Evaluation of Turnaround Times of Diagnostic Biopsies: A Metric of Quality in Surgical Pathology.

Introduction. Timely and accurate diagnosis of diseases is crucial for effective patient care. Turnaround time (TAT) in surgical pathology, defined as the time between accessioning the sample and reporting results, is a key performance indicator reflecting quality and efficiency. This study explores factors affecting TAT for diagnostic biopsies in a tertiary oncology hospital. Methods. A 1-month pilot study was conducted, focusing on 695 in-house diagnostic biopsies. Biopsies were categorized as routine (requiring only hematoxylin and eosin (H&E) staining) or complex cases (requiring additional tests). TAT was defined as the time between sample accessioning and report availability in the electronic medical record, with delays defined as exceeding 3 days for routine cases and 4 days for complex cases. Survival analysis using Kaplan-Meier plots was utilized to analyze TAT. Results. The overall mean TAT was 3.7 ± 2 days, with routine cases at 3.1 ± 2 days and complex cases at 4.8 ± 2 days (P < 0.001). Survival analysis revealed prolonged TAT for complex cases. Organ-specific analysis highlighted variations in TAT, with brain biopsies presenting the highest complexity and longest TAT. Surprisingly, malignant cases demonstrated slightly shorter TATs compared to benign cases (P = 0.026). Delays were observed in 34% of all cases. Conclusions. Laboratory TAT is crucial and is frequently used as a performance benchmark. We analyzed the various causes of delayed TAT in our hospital's histopathology department, with an emphasis on variables in the analytical phase. The results of this study demonstrate that cases involving ancillary techniques had significantly longer TATs compared to routine H&E cases.

Jonas Friedenwald, Eugene Wolff, and the Two Books That Helped Bridge the Specialty Gap.

Background: Eugene Wolff (1896-1954) and Jonas S. Friedenwald (1897-1955) were life-long students and educators of anatomic pathology and ophthalmology. Both contributed toward narrowing the gap between the two rapidly diverging specialties of pathology and ophthalmology. Friedenwald in 1929 and Wolff in 1934 each published textbooks of ophthalmic pathology that influenced medical education for decades to come. Summary: Friedenwald's The Pathology of the Eye and Wolff's A Pathology of the Eye introduced ophthalmologists in training and practice to anatomic pathology, while familiarizing pathologists with the nature of ocular disease. Both books appeared at the time when anatomic pathology was departing from its mostly academic roles in education and research to assume more active participation in clinical care by establishing diagnoses through biopsy. Key Messages: Wolff and Friedenwald dedicated their careers to teaching the art and science of anatomic pathology to clinical ophthalmologists. Their efforts helped anchor ophthalmology to the traditions of mainstream medicine.

A Comparison of Diagnostic and Immunohistochemical Workup and Literature Review Capabilities of Online Artificial Intelligence Assistance Models in Pathology.

Artificial intelligence (AI) is a suite of technologies that enables computers to learn and interpret information like human cognition. It has found applications across various fields, including healthcare, agriculture, astronomy, navigation, and robotics. Within healthcare, AI has the potential to enhance diagnostic accuracy, facilitate drug research, and automate patient experiences. This comparative study focuses on the proficiency of AI in generating accurate differential diagnoses in the field of pathology. Six medical vignettes were crafted, and each scenario was then input into three different AI platforms. The pathologist reviewed and determined the most accurate AI model.

Index tumor location affected early biochemical recurrence after radical prostatectomy in patients with negative surgical margin: a retrospective study.

BACKGROUND: Index tumors are the most aggressive tumors of the prostate. However, their clinical significance remains unclear. This study aimed to assess the incidence of index tumor location according to the zonal origin and whether these locations affect the prognosis after radical prostatectomy in patients with negative surgical margins. METHODS: This single-centered, retrospective study evaluated 1,109 consecutive patients who underwent radical prostatectomies. An index tumor was defined as the largest tumor in the prostate gland. We detected these locations based on McNeal's zonal origin using whole-mount sections. Biochemical recurrence (BCR) free survival curves were generated using the Kaplan-Meier method. Univariate and multivariate analyses using the Cox proportional hazards model were performed to determine the predictive factors for early BCR (within 1-year). RESULTS: A total of 621 patients with negative surgical margins who did not receive adjuvant therapy were included in this study. The index tumor were located in the transitional zone in 191 patients (30.8%), the peripheral zone in 399 patients (64.3%), and the central zone in 31 patients (5.0%). In total, 22 of 621 patients (3.5%) experienced early BCR and 70 patients (11.2%) experienced overall BCR at a median follow-up of 61.7 months. According to the index tumor location, the early BCR-free rates were 99.5%, 95.7 %, and 83.3% in the transitional, peripheral, and central zones, respectively. On multivariate analysis, the index tumor in the central zone was an independent predictor of early BCR with negative surgical margins following radical prostatectomy, followed by prostatectomy pathological grade, index tumor in the peripheral zone, and high prostate-specific antigen level. CONCLUSIONS: We assessed the significance of index tumor location in patients with negative surgical margins following radical prostatectomy. Index tumors located in the central zone, although infrequent, were the strongest predictive factors for early BCR. Our results may allow urologists and patients to reconsider the therapeutic strategies for prostate cancer.

Colitis Cystica Profunda of the Hepatic Flexure: A Case Report.

A 72-year-old woman with a prior sigmoid resection for colon cancer underwent a right hemicolectomy after a colonoscopy revealed a mass in the hepatic flexure. A preoperative biopsy at colonoscopy showed tubulovillous dysplasia with high-grade neoplasm. The final specimen pathology revealed benign mucosal elements with mucin pools consistent with colitis cystica profunda (CCP). CCP is a benign lesion; no further treatment was necessary after resection. To our knowledge, this is the first reported case of CCP in the right colon, presenting atypically in the hepatic flexure. This case report brings to light the difficulty and importance of making an accurate diagnosis of CCP.

Reducing reflex first-line prescriptions in a surgical pathology laboratory: toward sustainable practice with positive economic and clinical effects.

In surgical pathology departments, reflex first-line techniques (RFLTs) are aimed at reducing workloads and addressing recent shortages of medical personnel. However, the impacts thereof on economic and diagnostic factors have been poorly addressed. Also, in the era of global warming, environmental considerations are crucial. This study assessed the economic and diagnostic efficacies of routine pathological RFLT and the quality of care and sustainability. Ten RFLTs of the Bordeaux University Hospital pathology department (six special stains, one cytology technique, and three immunohistochemical tests) were studied. First, a retrospective economic analysis evaluated the average cost of these RFLTs per slide and per year. Second, diagnostic relevance was prospectively surveyed. Third, the effects of changes made were analyzed over 2 years. The ten RFLTs were associated with average annual costs of €46,708. Diagnostic relevance analysis indicated that most stains were unnecessary; only 17% were requested as second-line techniques. Elimination of 7/10 tests afforded annual cost savings of €22,522 and reduced the workload by 5568 tests/year, without compromising the workflow or diagnostic quality. Seven of ten RFLTs could be eliminated without compromising diagnostic quality or the workflow. This afforded not only financial benefits but also positive social and environmental impacts. We offer valuable insights into appropriate practices in surgical pathology laboratories. Collaboration between the medical and technical teams was crucial; other healthcare sectors would also benefit from our approach.

Amendments in surgical pathology reports: An 8-year institutional experience.

Surgical pathology reports may undergo revisions broadly categorized as addenda (supplementary information) or amendments (changes to finalized reports). Amendments indicate potential flaws in the diagnostic process and serve as important indicators of vulnerabilities in the histopathology workflow. This study analyzed the frequency and distribution of amendments in surgical pathology reports over 8 years to identify patterns highlighting opportunities for improvement. Surgical biopsies, excisions, and resections were included; cytology and molecular tests were excluded. Amended reports were categorized using previously used taxonomy documented in literature. Defects were classified as misinterpretations, misidentifications, defective specimens, or defective reports. Of 101,355 reports, 155 (0.15 %) were signed out with amendments. The amendment rate was approximately 1-2 cases per 1000 reports annually. Misinterpretations accounted for the majority (52 %) of amended reports, with undercalls (62 %) and overcalls (27 %) being predominant subtypes. Tumor staging was amended in 57 (37 %) cases, with 30 being upstaged and 11 downstaged clinically. The highest number of misinterpretation defects occurred in head and neck (36 %) and breast (21 %) specimens. Misinterpretation defects were present in 53 % of malignant cases versus 42 % of benign cases. In 18 cases, there were significant changes in pathological diagnosis (14 major and 4 minor). A standard taxonomy categorizing report defects is crucial for measuring and improving quality control. Accurate pathology reporting impacts patient care and guides workflow improvements. This taxonomy enables us to track variations and deficiencies in our pathology reporting processes in a reproducible way across the department.

Tumour budding as a prognostic biomarker in biopsies and resections of neoadjuvant-treated rectal adenocarcinoma.

BACKGROUND: Tumour budding (TB) is a marker of tumour aggressiveness which, when measured in rectal cancer resection specimens, predicts worse outcomes and response to neoadjuvant therapy. We investigated the utility of TB assessment in the setting of neoadjuvant treatment. METHODS AND RESULTS: A single-centre, retrospective cohort study was conducted. TB was assessed using the hot-spot International Tumour Budding Consortium (ITBCC) method and classified by the revised ITBCC criteria. Haematoxylin and eosin (H&E) and AE1/AE3 cytokeratin (CK) stains for ITB (intratumoural budding) in biopsies with PTB (peritumoural budding) and ITB (intratumoural budding) in resection specimens were compared. Logistic regression assessed budding as predictors of lymph node metastasis (LNM). Cox regression and Kaplan-Meier analyses investigated their utility as a predictor of disease-free (DFS) and overall (OS) survival. A total of 146 patients were included; 91 were male (62.3%). Thirty-seven cases (25.3%) had ITB on H&E and 79 (54.1%) had ITB on CK assessment of biopsy tissue. In univariable analysis, H&E ITB [odds (OR) = 2.709, 95% confidence interval (CI) = 1.261-5.822, P = 0.011] and CK ITB (OR = 2.165, 95% CI = 1.076-4.357, P = 0.030) predicted LNM. Biopsy-assessed H&E ITB (OR = 2.749, 95% CI = 1.258-6.528, P = 0.022) was an independent predictor of LNM. In Kaplan-Meier analysis, ITB identified on biopsy was associated with worse OS (H&E, P = 0.003, CK: P = 0.009) and DFS (H&E, P = 0.012; CK, P = 0.045). In resection specimens, CK PTB was associated with worse OS (P = 0.047), and both CK PTB and ITB with worse DFS (PTB, P = 0.014; ITB: P = 0.019). In multivariable analysis H&E ITB predicted OS (HR = 2.930, 95% CI = 1.261-6.809) and DFS (HR = 2.072, 95% CI = 1.031-4.164). CK PTB grading on resection also independently predicted OS (HR = 3.417, 95% CI = 1.45-8.053, P = 0.005). CONCLUSION: Assessment of TB using H&E and CK may be feasible in rectal cancer biopsy and post-neoadjuvant therapy-treated resection specimens and is associated with LNM and worse survival outcomes. Future management strategies for rectal cancer might be tailored to incorporate these findings.

Applications of Large Language Models in Pathology.

Large language models (LLMs) are transformer-based neural networks that can provide human-like responses to questions and instructions. LLMs can generate educational material, summarize text, extract structured data from free text, create reports, write programs, and potentially assist in case sign-out. LLMs combined with vision models can assist in interpreting histopathology images. LLMs have immense potential in transforming pathology practice and education, but these models are not infallible, so any artificial intelligence generated content must be verified with reputable sources. Caution must be exercised on how these models are integrated into clinical practice, as these models can produce hallucinations and incorrect results, and an over-reliance on artificial intelligence may lead to de-skilling and automation bias. This review paper provides a brief history of LLMs and highlights several use cases for LLMs in the field of pathology.

Frequency of Clinically Significant Findings in the Surgical Pathology Specimen Following Laparoscopic Sleeve Gastrectomy and Concordance with Preoperative Endoscopy: Insights from a Large Single-Center Experience.

INTRODUCTION: Endoscopy prior to bariatric surgery is not always performed, and in sleeve gastrectomy (SG), the surgical specimen is not always sent for pathological examination. There is limited data on the frequency of clinically significant findings in SG specimens or correlation with preoperative endoscopy. METHODS: We reviewed 426 consecutive SG patients to determine the concordance of preoperative endoscopy findings in patients with clinically significant postoperative pathology. RESULTS: Preoperative endoscopy was performed on 397 patients (93.2%). Three hundred seventy-three patients had preoperative endoscopy and surgical pathology results available. Then, 20/373 (5.4%) patients had potentially significant postoperative pathology, including intestinal metaplasia, autoimmune metaplastic atrophic gastritis (AMAG), gastrointestinal stromal tumors, and/or gastric cancer. The overall incidence of AMAG in the entire cohort was 2.3%. Preoperative gastric biopsies (to include gastric body) identified AMAG in nearly 1/2 of patients. Patients with clinically significant postoperative pathology results had a median [interquartile range] of 3 [3-5] tissue blocks examined as compared to 3 [1-3] for the remainder of the cohort (p < 0.001). CONCLUSION: This is one of the largest studies describing clinically significant postoperative pathology after SG. AMAG, in particular, is of particular importance as it is associated with a 3-fivefold increase in risk for gastric cancer. The incidence of significant postoperative pathology in this population is small but potentially clinically significant and requires validation in larger studies. We recommend wider sampling in preoperative endoscopy (body and antrum), especially in patients being planned for gastric bypass, consideration for routine pathological examination of SG surgical specimens, with careful gross examination and targeted sampling.

The carbon footprint of a surgical pathology laboratory in France.

OBJECTIVES: The health sector contributes to climate disruption through greenhouse gas (GHG) emissions. It accounts for 8% to 10% of France's GHG emissions. Although the medical community has been alerted to the problem, more data are needed. This study aimed to determine the carbon footprint of a surgical pathology laboratory. METHODS: The study was conducted in the surgical pathology laboratory at Saint Vincent hospital (Lille) in 2021. It represented 17,242 patient cases corresponding to 54,124 paraffin blocks. The 17 staff members performed cytology, immunohistochemistry, and in situ hybridization. The study included all inputs, capital equipment, freight, travel, energy consumption, and waste. Carbon emission factors were based on the French Agence De l'Environnement et de la Maîtrise de l'Energie database. RESULTS: In 2021, the pathology laboratory's carbon footprint was 117 tons of CO2 equivalent (t CO2e), corresponding to 0.5% of Saint Vincent hospital's total emissions. The most significant emissions categories were inputs (60 t CO2e; 51%), freight associated with inputs (24 t CO2e; 20%), and travel (14 t CO2e; 12%). Waste and energy generated 10 t CO2e (9%) and 9 t CO2e (8%), respectively. CONCLUSIONS: The pathology laboratory's carbon footprint was equivalent to the yearly carbon impact of 11 French inhabitants. This footprint is dominated by inputs and associated freight. This suggests an urgent need to develop ecodesign and self-sufficiency in our routine practices.

Topline/Final Diagnostic Inclusion of Relevant Histologic Findings in Surgical Pathology Reporting of Carcinoma in Prostate Biopsies.

INTRODUCTION: As the list of histologic parameters to include in surgical pathology reports of prostate cancer biopsies grows, some pathologists include this information in the microscopic description or summary sections of the report, whereas others include it in the "topline" or final diagnosis section. This prompted us to develop a multi-institutional survey to assess reporting trends among genitourinary (GU) pathologists. METHODS: A survey instrument was shared among 110 GU pathologists via surveymonkey.com. Anonymized respondent data was analyzed. RESULTS: Eighty-four (76%) participants completed the survey across four continents. Most participants report tumor volume quantitation (88%), number of cores involved (89%), and both Gleason grade and Grade group (93%) in their topline; 71% include percent of pattern 4, with another 16% including it depending on cancer grade; 58% include the presence of cribriform growth pattern 4, with another 11% including it depending on cancer grade. When present, most include extraprostatic extension (90%), prostatic intraductal carcinoma (77%), and perineural invasion (77%). Inclusion of atypical intraductal proliferation (AIP) in the topline diagnosis was cancer grade-dependent, with 74% including AIP in Grade group 1, 61% in Grade group 2, 45% in Grade group 3, 30% in Grade group 4, and 26% in Grade group 5 cancers. CONCLUSION: Certain histologic features such as Gleason grade and tumor volume/cores involved are frequently included in the topline diagnosis, whereas the incorporation of other findings are more variably included. Prostate biopsy reporting remains a dynamic process with stylistic similarities and differences existing among GU pathologists.

Role of sentinel lymph node evaluation during hysterectomy for preoperative pathology diagnosis of endometrial intraepithelial neoplasia in a community hospital setting.

OBJECTIVE: To determine the utility of sentinel lymph node (SLN) evaluation during hysterectomy for endometrial intraepithelial neoplasia (EIN) in a community hospital setting and identify descriptive trends among pathology reports from those diagnosed with endometrial cancer (EC). METHODS: We reviewed patients who underwent hysterectomy from January 2015 to July 2022 for a pathologically confirmed diagnosis of EIN obtained by endometrial biopsy (EMB) or dilation and curettage. Data was obtained via detailed chart review. Statistical testing was utilized for between-group comparisons and multivariate logistic regression modeling. RESULTS: Of the 177 patients with EIN who underwent hysterectomy during the study period, 105 (59.3%) had a final diagnosis of EC. At least stage IB disease was found in 29 of these patients who then underwent adjuvant therapy. Pathology report descriptors suspicious for cancer and initial specimen type obtained by EMB were independently and significantly associated with increased odds of EC diagnosis (aOR 8.192, p < 0.001;3.746, p < 0.001, respectively). Operative times were not increased by performance of SLN sampling while frozen specimen evaluation added an average of 28 min to procedure length. Short-term surgical outcomes were also similar between groups. CONCLUSION: Patients treated for EIN at community-based institutions might be more likely to upstage preoperative EIN diagnoses and have an increased risk of later stage disease than previous research suggests. Given no surgical time or short-term outcome differences, SLN evaluation should be more strongly considered in this practice setting, especially for patients diagnosed by EMB or with pathology reports indicating suspicion for EC.

Virtual Staining of Nonfixed Tissue Histology.

Surgical pathology workflow involves multiple labor-intensive steps, such as tissue removal, fixation, embedding, sectioning, staining, and microscopic examination. This process is time-consuming and costly and requires skilled technicians. In certain clinical scenarios, such as intraoperative consultations, there is a need for faster histologic evaluation to provide real-time surgical guidance. Currently, frozen section techniques involving hematoxylin and eosin (H&E) staining are used for intraoperative pathology consultations. However, these techniques have limitations, including a turnaround time of 20 to 30 minutes, staining artifacts, and potential tissue loss, negatively impacting accurate diagnosis. To address these challenges, researchers are exploring alternative optical imaging modalities for rapid microscopic tissue imaging. These modalities differ in optical characteristics, tissue preparation requirements, imaging equipment, and output image quality and format. Some of these imaging methods have been combined with computational algorithms to generate H&E-like images, which could greatly facilitate their adoption by pathologists. Here, we provide a comprehensive, organ-specific review of the latest advancements in emerging imaging modalities applied to nonfixed human tissue. We focused on studies that generated H&E-like images evaluated by pathologists. By presenting up-to-date research progress and clinical utility, this review serves as a valuable resource for scholars and clinicians, covering some of the major technical developments in this rapidly evolving field. It also offers insights into the potential benefits and drawbacks of alternative imaging modalities and their implications for improving patient care.

[The errors in intravital pathologicoanatomic diagnostic: a publications review].

The review presents analysis of scientific publications considering medical errors in intravital pathologicoanatomic diagnostic. The examples of classification, rate of diagnostic errors and possible ways of decreasing the number of diagnostic errors in pathological anatomy are considered.

Synchronous Pelvic Schwannoma With Metastatic Prostate Cancer: A Rare Case and Pathology Review.

Schwannomas are benign tumors arising from well-differentiated Schwann cells of peripheral nerves. They are usually found on the limbs, head, and neck. It is uncommon for schwannoma to occur in the pelvis and when it does, it is often diagnosed late. Pelvic schwannoma when diagnosed are often bigger in size (>5 cm) and may present with local symptoms such as constipation and bladder outlet obstruction. We hereby present a patient with concurrent metastatic prostate carcinoma and pelvic schwannoma. The patient is a 57-year-old man initially diagnosed with prostate cancer and was lost to follow-up. One year later, he presented with metastatic prostate disease and bladder outlet obstruction. Further evaluation revealed a concurrent pelvic mass that was increasing in size. The biopsy of this mass was suggestive of schwannoma. It was decided at the multidisciplinary tumor board conference to offer treatment for his metastatic prostate disease and observe the schwannoma. His obstructive symptoms worsened in the face of clinical evidence of regression of his prostatic disease, and it was decided to resect the pelvic mass. The surgery revealed a huge soft tissue mass within the pelvis that was adherent to the bladder, prostate, and rectum. Morphology and immunohistochemistry studies of the pelvic mass confirmed the diagnosis of ancient schwannoma. We hereby highlight the clinical importance of this presentation and the diagnostic and therapeutic dilemma involved in the management of this patient who presented with two pathologic conditions causing similar symptoms but of different prognostic and therapeutic significance.

Pathology focused review of morphologic subtypes and molecular variants of urothelial carcinoma with an emphasis on clinical/treatment relevance.

Urothelial carcinoma (UC) has significant morbidity, mortality, and remains the most financially costly carcinoma to manage and treat. This review will cover special morphologic features of UC that may be noted by the pathologist and any subsequent significance in terms of clinical management or treatment considerations as mentioned or recommended in the latest WHO 2022 classification of GU tumors. Many important potentially therapy altering morphologic findings can be consistently identified and reported on routine microscopic examination of hematoxylin and eosin (H&E) stained slides. Furthermore, there has been a rapid advancement of molecular diagnostics and tailored therapies throughout oncology, and we will briefly highlight some of these as they relate to the management of UC. We will actively attempt to limit the discussion of histologic descriptions or pathologic diagnostic criteria of these entities and focus rather on the recognition of their importance/implication for clinicians who must make clinical management decisions based upon these findings. Finally, the importance of open lines of communication with the pathologists who review clinical specimens as well as their practice and reporting methods cannot be overstated.

Small Intestine Gastrointestinal Clear Cell Sarcoma: A Case Report and Review of the Literature.

Gastrointestinal clear cell sarcoma (GICCS)/malignant gastrointestinal neuroectodermal tumor (GNET) is an extremely rare form of cancer with aggressive clinical behavior. It has distinct pathological, immunohistochemical, ultrastructural, and molecular features. Herein, we present the case of a 20-year-old woman with no notable medical history who presented to the outpatient department with complaints of abdominal pain and vomiting. Symptoms had been evolving for 3 months. The physical examination revealed slight abdominal tenderness and melena. Biological investigations revealed iron-deficiency anemia. The upper and lower endoscopies showed no abnormalities. Magnetic resonance enterography revealed small bowel wall thickening of 15 mm × 2 mm. Exploratory laparotomy revealed an ileal mass with mesenteric lymphadenopathy. A wide resection of the mass was then performed. The final pathological report confirmed the diagnosis of small bowel GICCS/GNET. After 11 months of follow-up, the patient presented with mesenteric lymph node metastases.

Acetone compression improves lymph node yield and metastasis detection in colorectal cancer.

Lymph node status is one of the most important prognostic factors in colorectal cancer, and accurate pathological nodal staging and detection of lymph node metastases is crucial for determination of post-operative management. Current guidelines, including the TNM staging system and European Society for Medical Oncology (ESMO) guidelines, recommend examination of at least 12 lymph nodes. However, identification of an adequate number of lymph nodes can be challenging, especially in the setting of neoadjuvant treatment, which may reduce nodal size. In this study, we investigated 384 colorectal cancer resections that were processed at our department of pathology between January 2012 and December 2022, in which the number of detected lymph nodes was less than 12 subsequent to conventional preparation of mesocolic fat tissue. By means of acetone compression, lymph node harvest increased significantly (p < 0.0001), and the intended number of ≥ 12 lymph nodes was achieved in 98% of resection specimens. The number of nodal positive cases increased significantly from n = 95 (24.7%) before versus n = 131 (34.1%) after acetone compression due to additionally identified lymph node metastases (p < 0.001). In 36 patients (9.4%) initially considered as nodal negative, acetone compression led to a staging adjustment to a nodal positive category and thereby drove a recommendation to offer post-operative therapy. In conclusion, acetone compression is a reliable and useful method implementable in routine surgical pathology for the retrieval of lymph nodes in colorectal cancer specimen, allowing for an adequate lymph node sampling and an increase in nodal staging reliability.

TLE1 Expression in NUT Carcinoma: A Case Report Highlighting a Potential Diagnostic Pitfall for the Pathologist.

NUT carcinoma is a rare, aggressive malignancy defined as a carcinoma with a chromosomal rearrangement affecting the nuclear protein in testis (NUTM1) gene. This small round blue cell tumor classically exhibits focal abrupt keratinization and immunohistochemical positivity for keratin and squamous markers. However, keratinization is not always present and reports of positivity for other markers that may obscure the diagnosis are increasing. It is also noteworthy that gene fusions involving NUTM1 are not restricted to NUT carcinoma. Herein, we report a NUT carcinoma arising in the mediastinum of a male patient in his 40 s with morphological and immunohistochemical overlap with Ewing family sarcoma and poorly differentiated synovial sarcoma given a round cell morphology, diffuse strong immunoreactivity for CD99, and patchy strong immunoreactivity for TLE1. Squamous differentiation by morphology and p40 expression were notably absent in this case. Classification as NUT carcinoma was ultimately possible when the morphological and immunohistochemical findings were considered in the context of a BRD4::NUTM1 gene fusion identified by next-generation sequencing. While the patient initially responded to palliative radiotherapy, he died approximately one month later. To our knowledge, this is the first report of TLE1 immunoreactivity in NUT carcinoma. This case highlights a potential diagnostic pitfall and emphasizes the need for molecular confirmation in equivocal situations.