Recent revisions to acute leukemia classification systems increasingly emphasize genetic drivers of disease; however, critical immunophenotypic features remain essential for accurate diagnosis and risk assessment. CD56, an isoform of the neural cell adhesion molecule, is expressed on natural killer (NK) cells but is aberrantly present across multiple acute leukemias, where it may contribute to diagnostic complexity and adverse clinical outcomes. This review highlights the diagnostic and clinical significance of CD56 expression in acute leukemias. We review current classification frameworks and relevant literature to examine the role of CD56 immunophenotyping in the evaluation and clinical significance of acute leukemias. CD56 expression is observed in a heterogeneous group of acute leukemias, including subsets of acute myeloid leukemia (AML), NK lymphoblastic leukemia, acute undifferentiated leukemia, and blastic plasmacytoid dendritic cell neoplasm (BPDCN). In AML, CD56 expression has been associated with extramedullary disease, treatment resistance, and inferior survival in several studies, including high-risk subgroups such as AML with the RAM immunophenotype. In NK lymphoblastic leukemia and BPDCN, CD56 represents a key but nonspecific marker that must be interpreted in conjunction with additional lineage-associated antigens and molecular features to avoid diagnostic misclassification. Acute undifferentiated leukemia frequently demonstrates CD56 expression in the absence of definitive lineage markers, further emphasizing the need for integrated diagnostic approaches. Acute leukemias with CD56 expression represent a diagnostically challenging and biologically diverse group of neoplasms. While CD56 lacks lineage specificity, its detection by flow cytometry provides valuable information for differential diagnosis, risk stratification, and, in some settings, minimal residual disease assessment. Accurate classification requires careful integration of immunophenotypic, morphologic, and genetic data. As therapeutic strategies become increasingly risk-adapted and targeted, precise recognition of CD56-positive blastoid neoplasms is essential to optimize patient management and outcomes.

The presence or absence of muscularis propria (MP) invasion carries significant implications in bladder carcinoma treatment. Identification of MP is mainly based on morphology, with occasional application of immunohistochemical stains, including desmin, H-caldesmon, and smooth muscle actin (SMA), all aimed to stain smooth muscle. The specificity of these stains is imperfect. We evaluate the ancillary role of D2-40 in separating MP from desmoplastic reaction. We first examined immunoreactivity of muscularis mucosae and MP with the above markers in nonneoplastic specimens to establish their staining patterns. We then studied 33 cases of muscle-invasive urothelial carcinoma in which a desmoplastic reaction and MP were both present and clearly identifiable by routine hematoxylin and eosin morphology, with the goal of determining the sensitivity and specificity of these stains in identifying MP vs desmoplastic reaction. Both muscularis mucosae and MP stained uniformly positive for desmin, H-caldesmon, and SMA and lacked expression of D2-40. The desmoplastic reaction showed positivity with desmin in 30 cases (91%), H-caldesmon in 29 cases (88%), SMA in 33 cases (100%), and D2-40 in 33 cases (100%). Expression of desmin, H-caldesmon, and SMA identified MP with 100% sensitivity, but their specificity dropped to 9%, 12%, and 0%, respectively. In contrast, D2-40 demonstrated 100% sensitivity and specificity in distinguishing MP vs desmoplastic reaction. We suggest using an immunohistochemical panel composed of D2-40 and a smooth muscle marker, preferably desmin or H-caldesmon, to help distinguish muscularis propria from desmoplastic reaction when morphology alone is equivocal.

Castleman disease represents a heterogeneous group of hematologic conditions with a broad spectrum of histopathologic features. Historically, 2 major Castleman disease subgroups have been described-unicentric Castleman disease and multicentric Castleman disease, based on the extent of the disease. More recently, oligocentric Castleman disease, a subtype that falls between unicentric and multicentric, has been identified. Due to the overlapping features of each category, similarities to other conditions, and a lack of objective diagnostic biomarkers, the diagnosis of Castleman disease remains a challenge. Here, we review the diagnostic criteria of each entity and discuss the features that distinguish them from overlapping diseases. This review highlights the clinicopathologic features of multiple cases to depict the wide spectrum of clinical and laboratory characteristics of Castleman disease. It also outlines the diagnostic workup for each subtype. In Castleman disease, the morphologic findings are nonspecific and represent a histologic pattern rather than a pathognomonic feature that allows definitive separation from other morphologically overlapping entities; therefore, correlation with clinical history, radiologic studies, and laboratory findings is essential to arrive at the correct diagnosis. Castleman disease represents a spectrum of clinical syndromes with overlapping morphological features.

To identify molecular markers that predict advanced disease in mucin-producing cystic neoplasms of the pancreas. In this single-institution study, 454 pancreatic cystic fluid specimens were examined by a clinical next-generation sequencing assay. TP53 mutations were detected in 25 (7.9%) of 318 specimens harboring KRAS (BRAF or GNAS) mutation(s). Of the 25, 12 had advanced cytology/histology (positive group), and 5 did not have advanced cytology/histology (negative group). Eight cases were classified as indeterminate. Since KRAS and TP53 variant allele frequencies (VAFs) can be as low as 1% to 5%, we analyzed the TP53/KRAS VAF ratio and demonstrated a significantly higher VAF ratio in the positive group (P = .005). A VAF ratio of 1 or higher, interpreted as an indicator of a dominant TP53 clone, and concurrent TP53, CDKN2A, and SMAD4 mutations were seen only in the positive group. Lower TP53 ratios, interpreted as a minor TP53 clone, were seen in all 5 negative group specimens. These results suggest that VAF ratios and concurrent mutations can be incorporated into multimodality assessments of pancreatic cysts. Longitudinal studies in patients with a lower initial TP53 VAF ratio are warranted to elucidate whether serial VAF ratios are more accurate markers than a single VAF measurement.

We sought to investigate the frequency of diagnostic changes in hematopathology cases referred to the University of Michigan during a 3-year period and explore which parameters contribute to diagnostic change. Pathology reports from hematology patients who came to the University of Michigan for a second opinion from 2017 to 2019 were reviewed. Diagnostic discrepancies were classified into major or minor. Specimen type, hematopathology board certification and practice time of the outside pathologists, referring practice type, and whether the second review was done at the referring institution were recorded too. Agreement in diagnosis by the above-listed specimen characteristics was analyzed. A total of 2786 cases were reviewed (2016 bone marrow and 770 tissue specimens). Disagreements in diagnosis were found in 263 cases (9.4% of total cases), and 163 (5.9%) were major disagreements. Among the major disagreements, 119 (73%) were in bone marrow specimens and 44 (27%) in tissue specimens. Among bone marrows, the most common revisions were myeloid neoplasm reclassifications (35.3%), whereas lymphoma subtype revisions comprised 70.4% of all changes in tissues. Univariate analysis showed that major disagreement rates were significantly higher in cases signed out by pathologists without hematopathology certification, those practicing for more than 10 years, and in cases from nonacademic institutions. When analyzing bone marrows and tissues separately, these differences remained significant only for bone marrows. Second review of pathology material serves as an important quality assurance and patient safety measure. Lack of hematopathology training of the referring pathologists may contribute to the rate of diagnostic discrepancy.

Abstract Introduction/Objective CML blast phase (CML-BP) is characterized by > 20% blasts in blood, bone marrow, or extramedullary sites. Although tyrosine kinase inhibitors (TKIs) have reduced progression, resistance to TKIs remains a key driver of CML-BP and is associated with atypical blast morphologies beyond myeloblastic and lymphoblastic types. Methods/Case Report This case report details a rare presentation of CML-BP in a 63-year-old woman on dasatinib, who presented with elevated WBC count of 73.4 x 109/L. Peripheral smear revealed panleukocytosis with profound basophilia(53%) and a mixed population of pronormoblasts and basophilic precursors. Her hypercellular bone marrow showed immature mononuclear cells negative for CD34 and MPO, with E-cadherin, CD71, and CD117 highlighting pronormoblasts. CD117 also marked basophilic precursors, which aberrantly expressed CD7. Flow cytometry revealed markedly increased basophils (44%) with an aberrant immunophenotype. Cytogenetic studies confirmed BCR::ABL1 translocation, complex karyotypic abnormalities, including monosomies of chromosomes 7 and 17 and trisomy 8. NGS studies identified ABL1 V299L mutation which confers resistance to dasatinib. Results NA Conclusion This case underscores the potential for atypical blast types in CML-BP under TKI therapy. Basophilic blast crisis is an extremely rare manifestation of CML. Additionally, this case also highlights the impact of TP53 loss via monosomy 17 on erythroid proliferation.

Abstract Introduction/Objective Histiocytic sarcoma (HS) is a rare hematolymphoid malignancy, accounting for less than 1% of all hematologic neoplasms. While most cases arise de novo, rare instances of HS have been reported as transformations from pre-existing B-cell lymphomas, particularly follicular lymphoma (FL). These transformations, though uncommon, illustrate lineage plasticity and are supported by molecular studies demonstrating clonal relationships between the original FL and subsequent HS. Transformation can occur metachronously, synchronously, or even within the same lymph node. The pathogenesis of this phenomenon is not fully understood but is thought to involve direct transdifferentiation, dedifferentiation followed by reprogramming, or evolution from a common progenitor clone. Epigenetic reprogramming and activation of the MAPK/ERK pathway have been implicated. Clinically, transformation often presents with rapid disease progression and extranodal involvement. Diagnosis requires careful histopathologic and immunophenotypic evaluation, often supplemented by molecular testing. HS can mimic other neoplasms such as myeloid sarcoma, adding to diagnostic complexity. Recognition of this rare transformation is critical, as HS carries a poor prognosis and requires intensive treatment strategies. We present a case of histiocytic sarcoma in a patient with a history of follicular lymphoma, with molecular studies confirming a clonal relationship between the two neoplasms. Methods/Case Report A 56-year-old man with a history of follicular lymphoma presented with an enlarged tonsil, lymphadenopathy, and a rapidly growing cervical mass. Imaging raised clinical suspicion for disease progression with transformation to diffuse large B-cell lymphoma (DLBCL). Tonsillar biopsy showed small atypical lymphoid cells in a nodular pattern, positive for CD20, PAX5, CD10, BCL6, and BCL2, and a low Ki67 proliferation index consistent with persistent low-grade follicular lymphoma. Due to ongoing concern for transformation, a cervical lymph node biopsy was performed. This revealed large, pleomorphic cells with amphophilic cytoplasm, negative for multiple B-cell markers (CD19, CD20, CD22, PAX5, CD79a). Additional immunohistochemical studies showed positivity for CD163, CD68, CD4, and lysozyme, and negativity for CD34, CD117, and myeloperoxidase, supporting a diagnosis of histiocytic sarcoma. FISH analysis detected t(14;18) in both the follicular lymphoma and histiocytic components, and clonality studies showed identical IGH gene rearrangements, establishing a clonal relationship and supporting transdifferentiation of follicular lymphoma to histiocytic sarcoma. Next-generation sequencing is underway to identify actionable mutations for potential future targeted therapies. Results NA Conclusion This case highlights a rare but clinically significant transformation of FL into HS, confirmed by molecular evidence of clonal relatedness. HS is an exceptionally rare malignancy with an aggressive clinical course and poor prognosis, even with intensive therapy. Diagnosis requires a high index of suspicion, especially in patients with a history of indolent B-cell lymphoma who present with rapid clinical progression or atypical extranodal disease. Management of HS remains challenging due to the absence of standardized treatment protocols. Current approaches often involve a combination of surgical resection and aggressive chemotherapy, although outcomes are generally poor. In cases where HS arises from FL, therapies typically used for B-cell lymphomas may offer clinical benefit. A case report demonstrated efficacy of rituximab and bendamustine, achieving durable remission in patient with HS transformed from FL. Establishing a clonal relationship between HS and the antecedent lymphoid neoplasm is not only diagnostically valuable but may also inform therapeutic decisions. This case underscores the importance of integrating clinical, histopathologic, immunophenotypic, and molecular data to accurately diagnose such cases and guide management in this rare and diagnostically challenging transformation.

Abstract Clinicians at our institution can request esoteric tests not included in the standard test menu through a miscellaneous order process. While this flexibility allows access to specialized testing, it also introduces possibilities of errors. To address these challenges, a trainee-led review process of miscellaneous requests for chemistry tests was established to systematically assess these requests. Since beginning the process, we have slowly expanded the review to include other disciplines of testing. With now over thirty months of data, we evaluated the effectiveness of the program and how it is changed since initialization. We conducted a retrospective analysis of miscellaneous test request reviews over thirty months. The review process was conducted using our laboratory information system (Epic Beaker) to flag tests for review after specimen collection. Pathology residents reviewed the flagged tests daily and logged them into a shared, secure Excel-based tracker. Residents consulted with medical directors from the relevant laboratory sections for guidance as needed, and a medical director conducted a final daily review to complete the process. Additionally, residents met with a medical director weekly to discuss test requests and evaluate the review workflow. Since establishment of the review process, 4,766 tests requests have been reviewed, increasing from 312 in the first quarter of the program to 569 in the most recent completed quarter due to gradual expansion of the review into non-chemistry disciplines. Requests originate from internal (61.3%) and external/client providers (38.7%). As expected, chemistry tests account for 89.4% of requests; the two next largest categories are microbiology (6.4%) and immunology (3.6%). Of all reviewed requests, 80.6% were approved without modification, while 5.2% were canceled due to redundancy or inappropriateness. An additional 11.7% were converted to a defined in-house test, 1.7% were redirected to an alternative reference laboratory, and 0.8% required modifications to the test ordered. Interventions in all categories except approvals increased over time, reflecting improved recognition of inappropriate or misrouted tests. The median time spent per review was 1 minute, with complex interventions, such as order modifications and redirections, requiring additional time (∼30 minutes). Expanding the scope of the review provided further insights into provider ordering patterns and opportunities for test stewardship initiatives. This review highlights the continued effectiveness of a trainee-led miscellaneous test request review process in optimizing test selection and utilization. The findings support ongoing efforts to engage trainees in laboratory stewardship initiatives to enhance efficiency, reduce inappropriate testing, and improve patient care. Additionally, this process has served as an educational tool, improving residents’ knowledge of esoteric testing and its appropriate use. To further strengthen this initiative, we plan to continue to expand this review process to other laboratory discipline, allowing residents to develop specialized expertise and contribute more effectively to test stewardship.