OBJECTIVES:Diagnosing adult T-cell leukemia/lymphoma (ATLL) remains challenging due to its clinical and histopathologic heterogeneity. While HTLV-1 serology is a key diagnostic criterion, it is often unavailable at the time of diagnosis and does not confirm viral integration in tumor cells. The HTLV-1-encoded HBZ RNA is a reliable diagnostic biomarker for ATLL. We evaluated the diagnostic performance of RNAscope in situ hybridization (ISH) for HBZ RNA in formalin-fixed, paraffin-embedded (FFPE) tissues to distinguish ATLL from other T-cell lymphomas. METHODS:HBZ ISH was evaluated in 72 cases, including 52 ATLLs and 20 non-ATLL T-cell lymphomas, to determine diagnostic sensitivity and specificity, interobserver agreement, interlaboratory reproducibility, and comparison with digital gene expression. RESULTS:This assay demonstrated 86.5% sensitivity and 100% specificity, with sensitivity improving to 97% when excluding bone marrow clot sections and FFPE blocks older than 5 years. Interobserver agreement among 3 pathologists was excellent (κ = 0.92, percent agreement: 94%). Interlaboratory reproducibility of HBZ ISH was high, with 93% to 95% concordance across these Clinical Laboratory Improvement Amendments-certified laboratories. HBZ RNA ISH-positive cases demonstrated significantly higher HBZ gene expression levels by digital gene expression profiling. CONCLUSIONS:We established HBZ ISH as a reliable tissue-based surrogate for HTLV-1 infection in ATLL. This assay will enhance diagnostic accuracy, expedite definitive diagnosis, and improve patient selection for targeted therapies and clinical trials, particularly in regions with a high prevalence of HTLV-1.

Rosai-Dorfman disease is an uncommon histiocytic neoplasm that is often self-limiting; however, rarely, it exhibits aggressive behavior and may undergo biological transformation to histiocytic sarcoma. Pathogenic genetic drivers of Rosai-Dorfman disease include recurrent genetic abnormalities in KRAS, NRAS, ARAF, and MAP2K1. Mutation of BRAF is a well-known oncogenic driver in histiocytic neoplasms, including up to 50% of Langerhans cell histiocytosis lesions, Erdheim-Chester disease, and adult and juvenile xanthogranuloma. However, BRAF mutations have been documented in only 4 reported Rosai-Dorfman disease specimens, and none were rearrangements. Herein, we report the first instance of Rosai-Dorfman disease with OSBPL11::BRAF gene rearrangement, a unique gene fusion, and describe its clinicopathological features.

The treatment of breast cancer is largely determined by protein expression assays of estrogen receptor, progesterone receptor, and Her2/neu (HER2) status. These prognostic markers may vary due to tumor heterogeneity or the evolution of prognostic markers throughout the course of treatment. This report presents a case of a patient who initially presented with HER2-negative breast cancer and had rapidly progressed on numerous lines of treatment. An analysis of cerebrospinal fluid via next-generation sequencing and biopsy of metastasis to the liver identified HER2-positive cancer, which allowed for the use of trastuzumab deruxtecan, a HER2-targeted therapy. This led to an excellent clinical response with improvement in performance status and quality of life. This case report demonstrates the importance of continuing to follow a patient's cancer pathology to open the doors for other opportunities for treatment. Cancer has the potential to evolve and there is a benefit of obtaining rebiopsies to ensure the correct targeted therapies are provided to the patient.

OBJECTIVES:Large B-cell lymphomas (LBCLs) are a heterogeneous group of lymphoid neoplasms whose molecular and cytogenetic profile has predictive and prognostic implications. The concept of double-hit lymphomas (DHLs) was recently updated in the fifth edition of the World Health Organization classification, with the exclusion of MYC and BCL6 rearranged tumors from the group. Now, DHLs are referred to as diffuse large B-cell lymphoma/high-grade B-cell lymphoma with MYC and BCL2 rearrangements. Fluorescence in situ hybridization (FISH) is the current gold standard for detecting rearrangements in LBCLs, but comprehensive genomic profiling (CGP) has recently been suggested to be at least as accurate as FISH in classifying these neoplasms and providing additional genetic information.METHODS:We analyzed a cohort of 131 patients in whom FISH and CGP studies were performed as part of our normal clinical workflow and compared the effectiveness of FISH and CGP in detecting these clinically relevant rearrangements.RESULTS:Our findings are in agreement with our previously published study, which analyzed a cohort of 69 patients, supporting our hypothesis that the best approach to maximize detection of DHLs while limiting waste seems to be a combination of CGP and MYC break-apart FISH testing, the latter to capture the presence of non-IGH::MYC events.CONCLUSIONS:Our study supports the combined use of FISH and GCP rather than either method alone to better detect MYC and BCL2 (and BCL6) gene rearrangements.

Context.— Although rare in everyday practice, the initial presentation of hematopoietic neoplasms other than lymphoma in the musculoskeletal system and other extranodal sites can generate challenging diagnostic problems for surgical pathologists. Objective.— To review the morphologic and immunophenotypic features of various nonlymphoid hematopoietic diseases presenting at extranodal sites, with emphasis on the inherent diagnostic pitfalls and differential diagnoses of these entities to aid surgical pathologists in their accurate recognition. Data Sources.— Cases reviewed herein represent both in-house and consult cases seen at our institution between 2010 and 2021. Conclusions.— Entities that present in this way include myeloid neoplasms and histiocytic/dendritic cell neoplasms. These tumors commonly cause nonspecific symptoms, and their histologic appearance can overlap with a variety of benign neoplasms and reactive processes. This can lead to delay in diagnosis and intervention with potentially lifesaving therapy; thus, accurate and expedient recognition is of paramount importance.

Fig. 1 Anteroposterior radiograph of left upper arm shows no periosteal reaction, suspicious lytic or blastic osseous lesions, or calcified soft tissue mass

Immunohistochemical analysis has become an integral component in the diagnostic work up of hematopoietic neoplasms. It is not uncommon that visualization of single protein expression by immunohistochemistry among cells of interest may become a difficult task. Common scenarios of such include extensive colonization of germinal centers in the differential diagnosis of marginal zone lymphoma and follicular lymphoma, low-level bone marrow involvement by lymphoma and paucity of neoplastic lymphocytes in the setting of numerous background reactive lymphocytes, among others. For this reason, we have developed a variety of easy-to-employ dual-color dual-antibody immunohistochemical assays to aid in solving these diagnostic dilemmas. Herein, we share examples of our use of dual immunohistochemistry to illustrate its beneficial and practical objective.

CONTEXT.—:The gastrointestinal (GI) tract is the most common site of extranodal non-Hodgkin lymphoma, accounting for 20% to 40% of all extranodal lymphomas. The majority of these are systemic processes secondarily involving the GI tract. Primary GI lymphomas are less common, accounting for approximately 10% to 15% of all non-Hodgkin lymphomas. Most non-Hodgkin lymphomas involving the GI tract are of B-cell lineage, of which diffuse large B-cell lymphoma is the most common subtype, irrespective of location.OBJECTIVE.—:To review the lymphoproliferative neoplasms of B-cell and T-cell lineage involving the luminal GI tract according to the most prevalent subtypes at each anatomic site.DATA SOURCE.—:Systematic search of the PubMed database for updated literature on GI lymphoma epidemiology, subtypes, clinical, endoscopic, and genetic findings. Histologic images are derived from our collection of clinical cases.CONCLUSIONS.—:The GI tract is the most common site of extranodal lymphoproliferative neoplasms. Recognition of the most frequently encountered GI lymphomas is imperative for patient management and treatment.

We reviewed 341 consecutive cases of marginal zone lymphoma (MZL) (47) or follicular lymphoma (FL) (294) of which 7 were difficult to distinguish due to perceived coexpression of BCL6 and BCL2 by tumor cells in follicular foci. This stimulated us to develop dual BCL6/BCL2 immunohistochemistry, allowing us to assess coexpression among individual cells. Dual staining confirmed coexpression in 6 of 7 cases, all extranodal MZL (ENMZL) based on overall features and representing 13% of MZL in this series. These findings confirm that MZL cells have plasticity regarding protein expression within the germinal center (GC) microenvironment, an important diagnostic pitfall. Intriguingly, in all MZL expressing BCL6, non-neoplastic GC B cells within colonized follicles showed diminished or absent CD10 expression but preserved BCL6 and high ki67. This finding suggests plasticity of CD10 expression in non-neoplastic GC B cells in the context of colonization by MZL, possibly related to NF-kB dysregulation.

Follicular lymphoma (FL) is one of the most frequently diagnosed lymphomas in the United States and Europe. The definition of and basic approach to diagnosis and grading of FL is essentially unchanged in the recently updated revision of the World Health Organization (WHO) classification. FL is a biologically and histopathologically heterogeneous disease. Although there is an improved understanding of some FL variants and specific subtypes, there are cases whose recognition is particularly challenging, either because they have unusual features or represent examples of new or rare variants. Herein, we share a series of unusual and difficult to recognize FLs with the goal of increasing awareness of the expanding histopathologic variability in FL. Unusual FL discussed here include: FL with Castleman-like changes, FL with plasmacytic differentiation, and immunoglobulin G4-positive plasma cells in the setting of immunoglobulin G4-related disease, FL with marginal zone differentiation and involving mucosa-associated lymphoid tissue sites, diffuse FL variant expressing CD23 with STAT6 mutation, large B-cell lymphoma with IRF4 rearrangement, CD10-negative and MUM1-positive aggressive FL, and Epstein-Barr virus-positive FL.

Involvement of the central nervous system by chronic lymphocytic leukemia/small lymphocytic lymphoma is exceedingly rare, and currently no risk factors have been described. We report the case of a patient with concomitant chronic lymphocytic leukemia/small lymphocytic lymphoma and an embolic cerebrovascular accident related to a cardiac myxoma, who developed parenchymal central nervous system involvement of lymphoma on the ischemic bed. The patient was successfully treated with a high-dose fludarabine-based chemotherapy regimen, achieving a sustained remission. We propose that embolic breakage of the blood-brain barrier may be a major risk factor in producing central nervous system involvement. We also propose that a high-dose fludarabine-based chemotherapy regimen may be adequate to achieve a better CNS penetration and improved outcomes.

Gaucher disease is the most common autosomal recessive lysosomal storage disorder, caused by mutations in the β-glucocerebrosidase gene GBA. Here we describe generation of iPSC from skin-derived fibroblasts from two unrelated individuals with neuronopathic forms of Gaucher disease. The donor for line iPSC-GBA-1, a 21 month old girl, carried the recurring GBA mutation c.1448 T > C, p.Leu483Pro at homozygous state; fibroblasts for line iPS-GBA-2 were obtained from a 4 year old girl compound heterozygous for the GBA mutations c.667 T > C, p.Trp223Arg and c.1226A > G, p.Asn409Ser. iPSCs were developed using integration free episomal vectors (OCT4, KLF4; L-MYC, SOX2 (OSKM) and LIN28).Resource table

Objectives: To compare fluorescence in situ hybridization (FISH) and a commercially available sequencing assay for comprehensive genomic profiling (CGP) to determine the best approach to identify gene rearrangements (GRs) in large B-cell lymphomas (LBCLs). Methods: Comparison of standard-of-care FISH assays (including a two-probe approach for MYC; break-apart and fusion probes) and an integrated genomic DNA/RNA sequencing CGP approach on a set of 69 consecutive LBCL cases. Results: CGP detected GRs, including those involving MYC (1), BCL-2 (3), and BCL-6 (3), not detected by FISH. FISH detected non-IgH-MYC (4) and BCL-6 (2) GRs that were not detected by CGP. In four instances, standalone CGP or FISH testing would have missed a double-hit lymphoma. Conclusions: CGP was superior to FISH in the detection of IgH-MYC rearrangements but was inferior for the detection of non-IgH-MYC rearrangements. Our study demonstrates the rationale for development of a customized approach to identify GRs in LBCLs.

![Figure][1] A 43-year-old Hispanic man with a history of lepromatous leprosy presented with lymphadenopathy. Biopsy was performed to exclude lymphoproliferative disorder. Histologic examination showed lymphoid tissue containing clustered histiocytes with vacuolated, “lipid-like”