [ABSTRACT FROM AUTHOR]

Aims Methods and results Conclusions CIC‐rearranged sarcomas (CRS) are clinically aggressive undifferentiated round cell sarcomas (URCS), commonly driven by CIC::DUX4. Due to the repetitive nature of DUX4 and the variability of the fusion breakpoints, CIC::DUX4 fusion may be missed by molecular testing. Immunohistochemical (IHC) stains have been studied as surrogates for the CIC::DUX4 fusion. We aim to assess the performance of DUX4 IHC in the work‐up of CRS and its expression in non‐CRS round cell or epithelioid neoplasms.Cases of molecularly confirmed CRS (n = 48) and non‐CRS (n = 105) were included. CRS cases consisted of 35 females and 13 males, with ages ranging from less than 1 year to 67 years (median = 41 years). Among the molecularly confirmed non‐CRS cases, C‐terminal DUX4 expression was investigated in Ewing sarcomas (38 cases), alveolar rhabdomyosarcomas (18 cases), desmoplastic small round cell tumours (12 cases) and synovial sarcomas (n = five), as well as in non‐mesenchymal neoplasms such as SMARCA4/SMARCB1‐deficient tumours (n = five), carcinomas of unknown primary (n = three) and haematolymphoid neoplasms (four cases). DUX4 IHC was considered positive when strong nuclear expression was detected in more than 50% of neoplastic cells. When used as a surrogate for the diagnosis of CRS, the sensitivity and specificity of DUX4 IHC was 98 and 100%, respectively. Only one CRS case was negative for DUX4 IHC and harboured a CIC::FOXO4 fusion.DUX4 IHC is a highly sensitive and specific surrogate marker for the presence of CIC::DUX4 fusion, demonstrating its utility in establishing a diagnosis of CRS. [ABSTRACT FROM AUTHOR]

Aims Methods Results Conclusion Mesonephric‐like adenocarcinoma (MLA) of the endometrium or ovary is a rare but distinct endometrial carcinoma which has a combination of characteristic morphological, immunohistochemical (IHC) and molecular features. SOX17 has been recently identified as a highly sensitive and specific marker for endometrial and ovarian carcinomas. In this study, we aimed to investigate SOX17 expression in MLA together with other IHCs to differentiate MLAs from other endometrial carcinomas.Seventeen previously diagnosed endometrial/ovarian MLAs were collected, and multiple IHCs were performed. Additionally, we performed SOX17, PAX8 and ER on tissue microarrays (TMAs) composed of 652 endometrial carcinomas from 2012 to 2015 when MLA diagnostic criteria were not established.All 17 MLAs showed diffuse strong positive PAX8, negative ER and variable TTF1/GATA3 staining. Notably, all MLAs showed negative (n = 10) or focal weak/moderate (n = 7) staining for SOX17, which is more diffuse and stronger than PAX8 in other endometrial carcinoma subtypes. This finding prompted us to screen TMAs with 652 endometrial carcinomas diagnosed before MLA by an approach of combined SOX17 and PAX8 IHCs, and 14 cases with positive PAX8 but negative/focal weak SOX17 were identified. We further studied the 14 cases by examining morphology and performing additional IHCs (TTF1, GATA3, ER and CD10) and would classify seven (50%) of them as MLAs based on morphological features and positive CD10, TTF1 and/or GATA3 staining.Our results suggest that a combination of SOX17 and PAX8 IHCs would aid in diagnosing MLA if the results show strong positive PAX8, but negative SOX17. [ABSTRACT FROM AUTHOR]

Aims: Over 50% of breast cancer cases are "Human epidermal growth factor receptor 2 (HER2) low breast cancer (BC)", characterized by HER2 immunohistochemistry (IHC) scores of 1+ or 2+ alongside no amplification on fluorescence in situ hybridization (FISH) testing. The development of new anti‐HER2 antibody‐drug conjugates (ADCs) for treating HER2‐low breast cancers illustrates the importance of accurately assessing HER2 status, particularly HER2‐low breast cancer. In this study we evaluated the performance of a deep‐learning (DL) model for the assessment of HER2, including an assessment of the causes of discordances of HER2‐Null between a pathologist and the DL model. We specifically focussed on aligning the DL model rules with the ASCO/CAP guidelines, including stained cells' staining intensity and completeness of membrane staining. Methods and Results: We trained a DL model on a multicentric cohort of breast cancer cases with HER2‐IHC scores (n = 299). The model was validated on two independent multicentric validation cohorts (n = 369 and n = 92), with all cases reviewed by three senior breast pathologists. All cases underwent a thorough review by three senior breast pathologists, with the ground truth determined by a majority consensus on the final HER2 score among the pathologists. In total, 760 breast cancer cases were utilized throughout the training and validation phases of the study. The model's concordance with the ground truth (ICC = 0.77 [0.68–0.83]; Fisher P = 1.32e‐10) is higher than the average agreement among the three senior pathologists (ICC = 0.45 [0.17–0.65]; Fisher P = 2e‐3). In the two validation cohorts, the DL model identifies 95% [93% ‐ 98%] and 97% [91% ‐ 100%] of HER2‐low and HER2‐positive tumours, respectively. Discordant results were characterized by morphological features such as extended fibrosis, a high number of tumour‐infiltrating lymphocytes, and necrosis, whilst some artefacts such as nonspecific background cytoplasmic stain in the cytoplasm of tumour cells also cause discrepancy. Conclusion: Deep learning can support pathologists' interpretation of difficult HER2‐low cases. Morphological variables and some specific artefacts can cause discrepant HER2‐scores between the pathologist and the DL model. [ABSTRACT FROM AUTHOR]

Background: Adenoid cystic carcinoma is a rare subtype of triple‐negative breast carcinoma. These low‐grade tumours, which are treated by simple mastectomy and have an excellent prognosis compared to other triple‐negative breast carcinomas. Solid‐variant adenoid cystic carcinomas have basaloid features and are difficult to distinguish morphologically from other triple‐negative breast cancers. Breast adenoid cystic carcinoma exhibits MYB protein overexpression, which can be detected by immunohistochemistry (IHC). Aim: We compared the IHC expression of MYB in solid‐variant adenoid cystic carcinoma with that in other triple‐negative breast cancers. Methods: We conducted IHC staining of 210 samples of triple‐negative breast cancers, including solid‐variant adenoid cystic carcinoma (n = 17), metaplastic breast carcinoma (n = 44), basaloid triple‐negative breast cancer (n = 21), and other triple‐negative invasive ductal carcinoma (n = 128). We classified nuclear staining of MYB as diffuse/strong (3+), focal moderate (2+), focal weak (1+), or none (0). Results: All 17 solid/basaloid adenoid cystic carcinoma cases exhibited 3+ MYB expression. Of the 21 solid/basaloid triple‐negative breast cancers, one (5%) had 2+ expression, seven (33%) 1+ expression, and 13 (62%) 0 expression. Of the 44 metaplastic carcinoma cases, 39 cases (89%) had no (0) staining, and the other five cases had focal weak (1+) or moderate (2+) staining. Among the 128 triple‐negative invasive ductal carcinoma cases, 92 cases (72%) had no (0) staining, 36 cases (28%) exhibited focal weak (1+) or moderate (2+) staining. Conclusions: Our study revealed diffuse/strong MYB staining (3+) only in solid/basaloid adenoid cystic carcinomas. Thus, we recommend routine MYB IHC staining in triple‐negative breast carcinoma with solid/basaloid morphology to improve diagnostic accuracy. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Human epidermal growth factor receptor 2 (HER2)‐enriched breast cancer benefits significantly from anti‐HER2 targeted therapies. This highlights the critical need for precise HER2 immunohistochemistry (IHC) interpretation serving as a triage tool for selecting patients for anti‐HER2 regimens. Recently, the emerging eligibility of patients with HER2‐low breast cancers for a novel HER2‐targeted antibody–drug conjugate (T‐DXd) adds challenges to HER2 IHC scoring interpretation, notably in the 0–1+ range, which shows high interobserver and interlaboratory staining platform variability. In this review, we navigate evolving challenges and suggest practical recommendations for HER2 IHC interpretation. [ABSTRACT FROM AUTHOR]

Aims: Superficial CD34‐positive fibroblastic tumour (SCD34FT) is an uncommon but distinctive low‐grade neoplasm of the skin and subcutis that shows frequent CADM3 expression by immunohistochemistry (IHC). In this study, prompted by an index case resembling 'atypical fibrous histiocytoma (FH)' that was positive for CADM3 IHC, we systematically examined a cohort of tumours previously diagnosed as 'atypical FH' by applying CADM3 and fluorescence in situ hybridization (FISH) for PRDM10 rearrangement, to investigate the overlap between these tumour types. Methods and Results: Forty cases of atypical FH were retrieved, including CD34‐positive tumours (n = 20) and CD34‐negative tumours (n = 20). All tumours were stained for CADM3. All CADM3‐positive tumours were evaluated by FISH to assess for PRDM10 rearrangement. Eleven CD34‐positive tumours (11/20, 55%) coexpressed CADM3 and were reclassified as SCD34FT. None (0/20) of the CD34‐negative atypical FH were CADM3‐positive. Reclassified SCD34FT (10/11) arose on the lower extremity, with frequent involvement of the thigh (n = 8). Features suggestive of atypical FH were observed in many reclassified cases including variable cellularity, spindled morphology, infiltrative tumour margins, collagen entrapment, epidermal hyperpigmentation, and acanthosis. Variably prominent multinucleate giant cells, including Touton‐like forms, were also present. An informative FISH result was obtained in 10/11 reclassified tumours, with 60% (6/10) demonstrating PRDM10 rearrangement. Conclusion: A significant subset of tumours that histologically resemble atypical FH, and are positive for CD34, coexpress CADM3 and harbour PRDM10 rearrangement, supporting their reclassification as SCD34FT. Awareness of this morphologic overlap and the application of CADM3 IHC can aid the distinction between SCD34FT and atypical FH. [ABSTRACT FROM AUTHOR]

Aims: Recently, human epidermal growth factor 2 (HER2)‐low (i.e. HER2 score 1+ or 2+ without amplification) breast cancer patients became eligible for trastuzumab–deruxtecan treatment. To improve assay standardisation and detection of HER2‐low in a quantitative manner, we conducted an external quality assessment‐like study in the Netherlands. Dynamic range cell lines and immunohistochemistry (IHC) calibrators were used to quantify HER2 expression and to assess interlaboratory variability. Methods and results: Three blank slides with a dynamic range cell line and an IHC calibrator were stained with routine HER2 assays by 35 laboratories. Four different antibody clones were used: 19 (54.3%) 4B5, six (17.1%) A0485, five (14.3%) DG44 (HercepTest) and five (14.3%) SP3. Laboratories used two different detection kits for 4B5 assays: 14 (73.7%) ultraView and five (26.3%) OptiView. Variability of HER2 expression in cell lines, measured with artificial intelligence software, was median (min–max) = negative core 0.5% (0.0–57.0), 1+ core 4.3% (1.6–71.3), 2+ core 42.8% (30.4–92.6) and 3+ core 96.2% (91.8–98.8). The calibrators DG44 and 4B5 OptiView had the highest analytical sensitivity, closely followed by 4B5 ultraView. SP3 was the least sensitive. Calibrators of A0485 assays were not analysable due to background staining. Conclusions: As assays were validated for detecting HER2‐amplified tumours, not all assays and antibodies proved suitable for HER2‐low detection. Some tests showed distinct expression in the negative cell line. Dynamic range cell line controls and quantitative analysis using calibrators demonstrated more interlaboratory variability than commonly appreciated. Revalidation of HER2 tests by laboratories is needed to ensure clinical applicable HER2‐low assays. [ABSTRACT FROM AUTHOR]

Background and aims: Gastric cancers (GC) are divided into subtypes based on molecular profile: Epstein–Barr virus (EBV)‐positive, microsatellite instability (MSI), chromosomal instability (CIN) and genomically stable (GS) tumours. The prognostic impact of this classification is unclear. The aim was to evaluate whether the molecular subtypes determined using in‐situ hybridisation (ISH) and immunohistochemistry (IHC) are associated with clinicopathological parameters and prognosis. Methods and results: The study included 503 GC patients. Based on ISH (EBV) and IHC (MSI and TP53), tumours were divided into EBV‐positive, MSI, CIN (EBVneg/MSS/TP53aberrant) and GS (EBVneg/MSS/TP53wild‐type) subgroups. Survival analyses with intestinal‐ and diffuse‐type tumours were examined separately. EBV‐positive tumours associated with male sex. Both EBV‐positive and MSI tumours associated with intestinal type. CIN tumours associated with intestinal‐type and positive lymph node status. GS tumours associated with diffuse‐type and negative lymph node status. In the total cohort, no significant differences in the 5‐year survival were observed. In intestinal tumours, the 5‐year survival was better in EBV‐positive tumours compared with GS tumours [hazard ratio (HR) = 0.57, 95% confidence interval (CI) = 0.33–0.99]. In diffuse tumours, the 5‐year survival was worse in CIN tumours compared with GS tumours (HR = 1.57, 95% CI = 1.14–2.18). In radically resected diffuse tumours, the 5‐year survival was worse in MSI tumours compared with GS tumours (HR = 3.26, 95% CI = 1.20–8.82). Conclusions: The molecular classification is associated with histological type but not prognosis in GC. As the prognostic effects of molecular subtypes in intestinal‐ and diffuse‐type cancers may differ, combining histological and molecular information is recommended for future studies. [ABSTRACT FROM AUTHOR]

Aims: Patients with recurrent or metastatic head and neck squamous cell carcinoma (HNSCC) are eligible for first‐line immune checkpoint inhibition if their tumour is positive for programmed death ligand 1 (PD‐L1) determined by the combined positive score (CPS). This nationwide study, using real‐world data, investigated the developing PD‐L1 testing landscape in the first 3 years after introduction of the test in HNSCC and examined interlaboratory variation in PD‐L1 positivity rates. Methods: Pathology reports of HNSCC patients mentioning PD‐L1 were extracted from the Dutch Pathology Registry (Palga). Tumour and PD‐L1 testing characteristics were analysed per year and interlaboratory variation in PD‐L1 positivity rates was assessed using funnel plots with 95% confidence limits around the overall mean. Results: A total of 817 PD‐L1 tests were reported in 702 patients among 19 laboratories; 85.2% of the tests on histological material were stated to be positive. The national PD‐L1 positivity rate differed significantly per year during the study period (79.7–89.9%). The use of the recommended 22C3 antibody increased from 59.9 to 74.3%. A total of 673 PD‐L1 tests on histological material from 12 laboratories were analysed to investigate interlaboratory variation. Four (33%) deviated significantly from the national mean of PD‐L1‐positive cases using CPS ≥ 1 cut‐off, while two (17%) deviated significantly for CPS ≥ 20 cut‐off. Conclusion: In the first 3 years of PD‐L1 assessment in HNSCC, the testing landscape became more uniform. However, interlaboratory variation in PD‐L1 positivity rates between Dutch laboratories was substantial. This implies that there is a need for further test standardisation to reduce this variation. [ABSTRACT FROM AUTHOR]

Background: Testicular Leydig cell tumours (LCTs) are the most common type of sex cord–stromal tumour in men, representing 1%–3% of all testicular neoplasms. Among testicular sex cord–stromal tumours, CTNNB1 mutations and nuclear expression of β‐catenin have been typically associated with Sertoli cell tumour. Recent genomic analyses have shown that CTNNB1 variants are also identified in a subset of LCTs; however, the frequency and clinicopathologic associations of β‐catenin alterations remain incompletely understood in this tumour type. Methods: In this study, we evaluated 32 LCTs (five malignant/metastasizing, 27 nonmetastasizing) using β‐catenin immunohistochemistry and DNA sequencing. Results: Immunohistochemistry revealed focal or multifocal nuclear β‐catenin expression in 47% of the tumours. Diffuse nuclear β‐catenin expression (in >50% of the tumour cells) was not detected in any of the cases analysed herein. Comparison of β‐catenin‐positive and β‐catenin‐negative cases did not show significant differences in the frequency of adverse histopathologic findings or malignant clinical behaviour. DNA sequencing performed de novo on a subset of seven cases revealed the presence of exon 3 CTNNB1 variants in four of them (4/7, 57%), with variant allele frequencies (VAF) ranging from 7 to 33%. Two additional β‐catenin‐positive cases that had been sequenced as part of a previous study harboured exon 3 CTNNB1 variants at VAF of 28% and 7%, respectively. Conclusion: These results demonstrate that β‐catenin alterations are relatively common in LCT, most likely occurring as subclonal events that are not enriched in cases with aggressive features. Further studies are needed to clarify the oncogenic role of β‐catenin in this tumour type. [ABSTRACT FROM AUTHOR]

In the last two decades or so, a spectrum of benign, premalignant and malignant cervical glandular lesions exhibiting gastric differentiation has been described, with gastric‐type adenocarcinoma representing the most common human papillomavirus (HPV)‐independent cervical adenocarcinoma. More recently, limited literature has reported a variety of gastric‐type glandular lesions at other sites within the female genital tract and, as in the cervix (the most common site for these lesions), a spectrum of benign, premalignant and malignant lesions has been proposed. We provide an update and review of the emerging spectrum of gastric‐type glandular lesions at female genital tract sites other than the cervix. In the endometrium, putative gastric‐type glandular lesions include mucinous metaplasia of gastric‐type, atypical mucinous proliferation of gastric‐type and gastric‐type adenocarcinoma. Similarly in the vagina, gastric‐type adenosis, atypical adenosis and adenocarcinoma have been described. There have also been occasional reports of gastric‐type lesions involving the ovary and fallopian tube. We provide guidance on how to recognise gastric‐type lesions morphologically and immunophenotypically and stress that sometimes these lesions occur at more than one site within the female genital tract (synchronous/multifocal gastric‐type lesions of the female genital tract), sometimes in association with Peutz–Jeghers syndrome. [ABSTRACT FROM AUTHOR]

Aims: Verruciform acanthotic vulvar intra‐epithelial neoplasia (vaVIN) is an HPV‐independent, p53 wild‐type lesion with distinct morphology and documented risk of recurrence and cancer progression. vaVIN is rare, and prospective distinction from non‐neoplastic hyperplastic lesions can be difficult. CK17, SOX2 and GATA3 immunohistochemistry has emerging value in the diagnosis of HPV‐independent lesions, particularly differentiated VIN. We aimed to test the combined value of these markers in the diagnosis of vaVIN versus its non‐neoplastic differentials in the vulva. Methods and results: CK17, SOX2 and GATA3 immunohistochemistry was evaluated on 16 vaVINs and 34 mimickers (verruciform xanthoma, lichen simplex chronicus, lichen sclerosus, psoriasis, pseudo‐epitheliomatous hyperplasia). CK17 was scored as 3+ = full‐thickness, 2+ = partial‐thickness, 1+ = patchy, 0 = absent; SOX2 as 3+ = strong staining ≥ 10% cells, 2+ = moderate, 1 + =weak, 0 = staining in < 10% cells; and GATA3 as pattern 0 = loss in < 25% basal cells, 1 = loss in 25–75% basal cells, 2 = loss in > 75% basal cells. For analysis, results were recorded as positive (CK17 = 3+, SOX2 = 3+, GATA3 = patterns 1/2) or negative (CK17 = 2+/1+/0, SOX2 = 2+/1+/0, GATA3 = pattern 0). CK17, SOX2 and GATA3 positivity was documented in 81, 75 and 58% vaVINs, respectively, versus 32, 17 and 22% of non‐neoplastic mimickers, respectively; ≥ 2 marker positivity conferred 83 sensitivity, 88 specificity and 86% accuracy in vaVIN diagnosis. Compared to vaVIN, SOX2 and GATA3 were differentially expressed in lichen sclerosus, lichen simplex chronicus and pseudo‐epitheliomatous hyperplasia, whereas CK17 was differentially expressed in verruciform xanthoma and adjacent normal mucosa. Conclusions: CK17, SOX2 and GATA3 can be useful in the diagnosis of vaVIN and its distinction from hyperplastic non‐neoplastic vulvar lesions. Although CK17 has higher sensitivity, SOX2 and GATA3 are more specific, and the combination of all markers shows optimal diagnostic accuracy. [ABSTRACT FROM AUTHOR]

Aims: The Lynch syndrome (LS) screening algorithm requires BRAF testing as a fundamental step to distinguish sporadic from LS‐associated colorectal carcinomas (CRC). BRAF testing by immunohistochemistry (IHC) has shown variable results in the literature. Our aim was to analyse concordance between BRAFV600EIHC and BRAF molecular analysis in a large, mono‐institutional CRC whole‐slide, case series with laboratory validation. Methods and results: MisMatch repair (MMR) protein (hMLH1, hPMS2, hMSH2, and hMSH6) and BRAFV600EIHC were performed on all unselected cases of surgically resected CRCs (2018–2023). An in‐house validation study for BRAFV600EIHC was performed in order to obtain optimal IHC stains. BRAFVV600EIHC was considered negative (score 0), positive (scores 2–3), and equivocal (score 1). Interobserver differences in BRAFV600EIHC scoring were noted in the first 150 cases prospectively collected. Nine‐hundred and ninety CRCs cases (830 proficient (p)MMR/160 deficient (d)MMR) were included and all cases performed BRAFV600EIHC (BRAFV600EIHC‐positive 13.5% of all series; 66.3% dMMR cases; 3.4% pMMR cases), while 333 also went to BRAF mutation analysis. Optimal agreement in IHC scoring between pathologists (P < 0.0001) was seen; concordance between BRAFV600EIHC and BRAF molecular analysis was extremely high (sensitivity 99.1%, specificity 99.5%; PPV 99.1%, and NPV 99.5%). Discordant cases were reevaluated; 1 score 3 + IHC/wildtype case was an interpretation error and one score 0 IHC/mutated case was related to heterogenous BRAFV600EIHC expression. Among the 12 IHC‐equivocal score 1+ cases (which require BRAF molecular analysis), three were BRAF‐mutated and nine BRAF‐wildtype. Conclusion: BRAFV600EIHC can be used as a reliable surrogate of molecular testing after stringent in‐house validation. [ABSTRACT FROM AUTHOR]

Tumour protein 63 (p63) is a transcription factor of the p53 gene family, encoded by the TP63 gene located at chromosome 3q28, which regulates the activity of genes involved in growth and development of the ectoderm and derived tissues. p63 protein is normally expressed in the nuclei of the basal cell layer of glandular organs, including breast, in squamous epithelium and in urothelium. p63 immunohistochemical (IHC) staining has several applications in diagnostic breast pathology. It is commonly used to demonstrate myoepithelial cells at the epithelial stromal interface to differentiate benign and in situ lesions from invasive carcinoma and to characterize and classify papillary lesions including the distinction of breast intraduct papilloma from skin hidradenoma. p63 IHC is also used to identify and profile lesions showing myoepithelial cell and/or squamous differentiation, e.g. adenomyoepithelioma, salivary gland‐like tumours including adenoid cystic carcinoma, and metaplastic breast carcinoma including low‐grade adenosquamous carcinoma. This article reviews the applications of p63 IHC in diagnostic breast pathology and outlines a practical approach to the diagnosis and characterization of breast lesions through the identification of normal and abnormal p63 protein expression. The biology of p63, the range of available antibodies with emphasis on staining specificity and sensitivity, and pitfalls in interpretation are also discussed. The TP63 gene in humans, which shows a specific genomic structure, resulting in either TAp63 (p63) isoform or ΔNp63 (p40) isoform. As illustrated in the figure, both isoforms contain a DNA‐binding domain (Orange box) and an oligomerization domain (Grey box). TAp63 contains an N‐terminal transactivation (TA) domain (Green box), while ΔNp63 has an alternative terminus (Yellow box). Antibodies against conventional pan‐p63 (TP63) bind to the DNA binding domain common to both isoforms (TAp63 and p40) and does not distinguish between them. Antibodies against TAp63 bind to the N‐terminal TA domain, while antibodies specific to ΔNp63 (p40) bind to the alternative terminus. Each isoform has variant isotypes (α, β, γ, δ, and ε). [ABSTRACT FROM AUTHOR]

Aims: T cell large granular lymphocytic leukaemia (T‐LGLL) is a rare disorder that may underlie otherwise unexplained cytopenias. The identification of T‐LGLL cells in bone marrow biopsies can be a challenge, because a robust immunohistochemistry marker is lacking. The markers currently in use (granzyme B, TIA‐1 and CD8) are difficult to interpret or lack specificity. Therefore, we investigated whether immunohistochemistry for thymocyte selection‐associated high‐mobility group box (TOX), a transcription factor that associates with chronic T cell stimulation, could be a reliable tool for the identification of T‐LGLL cells. Methods and results: In this retrospective study, expression of TOX in CD8+ cells in bone marrow biopsies of T‐LGLL patients (n = 38) was investigated and compared to bone marrow of controls with reactive T cell lymphocytosis (n = 10). All biopsies were evaluated for TOX staining within the CD8‐positive T cell population. The controls were essentially negative for TOX, whereas all T‐LGLL cases were positive (median = 80%, range = 10–100%), even when bone marrow involvement was subtle. Conclusion: TOX is a highly sensitive marker for the neoplastic cells of T‐LGLL and we recommend its use, especially in the diagnostic work‐up of patients with unexplained cytopenias. [ABSTRACT FROM AUTHOR]

Aims: NTRK‐rearranged sarcomas of the female genital tract mainly occur in the uterus (more commonly cervix than corpus) and are characterized by a "fibrosarcoma‐like" morphology and NTRK gene rearrangements. These neoplasms may exhibit histological overlap with other entities and can present diagnostic difficulties without molecular confirmation. Pan‐TRK immunohistochemistry was developed to identify tumours harbouring NTRK rearrangements. The aim of this study was to characterize pan‐TRK immunohistochemical expression in a large cohort of gynaecological mesenchymal neoplasms and investigate the utility of pan‐TRK immunohistochemistry to distinguish NTRK‐rearranged sarcoma from its mimics. Methods and results: A total of 473 gynaecological mesenchymal tumours (461 without known NTRK fusions and 12 NTRK‐rearranged sarcomas) were selected. Pan‐TRK immunohistochemistry (EPR17341, Abcam) was performed on whole tissue sections and tissue microarrays. Molecular interrogation of pan‐TRK positive tumours was performed by RNA sequencing or fluorescence in situ hybridization (FISH). Of the 12 NTRK‐rearranged sarcomas, 11 (92%) exhibited diffuse (≥70%) cytoplasmic pan‐TRK staining with moderate/marked intensity, while the other was negative. Eleven (2.4%) additional tumours also exhibited pan‐TRK immunohistochemical expression: three low‐grade endometrial stromal sarcomas, seven high‐grade endometrial stromal sarcomas, and an undifferentiated uterine sarcoma. Molecular confirmation of the absence of NTRK rearrangements was possible in nine of these tumours. Of these nine neoplasms, seven exhibited focal/multifocal (<70%) pan‐TRK cytoplasmic staining with weak/moderate intensity. Conclusion: Even though pan‐TRK immunohistochemical expression is not entirely sensitive or specific for NTRK‐rearranged sarcomas, these neoplasms tend to exhibit diffuse staining of moderate/strong intensity, unlike its mimics. Pan‐TRK should be performed in monomorphic uterine (corpus and cervix) spindle cell neoplasms that are negative for smooth muscle markers and hormone receptors and positive for CD34 and/ or S100. Ultimately, the diagnosis requires molecular confirmation. [ABSTRACT FROM AUTHOR]

Aims: Angiofibroma of soft tissue (AFST) is a benign, morphologically distinctive tumour type that harbours recurrent AHRR::NCOA2 fusions in 60–70% of cases and shows a non‐specific immunophenotype, expressing EMA in roughly half of cases. The AHRR::NCOA2 fusion results in increased expression of cytochrome P450 1A1 (CYP1A1); a recent study demonstrated CYP1A1 immunohistochemistry (IHC) to be moderately sensitive and highly specific for AFST. Methods and results: In this study, we sought to validate these findings in a larger independent cohort of 30 AFST, as well as 215 morphological mimics, including 30 solitary fibrous tumours, 29 myxoid liposarcomas, 28 low‐to‐intermediate grade myxofibrosarcomas (MFS), 20 atypical spindle cell lipomatous tumours (ASCLT), 20 cellular angiofibromas, 10 cases each of spindle cell lipoma, neurofibroma, malignant peripheral nerve sheath tumour, superficial angiomyxoma, cellular myxoma, soft tissue perineurioma and deep fibrous histiocytoma, and nine cases each of low‐grade fibromyxoid sarcoma and mammary‐type myofibroblastoma. We found CYP1A1 IHC to be 70% sensitive for AFST, with granular cytoplasmic staining in 21 of 30 tumours, and 98% specific, with staining in only five morphological mimics: two deep fibrous histiocytomas, one MFS, one cellular angiofibroma and one ASCLT. Conclusions: These findings confirm that CYP1A1 is 70% sensitive, consistent with the prevalence of AHRR::NCOA2 fusions that up‐regulate this protein, and that it is highly specific among morphological mimics. [ABSTRACT FROM AUTHOR]

Aims: While epithelioid trophoblastic tumour (ETT) primarily arises from the uterus, cases have been increasingly documented at extrauterine sites, originating from an ectopic gestation or presenting as a metastatic tumour, leading to the major differential diagnosis of somatic carcinoma with trophoblastic differentiation. The precise separation of a gestational trophoblastic tumour from its somatic carcinoma mimics is highly relevant and crucial for patient management and prognosis. Methods and results: We summarise the clinicopathological and molecular features of four challenging epithelioid malignancies presenting at extrauterine sites, with ETT as the main differential diagnosis. All four tumours demonstrated histological and immunohistochemical features overlapping between a somatic carcinoma and an ETT, combined with inconclusive clinical and imaging findings. Serum beta‐hCG elevation was documented in two cases. Short tandem repeat (STR) genotyping was performed and was informative in all cases. The presence of a unique paternal allelic pattern in the tumour tissue confirmed the diagnosis of ETT in two cases with an initial consideration of either somatic carcinoma or suspicion of a gestational trophoblastic tumour. The presence of matching genetic profile with the patient's paired normal tissue was seen in two other cases (both initially considered as ETT), confirming their somatic origin, including one metastatic triple‐negative breast carcinoma and one primary lung carcinoma. Conclusions: Diagnostic separation of ETT at an extrauterine site from its somatic carcinoma mimics can be difficult at the histological and immunohistochemical levels. STR genotyping offers a robust ancillary tool that precisely separates ETT from somatic carcinomas with trophoblastic differentiation. [ABSTRACT FROM AUTHOR]

Primary mesenchymal tumours of the pleura are uncommon and can be diagnostically challenging due to their overlapping histopathologic and immunophenotypic features. Herein we discuss selected mesenchymal tumours of the pleura, including solitary fibrous tumour, calcifying fibrous tumour, desmoid fibromatosis, synovial sarcoma, schwannoma, malignant peripheral nerve sheath tumour, inflammatory myofibroblastic tumour, follicular dendritic cell sarcoma, epithelioid hemangioendothelioma, and desmoplastic small round cell tumour. We review their clinicopathologic characteristics, along with an update on the relevant immunohistochemical and molecular features. [ABSTRACT FROM AUTHOR]

Aims: Wilson disease (WD) is a genetic disorder of copper metabolism caused by mutations in the ATP7B gene. Toxic copper accumulation leads to hepatic, neurologic, and psychiatric disorders with variable presentation. Metallothionein (MT) immunohistochemistry was proposed as a diagnostic marker. Methods: MT immunohistochemistry was performed on liver specimens of WD patients (n = 64) and control cases (n = 160) including acute liver failure, steatotic liver disease, autoimmune hepatitis, normal liver, primary biliary cholangitis, primary and secondary sclerosing cholangitis, and progressive familial intrahepatic cholestasis. The optimal cutoff for detection of WD was determined by receiver operating characteristic (ROC) analysis. Results: At least moderate staining in >50% of hepatocytes was observed in 81% of analysed liver specimens (n = 56/69) of WD patients, while only five control cases showed this staining pattern. The sensitivity, specificity, and accuracy for a new diagnosis of WD were 85.7%, 96.9%, and 94.9%, respectively. Sensitivity in nonfibrotic patients was 70.6% and this MT pattern was robust in small biopsies. The hepatic copper concentration was similar between MT‐positive and MT‐negative liver samples (P > 0.05). Zinc treatment may induce hepatocellular MT expression. Kayser–Fleischer rings (50% versus 15%) and neurologic disorders (50% versus 13%) were significantly more prevalent in MT‐negative compared to MT‐positive WD patients, respectively. Conclusion: MT immunostaining is an excellent biomarker for histological diagnosis of WD, should be incorporated in the diagnostic work‐up of patients with potential WD, and is useful in a modified Leipzig score. [ABSTRACT FROM AUTHOR]

Aims: The LUMINA trial demonstrated a very low local recurrence rate in women ≥55 years with low‐risk luminal A breast cancer (defined as grade I‐II, T1N0, hormone receptor positive, HER2 negative and Ki67 index ≤13.25%) treated with breast‐conserving surgery and endocrine therapy (but no other systemic therapy), supporting the safe omission of radiation in these women. Here we describe the protocol for Ki67 assessment, the companion diagnostic used to guide omission of adjuvant radiotherapy. Methods: Ki67 immunohistochemistry was performed on full‐face sections at one of three regional labs. Pathologists trained in the International Ki67 in Breast Cancer Working Group (IKWG) method demarcated tumour areas on scanned slides and scored 100 nuclei from each of at least five randomly selected 1‐mm fields. For cases with high Ki67 heterogeneity, further virtual cores were selected and scored in order to confidently assign a case as luminal A (≤13.25%) or B (>13.25%). Interlaboratory variability was assessed through an annual quality assurance programme during the study period. Results: From the quality assurance programme, the mean Ki67 index across all cases/labs was 13%. The observed intraclass correlation coefficient (ICC) and kappa statistics were ≥0.9 and ≥0.7, respectively, indicating a substantial level of agreement. Median scoring time was 4 min per case. The IKWG‐recommended scoring method, performed directly from slides, requiring up to four scored fields, is concordant with the LUMINA scoring method (ICC ≥ 0.9). Conclusion: Ki67 is a practical, reproducible, and inexpensive biomarker that can identify low‐risk luminal A breast cancers as potential candidates for radiation de‐escalation. Clinical trial registration: ClinicalTrials.gov number, NCT01791829 [ABSTRACT FROM AUTHOR]

AimsAdenomatoid tumours are mesothelial-derived benign neoplasms with a predilection for the genital tract. Extragenital sites are rare and can cause significant diagnostic challenges. Herein, we describe the clinicopathological features of a cohort of adenomatoid tumours involving the gastrointestinal tract and liver in order to more clearly characterise their histological findings and aid in diagnosis.Methods and resultsThe pathology databases at four institutions were searched for adenomatoid tumours involving the gastrointestinal tract or liver, yielding eight cases. Available clinicoradiological and follow-up data were collected from the medical records. Six tumours were incidentally discovered during imaging studies or at the time of surgical exploration for unrelated conditions; presenting symptoms were unknown in two patients. Histologically, the tumours were well-circumscribed, although focal ill-defined borders were present in four cases. No infiltration of adjacent structures was identified. Architectural heterogeneity was noted in five (63%) tumours; an adenoid pattern often predominated. The neoplastic cells were flattened to cuboidal with eosinophilic cytoplasm. Cytoplasmic vacuoles mimicking signet ring-like cells were present in five (63%) cases. Three (38%) cases showed involvement of the mesothelium with reactive mesothelial hyperplasia. Cytological atypia or increased mitotic activity was not identified. The surrounding stroma ranged from oedematous/myxoid to densely hyalinised. Immunohistochemistry confirmed mesothelial origin in all cases evaluated. No patients developed recurrence of disease.ConclusionsThe current study evaluates the clinicopathological findings in a collective series of gastrointestinal and hepatic adenomatoid tumours, correlating with those described in individually reported cases. We highlight common histological features and emphasise variable findings that could mimic a malignant neoplasm.

Aims: Invasive lobular carcinoma (ILC) has distinct morphology and association with loss of E‐cadherin function. It has special clinical and imaging features, and its proper recognition is important. Following a recent proposal, we tested the value of the routine use of E‐cadherin immunohistochemistry (IHC) in recognizing ILC. Methods and results: Five pathologists with experience in breast pathology from four Hungarian institutions histotyped 1001 breast cancers from diagnostic core biopsies or excision specimens randomly assigned to haematoxylin and eosin (HE) diagnosis first, followed by E‐cadherin IHC; or to immediate HE and E‐cadherin‐based diagnosis. Of 524 cases with HE diagnosis, 73(14%) were deemed uncertain. E‐cadherin made the initial histological type change in 14/524 cases (2.7%), including three with confident HE‐based type allocation. Use of E‐cadherin immunostaining was considered useful in 88/477 cases (18%) with immediate dual assessment, and typing uncertainty went down to 5% (25/477 cases), but was not zero. Collective assessment of 171 uncertain, difficult, nonclassical cases resulted in consensus diagnosis in most cases, but 15 cases were still doubtful as concerns their proper histological type. CDH1 gene sequencing was attempted and successful in 13; pathogenic genetic alterations were identified in seven cases. Conclusions: The routine use of E‐cadherin IHC decreases the uncertainty in typing and improves the typing accuracy at the cost of potentially redundant additional immunostains. Furthermore, this procedure does not exclude uncertainty due to E‐cadherin‐positive ILCs, which are occasionally difficult to confidently label as ILC, especially when the growth pattern is not classic. [ABSTRACT FROM AUTHOR]

Aims: The reliable classification of type A versus type B3 thymomas has prognostic and therapeutic relevance, but can be problematic due to considerably overlapping morphology. No immunohistochemical markers aiding in this distinction have been published so far. Methods and results: We identified and quantified numerous differentially expressed proteins using an unbiased proteomic screen by mass spectrometry in pooled protein lysates from three type A and three type B3 thymomas. From these, candidates were validated in a larger series of paraffin‐embedded type A and B3 thymomas. We identified argininosuccinate synthetase 1 (ASS1) and special AT‐rich sequence binding protein 1 (SATB1) as highly discriminatory between 34 type A and 20 type B3 thymomas (94% sensitivity, 98% specificity and 96% accuracy). Although not the focus of this study, the same markers also proved helpful in the diagnosis of type AB (n = 14), B1 (n = 4) and B2 thymomas (n = 10). Conclusions: Mutually exclusive epithelial expression of ASS1 in 100% of type B3 thymomas and ectopic nuclear expression of SATB1 in 92% of type A thymomas support the distinction between type A and type B3 thymomas with 94% sensitivity, 98% specificity and 96% accuracy. [ABSTRACT FROM AUTHOR]

Aims: Our understanding of dedifferentiated endometrial carcinoma (DEC), a rare and aggressive malignancy, mainly reflects undifferentiated carcinomas (UC) arising in the setting of low‐grade endometrial cancer (DEC‐LG). However, cases of UC arising in the setting of high‐grade EC (DEC‐HG) have been noted in the literature. Our knowledge of the genomics of DEC‐HG is limited. To characterise the molecular landscape of DEC‐HC, targeted genomic sequencing and immunohistochemical analysis was carried out on seven DEC‐HG and four DEC‐LG. Methods and results: DEC‐HG and DEC‐LG, including undifferentiated and differentiated components, both showed a similar frequency and spectrum of mutations. ARID1A mutations were identified in 6/7 (86%) DEC‐HG and 4/4 (100%) DEC‐LG, while SMARCA4 mutations were present in 4/7 (57%) DEC‐HG and in 1/4 (25%) DEC‐LG. Concurrent SMARCA4/BRG1 protein loss by immunohistochemistry was observed in 3/4 and 1/1 SMARCA4 mutated DEC‐HG and DEC‐LG, respectively. Neither genomic alterations nor protein loss in SMARCB1/INI1 were observed in any of our cases. TP53 mutations were detected in 4/7 (57%) DEC‐HG and in 2/4 (50%) DEC‐LG, while mutation‐pattern p53 immunohistochemistry expression was observed in 2/7 (29%) DEC‐HG and none of the DEC‐LG. MLH1 mutations were observed in 1/7 (14%) DEC‐HG and 1/4 (25%) DEC‐LG. MSH2 and MSH6 mutations were each detected in 1/7 (14%) DEC‐HG, but neither was associated with corresponding loss of protein expression. Conclusion: The findings support expanding the definition of DEC to include DEC‐HG, a previously under‐recognised phenomenon with genomic similarities to DEC‐LG. [ABSTRACT FROM AUTHOR]

Aims: Due to its rarity and non‐specific clinical and pathological features, low‐grade adenosquamous carcinoma (LGASC) of the breast continues to pose diagnostic challenges. Unlike other triple‐negative breast carcinomas, LGASC tends to have an indolent clinical behaviour. It is essential to recognise this lesion for accurate diagnosis and appropriate management. Methods and results: Twenty‐five cases of LGASC were identified in our archives and collaborating institutes. Cases of LGASC with dominant coexisting other type carcinomas were excluded. We studied the clinical presentation, morphological features, patterns of the commonly used immunohistochemical stains and follow‐up. In our cohort, LGASC was commonly located at the outer aspect of the breast and associated with intraductal papilloma. The morphology of LGASC is characterised by infiltrating small glands and nests with variable squamous differentiation. We also found cuffing desmoplastic (fibrolamellar) stromal change in 75% of patients and peripheral lymphocytic aggregates in 87.5% of patients. P63 and smooth muscle myosin (SMM) were the most common myoepithelial markers used to assist in diagnosis. P63 often stained peripheral tumour cells surrounding invasive glands (circumferential staining in 80% of the cases), mimicking myoepithelial cells. It also stained the small nests with squamous differentiation. However, SMM was negative in 63% of the cases. The vast majority of our cases were triple‐negative; only a few had focal and weak expressions of ER and PR. One patient who did not have excision developed lymph node metastasis. Most patients underwent excision or mastectomy with negative margins as surgical treatment; there were no recurrences or metastases in these patients with clinical follow‐ups up to 108 months. Conclusions: LGASC has some unique, although not entirely specific, morphological features and immunohistochemical staining patterns. Fibrolamellar stromal change, peripheral lymphocytic aggregates and variable staining of p63 and SMM are valuable features to facilitate the diagnosis. [ABSTRACT FROM AUTHOR]

Aims: Mesothelin (MSLN) is a cancer‐associated antigen that is overexpressed in malignancies such as mesothelioma, pancreatic and ovarian cancer. It is also a target for novel personalised therapies, including antibodies, antibody–drug conjugates and chimeric antigen receptor T cells. Immunohistochemistry may predict those who would best respond to anti‐mesothelin therapies and guide decisions in therapeutic strategy. This study aimed to assess the intensity and distribution of MSLN immunostaining in mesothelioma, and to determine the prognostic value of MSLN expression by histochemical‐score (H‐score). Methods and results: The MN1 anti‐MSLN antibody was used to stain a formalin‐fixed paraffin‐embedded tissue microarray of histologically confirmed mesothelioma from 75 consecutive patients who had undergone pleurectomy with or without decortication. MSLN positivity, the staining intensity, distribution of staining and H‐score were evaluated. The correlation of H‐score with prognosis was investigated. Sixty‐six per cent of epithelioid tumours were MSLN‐positive (with expression in > 5% tumour cells). Of MSLN‐expressing epithelioid tumours, 70.4% had moderate (2+) or strong (3+) intensity MSLN immunostaining, although only 37% of samples had staining in ≥ 50% of tumour cells. In multivariate analysis, MSLN H‐score as a continuous variable and an H‐score ≥ 33 were independent predictors of improved survival (P = 0.04 and P < 0.001, respectively). Conclusions: MSLN expression was more heterogenous in epithelioid mesothelioma than reported previously. Therefore, it would be appropriate to perform an immunohistochemical assessment of MSLN expression to stratify and assess patient suitability for mesothelin‐targeted personalised therapies, such as chimeric antigen receptor T cells. [ABSTRACT FROM AUTHOR]

Aims: Rhabdomyosarcomas currently are classified into one of four subtypes (alveolar, embryonal, spindle cell/sclerosing, or pleomorphic) according to their morphological, immunohistochemical, and molecular genetic features. The alveolar subtype is characterised by a recurrent translocation involving PAX3 or PAX7 and FOXO1; identification of this translocation is important for appropriate classification and prognostication. In this study, we aimed to explore the diagnostic utility of FOXO1 immunohistochemistry for rhabdomyosarcoma classification. Methods/results: A monoclonal antibody targeting a FOXO1 epitope retained in the fusion oncoprotein was used to study 105 rhabdomyosarcomas. FOXO1 was positive for expression by immunohistochemistry in all 25 alveolar rhabdomyosarcomas, with 84% showing diffuse expression in greater than 90% of neoplastic cells; the remainder of alveolar rhabdomyosarcomas displayed at least moderate staining in a minimum of 60% of lesional cells. Apart from three spindle cell rhabdomyosarcomas showing heterogeneous nuclear immunoreactivity in 40–80% of tumour cells, the 80 cases of embryonal, pleomorphic, and spindle cell/sclerosing rhabdomyosarcoma were negative for FOXO1 expression (96.3% specific) when using a threshold of nuclear staining in 20% of neoplastic cells to determine positivity. Variable cytoplasmic staining was present in a fraction of all rhabdomyosarcoma subtypes. Nonneoplastic lymphocytes, endothelial cells, and Schwann cells also showed variably intense nuclear anti‐FOXO1 immunoreactivity. Conclusion: Taken together, our findings suggest that FOXO1 immunohistochemistry is a highly sensitive and relatively specific surrogate marker of the PAX3/7::FOXO1 fusion oncoprotein in rhabdomyosarcoma. Cytoplasmic immunoreactivity, expression in nonneoplastic tissues, and limited nuclear staining of nonalveolar rhabdomyosarcomas represent potential pitfalls in interpretation. [ABSTRACT FROM AUTHOR]

PRAME is a novel immunohistochemical marker that aids the diagnosis of melanocytic lesions. Diffuse PRAME positivity suggests melanoma, whereas benign naevi are negative or only weakly positive. However, the factual diagnostic accuracy of PRAME is not well established. Moreover, some studies have suggested that the threshold of 3+/50% positive cells may be more useful in practice than the most widely used cut‐off (4+/75% of positive cells). Hence, we performed a systematic review and diagnostic accuracy meta‐analysis to evaluate sensitivity, specificity, likelihood ratios and optimal threshold for PRAME in distinguishing benign melanocytic proliferations from melanomas. Twenty‐six studies were enrolled into the meta‐analysis. A total of 2915 melanocytic lesions were analysed. The optimal threshold for PRAME positivity was estimated at 3.11, which translates into 3+ in practice. Sensitivity and specificity calculated from SROC at the 3+ threshold were 0.735 (0.631–0.818) and 0.915 (0.834–0.958), respectively, compared to 0.679 (0.559–0.957) and 0.957 (0.908–0.981) at the 4+ cut‐off. In subgroup analysis, the spitzoid subgroup was characterised by the lowest sensitivity and diagnostic odds ratio of PRAME. Our findings indicate that PRAME immunohistochemistry may serve as an ancillary marker to support the diagnosis of melanoma. Nevertheless, the accuracy of PRAME may be lower in spitzoid neoplasms. Our meta‐analysis suggests that the 3+/50% threshold might be more useful in practice than the 4+/75% cut‐off, as it shows higher sensitivity with retained satisfactory specificity. [ABSTRACT FROM AUTHOR]

Aims: Angiofibroma of soft tissue (AFST) is a benign tumour characterised by prominent arborizing blood vessels throughout the lesion. Approximately two‐thirds of AFST cases were reported to have AHRR::NCOA2 fusion, and only two cases have been reported to have other gene fusions: GTF2I::NCOA2 or GAB1::ABL1. Although AFST is included in fibroblastic and myofibroblastic tumours in the World Health Organization's 2020 classification, histiocytic markers, especially CD163, have been reported to be positive in almost all examined cases, and it still remains the possibility of a fibrohistiocytic nature of the tumour. Therefore, we aimed to clarify the genetic and pathological spectrum of AFST and identify whether histiocytic marker‐positive cells were true neoplastic cells. Methods and results: We evaluated 12 AFST cases, which included 10 cases with AHRR::NCOA2 and two with AHRR::NCOA3 fusions. Pathologically, nuclear palisading, which has not been reported in AFST, was detected in two cases. Furthermore, one tumour resected by additional wide resection revealed severe infiltrative growth. Immunohistochemical analysis indicated varying levels of desmin‐positive cells in nine cases, whereas CD163‐ and CD68‐positive cells were diffusely distributed in all 12 cases. We also performed double immunofluorescence staining and immunofluorescence in situ hybridisation in four resected cases with >10% desmin‐positive tumour cells. The results suggested that the CD163‐positive cells differed from desmin‐positive cells with AHRR::NCOA2 fusion in all four cases. Conclusion: Our findings suggested that AHRR::NCOA3 could be the second most frequent fusion gene, and histiocytic marker‐positive cells are not genuine neoplastic cells in AFST. [ABSTRACT FROM AUTHOR]

Aims: NTRK rearranged tumours are rare but can be successfully treated using anti‐TRK‐targeted therapies, making NTRK testing important for treatment choices in patients with advanced cancers. Pan‐Trk immunohistochemistry (IHC) has become a valuable and affordable screening tool in many laboratories. Unfortunately, the choice of antibodies and IHC protocols to investigate biomarkers is not standardised. In this study, we compared the performance of four pan‐Trk IHC methods, using three different clones, primarily in NTRK fusion‐positive tumours. Methods and results: We studied the performance of four pan‐Trk IHC methods using three different clones: EPR17341 (Abcam and Ventana), EP1058Y (Abcam) and A7H6R (Cell Signaling) in 22 molecularly confirmed NTRK rearranged tumours. Additionally, selected NTRK fusion‐negative tumours were further included: NTRK mutated (n = 8) and amplified (n = 15) tumours as well as NTRK fusion‐negative tumours driven by other gene fusions, such as ALK, ROS1 and BCOR (n = 20), as well as salivary gland tumours (n = 16). Inter‐rater agreement of three pathologists was additionally calculated, including H‐score. With clone EPR17341 (Abcam in‐house and ready‐to‐use Ventana protocol), all molecularly confirmed NTRK1–3 rearranged tumours were positively detected by immunohistochemistry, while the other clones missed NTRK2–3 rearranged tumours. For the fusion‐negative cohort we found the best performance (least false‐positive cases) using the clone A7H6R (Cell Signalling). Conclusion: Given the therapeutic importance, testing for NTRK rearrangements in daily practice has become necessary and, despite IHC being a fast and affordable tool, using it in routine diagnostics is complicated and requires a high level of expertise. [ABSTRACT FROM AUTHOR]

Background: SMAD4 is a tumour suppressor gene that is mutated in a variety of cancers. SMAD4 nonstop mutations, which convert stop codons to sense codons that extend transcription, have been identified in genomic databases but have not been characterised in human pathology samples. The frequency of SMAD4 nonstop mutations and the consequences of nonstop mutations on SMAD4 protein expression are unknown. Methods: We retrospectively analysed our cancer sequencing database of 38,002 tumour specimens and evaluated the spectrum of SMAD4 mutations. SMAD4 protein expression was evaluated by immunohistochemistry in tumours with SMAD4 nonstop mutations. Results: In total, 1956 SMAD4 mutations were identified in 1822 tumours. SMAD4 mutations were most common in tumours of the gastrointestinal tract and included nonsense variants (n = 344), frameshift indels (n = 258), splice site variants (n = 104), and missense variants at codon R361 (n = 245). In a subset of cases with immunohistochemistry, SMAD4 expression was lost in 23 of 25 tumours (92%) with protein truncating variants and in 7 of 27 tumours (26%) with missense variants. Four cases harboured SMAD4 nonstop mutations. SMAD4 nonstop mutations were identified in two pancreatic adenocarcinomas, one colonic adenocarcinoma, and one non‐small cell lung carcinoma. Immunohistochemistry demonstrated loss of SMAD4 protein expression in each of the four tumours with SMAD4 nonstop mutations. Conclusion: SMAD4 nonstop mutations are associated with loss of SMAD4 protein expression in multiple tumour types. SMAD4 nonstop mutations should be clinically interpreted as pathogenic loss of function alterations when identified in cancer sequencing panels. [ABSTRACT FROM AUTHOR]

Aims: To morphologically and immunophenotypically characterize dedifferentiated uterine leiomyosarcoma (LMS). Methods and results: We identified 23 dedifferentiated uterine LMS, defined as a malignant uterine smooth muscle tumour containing discrete differentiated and dedifferentiated components (i.e. with and without morphologic and immunophenotypic evidence of smooth muscle differentiation, respectively). The differentiated component was leiomyosarcoma in most cases (17/23), though some arose from a leiomyoma (n = 4) or smooth muscle tumour of uncertain malignant potential (n = 2). The dedifferentiated tumour component showed noncohesive polygonal cells with moderate to abundant cytoplasm, pleomorphic nuclei with coarse vesicular to smudged chromatin, one or more macronucleoli, frequent multinucleation, and atypical mitoses. Three cases showed heterologous osteosarcomatous or chondrosarcomatous differentiation. Immunohistochemistry revealed alterations characteristic of uterine LMS, including Rb loss (18/19); strong diffuse p16 (17/19); strong diffuse (9/19) or complete absence of (5/19) p53; and ATRX loss (6/16). Compared to a control cohort of uterine LMS without dedifferentiation, dedifferentiated uterine LMS showed significantly shorter disease‐specific (median, 54 versus 20 months; 5‐year DSS, 46% versus 36%; P = 0.04) and disease‐free (median, 31 versus 8 months; 5‐year DFS, 42% versus 8%; P = 0.002) survival. Of 19 dedifferentiated uterine LMS with follow‐up, 12 had died of disease at median 14 (range, 2–73) months; four were alive with disease at 4, 12, 44, and 50 months; and three were alive with no evidence of disease at 56, 109, and 114 months. Conclusion: Routine prospective recognition of dedifferentiated uterine LMS and distinction from mimics is advocated for accurate prognostication and for further characterisation of these tumours. [ABSTRACT FROM AUTHOR]

Primary bone tumours can pose diagnostic problems due to their overlapping radiologic and histologic features. Given the recent advancement in our understanding of the biology of bone tumours, multiple immunohistochemical and molecular markers have been devised to aid in their diagnosis. This review provides brief updates on select bone tumours, including chondrosarcomas, benign chondrogenic tumours, osteosarcomas, benign osteogenic tumours, fibroosseous lesions, vascular tumours, osteoclastic giant cell‐rich or cystic tumours, chordoma, adamantinoma, small round blue cell sarcomas, and others. We discuss their salient molecular features and novel immunohistochemical correlates, along with some tips to avoid common diagnostic pitfalls. [ABSTRACT FROM AUTHOR]

Squamous cell carcinoma (SCC) is the most common malignant tumour of the penis. The 2022 WHO classification reinforces the 2016 classification and subclassifies precursor lesions and tumours into human papillomavirus (HPV)‐associated and HPV‐independent types. HPV‐associated penile intraepithelial neoplasia (PeIN) is a precursor lesion of invasive HPV‐ associated SCC, whereas differentiated PeIN is a precursor lesion of HPV‐independent SCC. Block‐type positivity of p16 immunohistochemistry is the most practical daily utilised method to separate HPVassociated from HPVindependent penile SCC. If this is not feasible, the term SCC, not otherwise specified (NOS) is appropriate. Certain histologies that were previously classified as "subtypes" are now grouped, and coalesced as "patterns", under the rubric of usual type SCC and verrucous carcinoma (e.g. usual‐type SCC includes pseudohyperplastic and acantholytic/pseudoglandular carcinoma, and carcinoma cuniculatum is included as a pattern of verrucous carcinoma). If there is an additional component of the usual type of invasive SCC (formerly termed hybrid histology), the tumour would be a mixed carcinoma (e.g. carcinoma cuniculatum or verrucous carcinoma with usual invasive SCC); in such cases, reporting of the relative percentages in mixed tumours may be useful. The consistent use of uniform nomenclature and reporting of percentages will inform the refinement of future reporting classification schemes and guidelines/recommendations. The classification of scrotal tumours is provided for the first time in the fifth edition of the WHO Blue book, and it follows the schema of penile cancer classification for both precursor lesions and the common SCC of the scrotum. Basal cell carcinoma of the scrotum may have a variable clinical course and finds a separate mention. [ABSTRACT FROM AUTHOR]

Aims: Classic Hodgkin lymphoma (cHL) should be distinguished from its wide variety of histological mimics, including reactive conditions and mature B and T cell neoplasms. Thymus and activation‐related chemokine (TARC) is produced in extremely high quantities by the Hodgkin/Reed–Sternberg (HRS) tumour cells and is largely responsible for the attraction of CD4+ T cells into the cHL tumour micro‐environment. In the current study we evaluated the diagnostic potential of TARC immunohistochemistry in daily practice in a tertiary referral centre in the Netherlands. Methods and results: A total of 383 cases, approximately half of which were cHL mimics, were prospectively evaluated in the period from June 2014 to November 2020. In 190 cHL cases, 92% were TARC‐positive and the majority of cases showed strong and highly specific staining in all HRS cells (77%). In most cases, TARC could discriminate between nodular lymphocyte‐predominant and lymphocyte‐rich Hodgkin lymphoma. HRS‐like cells in mature lymphoid neoplasms were rarely positive (6.4%) and there was no TARC staining at all in 64 reactive lymphadenopathies. Conclusions: TARC immunohistochemistry has great value in differentiating between cHL and its mimics, including nodular lymphocyte‐predominant Hodgkin lymphoma, reactive lymphadenopathies and mature lymphoid neoplasms with HRS‐like cells. [ABSTRACT FROM AUTHOR]

Microsatellite instability (MSI) is a major carcinogenic pathway with prognostic and predictive implications. The validity of polymerase chain reaction (PCR)‐based MSI testing is well established in colorectal cancer; however, the data are limited in non‐colorectal gastrointestinal cancers. The aim of this study is to clarify the detailed MSI profiles of non‐colorectal gastrointestinal cancers and to investigate the differences from those of colorectal cancers. MSI testing was performed using paired tumour/normal tissues of 123 mismatch repair‐deficient cancers detected by immunohistochemistry including 80 non‐colorectal cancers (eight oesophagogastric junction (EGJ), 57 gastric and 15 small intestine) and 43 colorectal cancers. Fragment size analysis revealed that the mean nucleotide shifts of five markers (Promega panel) were the highest in the stomach (6.4), followed by colorectum (5.7), small intestine (5.0) and EGJ cancers (mean = 4.0; P = 0.015, versus stomach). All cases showed ≥ 1 nucleotide shift in ≥ 2 markers and were considered as MSI‐high. However, when the cut‐off was set to ≥ 3 nucleotide shifts in ≥ 2 markers, three EGJ (37.5%), two small intestine (13.3%) and two gastric (3.5%) cancers showed false‐negative results. In addition, cases with isolated loss of MSH6 or PMS2 showed smaller nucleotide shifts than those in others. MSI testing is applicable to non‐colorectal gastrointestinal cancers; however, a subset can yield false‐negative results due to subtle nucleotide shift in multiple markers. Analysis of paired tumour/normal tissues and careful interpretation is necessary to avoid false‐negative results and ensure appropriate treatment. [ABSTRACT FROM AUTHOR]

Cytokeratin 5 (CK5) is a marker for pulmonary squamous cell carcinoma; however, CK5 is sometimes present in pulmonary adenocarcinoma (ADC), and there is insufficient information regarding the clinicopathological features of CK5‐positive ADC. We aimed to explore the clinicopathological characteristics of CK5‐positive ADC using immunohistochemistry. We prepared the following two cohorts: a resected cohort containing 220 resected tumours for primarily studying the detailed morphological characteristics, and a tissue microarray (TMA) cohort containing 337 samples for investigating the associations of CK5 expression with other protein expressions, genetic and prognostic findings. CK5‐positive ADC was defined to have ≥ 10% tumour cells and presence of CK5‐positive tumour cells in the resected and TMA cohorts, respectively. CK5‐positive ADCs were identified in 91 (16.3%) patients in the combined cohort. CK5‐positive ADCs had male predominance (P = 0.012), smoking history (P = 0.001), higher stage (P < 0.001), histological high‐grade components (P < 0.001), vascular invasion (P < 0.001), mucinous differentiation (P < 0.001), spread through airspaces (P < 0.001), EGFR wild‐type (P < 0.001), KRAS mutations (P < 0.001), ALK rearrangement (P < 0.001) and ROS1 rearrangement (P = 0.002). In the resected cohort, more than half the CK5‐positive ADCs (19 cases, 65.5%) showed mucinous differentiation; the remaining cases harboured high‐grade components. In the TMA cohort, CK5‐positive ADCs correlated with TTF‐1 negativity (P = 0.002) and MUC5B, MUC5AC and HNF4alpha positivity (P < 0.001, 0.048, < 0.001). Further, CK5‐positive ADCs had significantly lower disease‐free and overall survival rates than CK5‐negative ADCs (P < 0.001 for each). Additionally, multivariate analysis revealed that CK5 expression was an independent poor prognostic factor. CK5‐positive ADCs showed aggressive clinical behaviour, with high‐grade morphology and mucinous differentiation. [ABSTRACT FROM AUTHOR]

Primary cutaneous apocrine carcinoma (PCAC) is a rare cutaneous malignancy that is derived from apocrine glands. Histologically, these tumours can appear welldifferentiated where diagnosis should be relatively straightforward. However, occasionally these tumours can exhibit high-grade features, and in such instances the diagnosis can be challenging. A retrospective analysis of 12 cases of poorly differentiated PCAC, obtained from large academic institutions, was performed, and summarised below. Immunohistochemical studies were performed in all cases with antibodies against CK7, p63, CAM 5.2, GCDFP-15, GATA3, CEA, PR, ER, HER2, calponin, SMA, androgen receptor and EMA. All 12 cases were poorly differentiated; however, there were some histopathological clues to the diagnosis of apocrine carcinoma; namely, the presence of focal glandular formation, acrosyringial involvement and the presence of single ‘pagetoid’ cells within epidermis. All tumours were consistently positive for CK7, GATA3 and GCDFP15 and negative for p63. The tumours had variable expression of CAM5.2, CEA, ER, PR, HER2, androgen receptor and EMA. In three cases, there was a preservation of the myoepithelial cell layer (with calponin and SMA), which also confirmed the primary cutaneous origin. PCAC is a difficult neoplasm to diagnose, as it can appear identical to metastatic carcinomas. We describe 12 cases of poorly differentiated PCAC, highlighting their salient clinical, histopathological and immunohistochemical features, and discuss the potential diagnostic pitfalls in distinguishing this entity from other malignant neoplasms. Our results indicate that a combination of thorough histological inspection coupled with an adequate battery of immunohistochemical stains is necessary to support the diagnosis of PCAC. [ABSTRACT FROM AUTHOR]

Immunohistochemistry for hepatitis E virus (HEV) ORF2 (capsid) protein is a powerful tool for tissue-based diagnosis of hepatitis E, particularly useful in evaluating abnormal liver values in immunocompromised patients. We report here a previously unobserved reactivity of the HEV ORF2 antibody to human cytomegalovirus (CMV) proteins and contrast the staining patterns encountered in HEV and CMV infection, respectively. As part of a routine diagnostic work-up, the liver biopsy of an immunocompromised patient with elevated liver values was examined histologically for infection with viruses including CMV and HEV. Cytopathic changes were found, suggestive of CMV infection, which was confirmed by immunohistochemistry. Surprisingly, reactivity of a portion of CMV-infected cells with a mouse monoclonal antibody (clone 1E6) against HEV ORF2 protein was also detected. This observation prompted a screening of 22 further specimens (including liver, gastrointestinal, lung, brain and placental biopsies) with confirmed CMV infection/reactivation. Immunoreactivity of CMV-infected cells with HEV ORF2 antibody was observed in 18 of 23 specimens. While the HEV ORF2 antibody showed cytoplasmic, nuclear and canalicular positivity in hepatitis E cases, positivity in CMV-infected cells was limited to the nucleus. In conclusion, the HEV ORF2 antibody (clone 1E6) shows unexpected immunoreactivity against CMV proteins. In contrast to the hepatitis E staining pattern with cytoplasmic, nuclear and occasional canalicular positivity, reactivity in CMV-infected cells is restricted to the nucleus. Awareness of this cross-reactivity and knowledge of the differences in staining patterns will prevent pathologists from misinterpreting positive HEV ORF2 immunohistochemistry in liver specimens. [ABSTRACT FROM AUTHOR]

Evaluation of oestrogen receptor (ER) expression by immunostaining is essential in the pathological assessment of breast cancer. Its expression is intercorrelated with clinicopathological features, molecular typing, and treatment selection. The development of novel therapeutic agents related to ER status, the recent ASCO introduction of an ER-low positive category of breast cancers, and the ever-increasing plethora of diagnostic and theragnostic markers call for a timely update. In this article we aim to review the clinicopathological features of ER-positive breast cancers, with an emphasis on ER-low positive breast cancers, and a focus on updating the (i) assessment, reporting and interpretation of ER immunohistochemical (IHC) staining, (ii) correlations of ER status with other diagnostic and theragnostic markers, and (iii) implications for treatment selection and response. In the face of the developments in IHC and molecular techniques and targeted therapy, ER immunostaining is still expected to remain as the core component of prognostic and theragnostic assessment of breast cancers. [ABSTRACT FROM AUTHOR]

WHO classification of Thoracic Tumours defines lung carcinoid tumours (LCTs) as well-differentiated neuroendocrine neoplasms (NENs) classified in low grade typical (TC) and intermediate grade atypical carcinoids (AC). Limited data exist concerning protein expression and morphologic factors able to predict disease aggressiveness. Though Ki-67 has proved to be a powerful diagnostic and prognostic factor for Gastro-entero-pancreatic NENs, its role in lung NENs is still debated. A retrospective series of 370 LCT from two oncology centers was centrally reviewed. Morphology and immunohistochemical markers (Ki-67, TTF-1, CD44, OTP, SSTR-2A, Ascl1, and p53) were studied and correlated with Overall Survival (OS), Cancer-specific survival (CSS) and Disease-free survival (DFS). Carcinoid histology was confirmed in 355 patients: 297 (83.7%) TC and 58 (16.3%) AC. Ki-67 at 3% was the best value in predicting DFS. Ki67 ≥ 3% tumours were significantly associated with AC histology, stage III-IV, smoking, vascular invasion, tumour spread through air spaces OTP negativity, and TTF-1, Ascl1 and p53 positivity. After adjustment for center and period of diagnosis, both Ki-67 (≥3 versus <3) and histology (AC versus TC) alone significantly added prognostic information to OS and CSS multivariable model with age, stage and OTP; addition of both variables did not provide further prognostic information. Conversely, an improved significance of the DFS prediction model at multivariate analysis was seen by adding Ki-67 (≥3 versus <3, P adj = 0.01) to TC and AC histological distinction, age, lymph node involvement, residual tumour and OTP. Ki-67 ≥ 3% plays a potentially pivotal role in LCT prognosis, irrespective of histological grade. [ABSTRACT FROM AUTHOR]

Mesenchymal lesions of the breast are a heterogeneous range of entities with diverse clinical, histological, and molecular features, as well as biological behaviour. Their morphologic overlap with non‐mesenchymal lesions (such as metaplastic carcinoma and phyllodes tumour) and relative rarity also pose significant diagnostic challenges. In this review, we summarize the salient features of selected mesenchymal lesions of the breast, emphasizing those that are the most common and problematic. Vascular, fibroblastic/myofibroblastic, adipocytic, and smooth muscle lesions are each covered with regard to their clinicopathological features and differential diagnosis, while recent advances and the role of immunohistochemistry and molecular tests are also highlighted. [ABSTRACT FROM AUTHOR]

Breast pathology is an ever‐expanding database of information which includes markers, or biomarkers, that detect or help treat the disease as prognostic or predictive information. This review focuses on these aspects of biomarkers which are grounded in immunohistochemistry, liquid biopsies and next‐generation sequencing. [ABSTRACT FROM AUTHOR]