Histiocytic and dendritic cell neoplasms: what have we learnt by studying 67 cases

Favorable outcome of a histiocytic sarcoma patient treated with immune checkpoint inhibitor: a case report

Introduction and Importance

Histiocytic sarcoma (HS) is an extremely rare malignancy in which there has been no standard treatment approach. Some preclinical studies have provided rationales for the application of immunotherapy in advanced HS.

Case Presentation

The authors reported a case of a 61-year-old patient with metastatic HS who had a rapid progression on ifosfamide, carboplatin, and etoposide chemotherapy. The authors performed PD-L1 testing, which showed a strong positivity in 90% of tumor cells. The patient was then treated with pembrolizumab 200 mg every 3 weeks. He refused palliative radiotherapy. A dramatic response in all sites was recorded on the PET-CT scan after three cycles. He was maintained on pembrolizumab, reaching over 30 months without disease progression.

Clinical Discussion

Recent molecular data suggests there could be a role of immunotherapy in HS. In our patient, the disease was refractory to chemotherapy and pembrolizumab has been given based on the strong PD-L1 expression. Response to immunotherapy has also been recorded in several cases with malignant histiocytic neoplasm.

Conclusion

Immunotherapy might bring sustained disease remission in PD-L1 high expression HS and further studies evaluating the role of immune checkpoint inhibitor in this disease are warranted.

MYC and TP53 Alterations but Not MAPK Pathway Mutations Are Common Oncogenic Mechanisms in Follicular Dendritic Cell Sarcomas

CONTEXT.—

Despite their stromal origin, follicular dendritic cells (FDCs) share many functions with hematopoietic system cells. FDC neoplasms are currently classified by the World Health Organization along with those of a histiocytic nature. However, the molecular alterations driving oncogenesis in FDC sarcomas (FDCSs) are beginning to be unveiled and do not seem to concur with those described in histiocytic neoplasms, namely MAPK pathway activation.

OBJECTIVE.—

To identify molecular alterations driving tumorigenesis in FDCS.

DESIGN.—

We investigated the role of MYC and TP53 in FDC-derived tumor oncogenesis and assessed comprehensively the status of the MAPK pathway in 16 FDCSs, 6 inflammatory pseudotumor (IPT)-like FDCSs, and 8 IPTs.

RESULTS.—

MYC structural alterations (both amplifications and rearrangements) were identified in 5 of 14 FDCSs (35.7%), all associated with MYC overexpression. TP53 mutations were identified in 4 of 14 FDCSs (28.6%), all of which displayed intense and diffuse p53 expression. None of these alterations were identified in any IPT-like FDCSs or in IPT cases. No MAPK pathway gene alterations were identified in any of the cases studied.

CONCLUSIONS.—

The presence of MYC and TP53 alterations and the lack of association with Epstein-Barr virus segregate classical FDCS from IPT-like FDCS, pointing at different oncogenic mechanisms in both entities. Our results suggest a possible oncogenic role of MYC and TP53 alterations in FDCS. The absence of MAPK pathway alterations confirms the lack of a significant role of this pathway in the oncogenesis of FDC-derived neoplasms.

Progression in Myeloid Neoplasms: Beyond the Myeloblast

Disease progression in myelodysplastic syndromes (MDS), myelodysplastic-myeloproliferative neoplasms (MDS/MPN), and myeloproliferative neoplasms (MPN), altogether referred to as myeloid neoplasms (MN), is a major source of mortality. Apart from transformation to acute myeloid leukemia, the clinical progression of MN is mostly due to the overgrowth of pre-existing hematopoiesis by the MN without an additional transforming event. Still, MN may evolve along other recurrent yet less well-known scenarios: (1) acquisition of MPN features in MDS or (2) MDS features in MPN, (3) progressive myelofibrosis (MF), (4) acquisition of chronic myelomonocytic leukemia (CMML)-like characteristics in MPN or MDS, (5) development of myeloid sarcoma (MS), (6) lymphoblastic (LB) transformation, (7) histiocytic/dendritic outgrowths. These MN-transformation types exhibit a propensity for extramedullary sites (e.g., skin, lymph nodes, liver), highlighting the importance of lesional biopsies in diagnosis. Gain of distinct mutations/mutational patterns seems to be causative or at least accompanying several of the above-mentioned scenarios. MDS developing MPN features often acquire MPN driver mutations (usually JAK2), and MF. Conversely, MPN gaining MDS features develop, e.g., ASXL1, IDH1/2, SF3B1, and/or SRSF2 mutations. Mutations of RAS-genes are often detected in CMML-like MPN progression. MS ex MN is characterized by complex karyotypes, FLT3 and/or NPM1 mutations, and often monoblastic phenotype. MN with LB transformation is associated with secondary genetic events linked to lineage reprogramming leading to the deregulation of ETV6, IKZF1, PAX5, PU.1, and RUNX1. Finally, the acquisition of MAPK-pathway gene mutations may shape MN toward histiocytic differentiation. Awareness of all these less well-known MN-progression types is important to guide optimal individual patient management.

Genomic profiling for clinical decision making in lymphoid neoplasms

With the introduction of large-scale molecular profiling methods and high-throughput sequencing technologies, the genomic features of most lymphoid neoplasms have been characterized at an unprecedented scale. Although the principles for the classification and diagnosis of these disorders, founded on a multidimensional definition of disease entities, have been consolidated over the past 25 years, novel genomic data have markedly enhanced our understanding of lymphomagenesis and enriched the description of disease entities at the molecular level. Yet, the current diagnosis of lymphoid tumors is largely based on morphological assessment and immunophenotyping, with only few entities being defined by genomic criteria. This paper, which accompanies the International Consensus Classification of mature lymphoid neoplasms, will address how established assays and newly developed technologies for molecular testing already complement clinical diagnoses and provide a novel lens on disease classification. More specifically, their contributions to diagnosis refinement, risk stratification, and therapy prediction will be considered for the main categories of lymphoid neoplasms. The potential of whole-genome sequencing, circulating tumor DNA analyses, single-cell analyses, and epigenetic profiling will be discussed because these will likely become important future tools for implementing precision medicine approaches in clinical decision making for patients with lymphoid malignancies.

Case report: Common clonal origin of concurrent langerhans cell histiocytosis and acute myeloid leukemia

Langerhans cell histiocytosis (LCH) and acute myeloid leukemia (AML) are distinct entities of blood neoplasms, and the exact developmental origin of both neoplasms are considered be heterogenous among patients. However, reports of concurrent LCH and AML are rare. Herein we report a novel case of concurrent LCH and AML which shared same the driver mutations, strongly suggesting a common clonal origin.An 84-year-old female presented with cervical lymphadenopathy and pruritic skin rash on the face and scalp. Laboratory tests revealed pancytopenia with 13% of blasts, elevated LDH and liver enzymes, in addition to generalised lymphadenopathy and splenomegaly by computed tomography. Bone marrow specimens showed massive infiltration of MPO-positive myeloblasts, whereas S-100 and CD1a positive atypical dendritic cell-like cells accounted for 10% of the atypical cells on bone marrow pathology, suggesting a mixture of LCH and AML. A biopsy specimen from a cervical lymph node and the skin demonstrated the accumulation of atypical cells which were positive for S-100 and CD1a. LCH was found in lymph nodes, skin and bone marrow; AML was found in peripheral blood and bone marrow (AML was predominant compared with LCH in the bone marrow).

Next generation sequencing revealed four somatic driver mutations (NRAS-G13D, IDH2-R140Q, and DNMT3A-F640fs/-I715fs), equally shared by both the lymph node and bone marrow, suggesting a common clonal origin for the concurrent LCH and AML. Prednisolone and vinblastine were initially given with partial response in LCH; peripheral blood blasts also disappeared for 3 months. Salvage chemotherapy with low dose cytarabine and aclarubicin were given for relapse, with partial response in both LCH and AML. She died from pneumonia and septicemia on day 384. Our case demonstrates a common cell of origin for LCH and AML with a common genetic mutation, providing evidence to support the proposal to classify histiocytosis, including LCH, as a myeloid/myeloproliferative malignancy.

Histiocytic and dendritic cell neoplasms

BACKGROUND

Histiocytic and dendritic cell neoplasms (HDCNs) represent very rare tumors, which have been the subject of debate during the last few years.

OBJECTIVES

We aimed to provide a comprehensive review of the subject.

MATERIALS AND METHODS

The experience gained by the authors in large international studies and as a national reference center has been summarized to highlight the characteristics of each entity.

RESULTS

The clinical, morphologic, phenotypic, and molecular data of the different entities included under the heading of HDCNs are extensively discussed.

CONCLUSION

Currently, HDCNs are classified in the group of orphan diseases for which a standardized therapy is lacking. An international registry would facilitate expansion and dissemination of knowledge of these diseases and improve their treatment.

Histiocytic Sarcoma Involving Cervical Vertebra: A Case Report and Review of the Literature

Histiocytic sarcoma (HS) is a rare neoplasm composed of cells with immunohistochemical characteristics of mature histiocytes. It can be disseminated or localized and usually involves the skin, spleen, and gastrointestinal tract. Primary involvement of the vertebral column is extremely rare. We report a 29-year-old female who presented with neck pain and had a destructive 35*43*48 mm lesion in C2 with a paravertebral extension. The initial biopsy did not lead to the correct diagnosis. She later developed dysphagia, and the anterior approach was used for tumor decompression. The diagnosis of cervical histiocytic sarcoma was made, and she underwent radiotherapy. The follow-up MRI showed a marked response to radiotherapy. Here, we report the first case of cervical HS, review all cases of vertebral HS, compare patients’ characteristics and clinical courses, and discuss diagnostic nuances and treatment options.

Hodgkinoid histiocytosis: an atypical nodal CD30 and S100-positive histiocytosis with eosinophilia

Histocytes and dendritic cells may display cytological atypia and aberrant immunophenotype even in reactive processes. Herein, we describe two cases of "Hodgkinoid histiocytosis" that show distinctive clinicopathological features, mimicking the morphologically classic Hodgkin lymphoma (CHL) but suggesting reactive histiocytic/dendritic cell proliferation in lymph nodes. Both the patients presented with peripheral lymphadenopathy and blood eosinophilia with skin manifestations. Lymph node biopsy revealed scattered large histiocytes resembling Hodgkin cells with a round or stellate shape, abundant cytoplasm, and distinct nucleoli admixed in a predominant inflammatory background. The Hodgkinoid histiocytes occasionally showed emperipolesis. They expressed CD30, S100, and PD-L1 proteins but lacked PAX5 and CD1a expressions, Epstein-Barr association, BRAF V600E mutation, and PD-L1 gene amplification. Neither of the patients showed overt progression to malignant hematopoietic neoplasms during the disease course. An identical case series of four patients has been reported to date. Both these series highlight the potential of being interpreted as CHL due to the presence of Hodgkinoid histiocytes with CD30 positivity.

Diagnostic utility of flow cytometry in identifying histiocytic sarcoma-A case report

Histiocytic sarcoma (HS) is a malignant neoplasm of hematopoietic origin. It is an exceedingly rare and aggressive malignancy commonly seen in adults. Diagnosis is difficult owing to lack of specific clinical manifestations with the absence of precursor lesions or causative agents. Hence, it primarily relies on histopathological morphology combined with immunohistochemistry, which is time-consuming, hence resulting in delayed treatment. However, diagnostic utility of flow cytometry is not well established in this. We report a case of a 45-year-old man who presented with right axillary lymphadenopathy for 1 month. FNAC was performed on the axillary lymph node, which showed large, atypical lymphoid/histiocyte-like cells. On flow cytometry, these cells were CD64+, CD11c+, and CD45+ suggesting histiocytic sarcoma. Similar morphology was seen on incisional biopsy. On immunohistochemistry, the cells were negative for B and T cell markers, PAX5, EMA, CK, ALK, and CD1a and expressed CD68, S100, and CD11c. A diagnosis of histiocytic sarcoma was made. Hence, flow cytometry can be a highly effective and powerful tool for the early detection of HS and can help in prompt treatment, given its aggressive clinical course and low survival interval.

Histiocytic Sarcoma: Clinical Features and Outcomes of Patients Treated at a Tertiary Cancer Care Center

Histiocytic sarcoma (HS) is an extremely rare histiocytic disorder of unknown etiology. It is not a true sarcoma and is named so, due to the pathological resemblance to mature histiocytes. The clinical presentation of HS is diverse and is related to the involved organs. Due to its aggressive nature, with poor prognosis and lack of a standard treatment regimen of choice, its diagnosis and management pose a challenge to the clinician. Limited literature is available on the management of this entity. Here, we report four patients with HS, diagnosed over 15 years in a tertiary cancer center, with varied clinical presentation, management, and outcomes. The first patient presented with a localized unresectable esophageal mass. He was treated with cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) combination chemotherapy and attained complete remission. The second patient had a painless mass of the hand, treated with wide excision and adjuvant Radiotherapy. She is disease-free for the past 12 years. The third patient had presented with an anterior mediastinal mass. He had progressive disease on chemotherapy. The fourth patient had multifocal disease with generalized lymphadenopathy. She was treated with CHOP chemotherapy and is now disease-free at 13 months. To summarize, the patients with the localized resectable disease did well, with surgical excision and adjuvant radiotherapy, while patients with the multifocal disease did well on CHOP chemotherapy. The take-home message from this case series is - CHOP off whenever you can and if not give CHOP to chop off the disease.

A case of primary histiocytic sarcoma arising from a neck lymph node

Histiocytic sarcoma (HS) is an aggressive and rare hematological malignancy. Its treatment has not been established, and most patients die within 2 years of diagnosis. Resection can provide a favorable prognosis for solitary lesions. We present the case of an 80-year-old Japanese man with HS. He presented a history of a slow-growing painless mass in the lower part of his right jaw. Ultrasonography showed a swollen lymph node in the vicinity of the right submandibular gland. Contrast-enhanced computed tomography revealed a heterogeneous, low-contrast mass on the right of the neck. Magnetic resonance imaging revealed a heterogeneously enhanced mass in gadolinium-enhanced T1-weighted images. The fine needle biopsy showed spindle-shaped cells and HS was suspected. Fluorodeoxyglucose positron emission tomography revealed uptake by the tumor alone. The patient underwent right upper neck dissection and resection of the submandibular salivary glands. No postoperative adjuvant treatment was administered, but 2-year survival was achieved. Histopathological examination showed proliferation of large, pleomorphic atypical cells without differentiation into lymphocytes, which proved their differentiation into histiocytes. A bone marrow biopsy showed no evidence of monocytic leukemia. Thus, a diagnosis of HS was made. With local treatment alone, our patient achieved long-term survival, maintaining his quality of life.

Blastic Indeterminate Dendritic Cell Tumor Associated With Chronic Myelomonocytic Leukemia

ABSTRACT

Indeterminate dendritic cell tumor (IDCT) is an exceedingly rare neoplasm that can be associated with hematopoietic malignancies. We report a case of multifocal cutaneous blastic indeterminate dendritic cell tumor (BIDCT) in a 75-year-old man with chronic myelomonocytic leukemia showing blastic histiocytoid morphology, positivity for CD1a and S100, and no expression of langerin. We present a literature review on the 11 reported cases of IDCTs/BIDCTs associated with chronic myelomonocytic leukemia (CMML), including this case. The clinicopathological characteristics have been summarized. The IDCT and CMML cells are clonally related in 4 tested cases. Patients with IDCT/BIDCT associated with CMML seem to have worse clinical outcomes compared with patients with IDCT not associated with CMML.

ALK-positive histiocytosis: a new clinicopathologic spectrum highlighting neurologic involvement and responses to ALK inhibition

ALK-positive histiocytosis is a rare subtype of histiocytic neoplasm first described in 2008 in 3 infants with multisystemic disease involving the liver and hematopoietic system. This entity has subsequently been documented in case reports and series to occupy a wider clinicopathologic spectrum with recurrent KIF5B-ALK fusions. The full clinicopathologic and molecular spectra of ALK-positive histiocytosis remain, however, poorly characterized. Here, we describe the largest study of ALK-positive histiocytosis to date, with detailed clinicopathologic data of 39 cases, including 37 cases with confirmed ALK rearrangements. The clinical spectrum comprised distinct clinical phenotypic groups: infants with multisystemic disease with liver and hematopoietic involvement, as originally described (Group 1A: 6/39), other patients with multisystemic disease (Group 1B: 10/39), and patients with single-system disease (Group 2: 23/39). Nineteen patients of the entire cohort (49%) had neurologic involvement (7 and 12 from Groups 1B and 2, respectively). Histology included classic xanthogranuloma features in almost one-third of cases, whereas the majority displayed a more densely cellular, monomorphic appearance without lipidized histiocytes but sometimes more spindled or epithelioid morphology. Neoplastic histiocytes were positive for macrophage markers and often conferred strong expression of phosphorylated extracellular signal-regulated kinase, confirming MAPK pathway activation. KIF5B-ALK fusions were detected in 27 patients, whereas CLTC-ALK, TPM3-ALK, TFG-ALK, EML4-ALK, and DCTN1-ALK fusions were identified in single cases. Robust and durable responses were observed in 11/11 patients treated with ALK inhibition, 10 with neurologic involvement. This study presents the existing clinicopathologic and molecular landscape of ALK-positive histiocytosis and provides guidance for the clinical management of this emerging histiocytic entity.

Potential role of MAP2K1 mutation in the trans-differentiation of interdigitating dendritic cell sarcoma: Case report and literature review

A 5-year-old male child was diagnosed with interdigitating dendritic cell sarcoma (IDCS) during his maintenance therapy for B-cell precursor acute lymphoblastic leukemia (B-ALL). Multiplex lymph node involvements of the neck were found by positron emission tomography CT (PET-CT). Treatments, including surgical and chemotherapy, resulted in complete remission. Four years later, systemic bone infiltration was discovered. Surgical resection of the IV rib and intensive chemotherapy led to a complete morphological remission, and allogeneic bone marrow transplantation was performed. Comprehensive genomic profiling of the formalin fixed the tumor tissue, and the cryopreserved leukemic cells revealed several common alterations and divergent clonal evolution with a novel MAP2K1 mutation of the IDCS, which is responsible for the trans-differentiation of the common lymphoid-committed tumor progenitor.

Nonlymphoid Hematopoietic Diseases Presenting in Bone, Soft Tissue, and Other Extranodal Sites

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.

Follicular dendritic cell sarcoma

Follicular dendritic cells (FDC) are mesenchymal-derived dendritic cells located in B-follicles where they play a pivotal role in triggering and maintaining B-cell adaptive immune response. In 1986 Dr. Juan Rosai first reported a series of neoplasms showing features of FDC and defined it as Follicular Dendritic Cell Tumor, subsequently renamed as "sarcoma" (FDCS). In its seminal and subsequent articles Rosai and colleagues highlighted the heterogeneous microscopic appearance of FDCS and its immunohistochemical and ultrastructural features.

FDCS mostly occurs in extranodal sites (79.4% of cases) and lymph nodes (15.1%); in about 7%-10% of cases it is associated with hyaline-vascular Castleman disease. Given its significant growth pattern and cytological variability, FDCS can be confused with various neoplasms and even inflammatory processes. The diagnosis requires the use of a broad spectrum of FDC markers (e.g. CD21, CD23, CD35, clusterin, CXCL13, podoplanin), particularly considering that tumor antigen-loss is frequent. The inflammatory-pseudotumor-like (IPT-like) variant of FDCS, in addition to its peculiar histopathological and clinical features, is characterized by positivity of tumor cells for Epstein-Barr virus, representing a diagnostic requisite.

No distinctive genetic and molecular anomalies have been identified in FDCS. It often carries an aberrant clonal karyotype and chromosomal structural alterations, frequently involving onco-suppressor genes. Direct or next generation sequencing showed alterations on genes belonging to the NF-κB regulatory pathway and cell-cycle regulators. In contrast to hematopoietic-derived histiocytic and dendritic cells tumors, FDCS typically lacks mutations in genes related to the MAPK pathway.

FDCS recurs locally in 28% and metastasizes in 27% of cases. Extent of the disease, surgical resectability and histopathological features are significantly associated with the outcome. IPT-like FDCS behaves as an indolent tumor, even if it often recurs locally over years.

Complete surgical excision is the gold standard of treatment. Data on targeted therapies (e.g.: tyrosine kinase inhibitors) or immune checkpoint inhibitors are very limited and responses are variable. A better understanding of the molecular drivers of this tumor may lead to potential new therapeutic strategies.

Case Report: Eighteen Month Relapse- Free Survival Following Radical Multidisciplinary Oncological Treatment in a 68-Year-Old Male Patient With Histiocytic Sarcoma

Introduction

Histiocytic Sarcoma (HS) is a rare and aggressive malignancy, and patients can present with rapid tumor growth and invasion. The optimal diagnostic and therapeutic management is unknown since only a few cases have been published. Here we report a patient with histiocytic sarcoma of the right groin.

Case

A 68 year-old male patient presented to our hospital with suspicion of a superinfected atheroma of the right groin. Computed tomography showed an abdominal tumor of unknown entity. Detailed assessment including immunohistochemically evaluation of biopsy material confirmed HS. The patient underwent radical tumor resection including compartment-resection of the right thigh. During five additional cycles of chemotherapy over a period of 1.5 years he remained relapse-free.

Summary

Diagnostic work up and treatment of HS is challenging, as there is a paucity of clinical reports and lack of standard guidelines for care. In the present case report, aggressive multidisciplinary treatment resulted in good clinical outcome, however, further studies evaluating this approach in similar patients are needed.

ALK-rearranged histiocytosis: Report of two cases with involvement of the central nervous system

AIMS

Histiocytoses are a heterogeneous group of localized or disseminated diseases. Clinical presentation and patients' outcome vary greatly, ranging from mild to life-threatening disorders. Rare cases of systemic or localized histiocytosis harboring ALK rearrangement have been reported.

METHODS

Two cases of CNS histiocytosis were thoroughly investigated by implementing multiple molecular tests, i.e. FISH, RT-qPCR, NGS analysis.

RESULTS

In a 10-month old girl (patient #1), MRI showed two left hemispheric lesions and a right fronto-mesial lesion histologically consisting of a moderately cellular infiltrative proliferation, composed by CD68(PGM1)+/CD163+ spindle cells. ALK 5'/3'-imbalance and a KIF5B(exon 24)-ALK(exon 20) fusion were documented by RT-qPCR and NGS analysis, respectively. A subsequent CT scan showed multiple hepatic and pulmonary lesions. The patient was started on chemotherapy (vinblastine) associated to an ALK-inhibitor (Alectinib) with remarkable response. In a 11-year-old girl (patient #2), MRI showed a right frontal 1.5 cm lesion. Neuropathological examination revealed a histiocytic proliferation composed by medium sized CD68(PGM1)+/HLA-DR+ cells, showing moderate ALK1 positivity. ALK rearrangement and a KIF5B(exon 24)-ALK(exon 20) fusion were demonstrated also in this case. Subsequent CT, 18F-FDG-PET and MRI scans showed the presence of a single right femoral lesion, proved to be a fibrous cortical defect.

CONCLUSIONS

In ALK-histiocytoses, CNS involvement may occur as part of a systemic disease or, rarely, as its only primary disease localization, which could remain otherwise asymptomatic. The diagnosis often relies on neuropathological examination of brain biopsy, which may pose a diagnostic challenge due to the variable histopathological features. An integrated histological and molecular approach in such cases is recommended.

ALK-positive Histiocytosis of the Breast: A Clinicopathologic Study Highlighting Spindle Cell Histology

Originally described as a systemic self-limiting disease in infancy, the spectrum of ALK-positive histiocytosis has recently been broadened to include localized diseases in older children and young adults. Despite different manifestations, these tumors share histologic characteristics and a highly recurrent KIF5B-ALK fusion. ALK-positive histiocytosis is poorly characterized in the breast. In this study, we report 3 cases of ALK-positive histiocytosis of the breast. The patients were Asian women, aged 16 to 45 years. Two patients presented with an isolated breast mass, while 1 exhibited multiorgan involvement. The latter patient received ALK inhibitor after surgery, which led to complete remission. Histologically, well-circumscribed tumors displayed fascicular and storiform growth of uniform, nonatypical spindle cells admixed with lymphocytic infiltrates. Fewer conventional epithelioid histiocytes with lobulated or clefted nuclei were observed within the same breast tumors in 2 cases or within a concomitant brain tumor in the third case. Touton-type giant cells were focally present in 2 cases. Immunohistochemically, tumor spindle, and epithelioid cells were diffusely positive for CD163 and ALK in all cases and focally positive for S100 protein in 1 of the cases. CD1a and langerin were negative. Actin-positive myofibroblasts were admixed within the tumor in 2 cases, and their reactive nature was highlighted using double immunostaining. Break-apart fluorescence in situ hybridization assay demonstrated gene rearrangements involving KIF5B and ALK in all the 3 cases. ALK-positive histiocytosis rarely occurs as a spindle cell breast tumor, and should be distinguished from other diseases such as inflammatory myofibroblastic tumors and spindled histiocytic reaction.

The mutational landscape of histiocytic sarcoma associated with lymphoid malignancy

Histiocytic sarcoma and tumors with dendritic cell differentiation (HDT) are uncommon neoplasms often with an aggressive clinical course that may occur in association with another hematologic malignancy or mediastinal germ cell tumor (secondary HDT, sHDT). Previous studies have shown mutations in the RAS/MAPK pathway in HDT and have demonstrated a clonal relationship between HDT and associated lymphoid malignancies through common translocations or identical immunoglobulin or T-cell receptor gene rearrangements. We performed whole exome sequencing on 16 cases of sHDT to further evaluate the spectrum of mutations that occur in sHDT in the context of an associated lymphoid malignancy, including cases associated with follicular lymphoma (FL), chronic lymphocytic leukemia/small lymphocytic lymphoma, B- and T-cell acute lymphoblastic leukemia/lymphoma and peripheral T-cell lymphoma, NOS. In addition, we assessed the clonal relationship between the HDT and the associated lymphoid malignancy in three cases for which matched samples were available. We found mutations in RAS/MAPK pathway genes in 14/16 cases of sHDT associated with diverse mature and precursor B-cell and T-cell neoplasms, involving KRAS (8/16), BRAF (2/16), NRAS (2/16), MAP2K1 (1/16), and NF1 (1/16). In addition, we note that FL-associated sHDT frequently shares a similar mutational profile to the associated malignancy, identifying mutations in CREBBP or KMT2D in all cases and "aberrant" somatic hypermutation in 5/6 cases. Our study confirms the role of the RAS/MAPK pathway in the pathogenesis of sHDT, provides further evidence of a common neoplastic precursor and, in the case of FL, gives additional insight into the stage in lymphomagenesis at which transdifferentiation may occur.

Original Observation of Primary Bladder Histiocytic Sarcoma: First Case Report

Histiocytic sarcoma (HS) is a rare malignant lymphohematopoietic neoplasm; it has been cited in the recent World Health Organization (WHO) classification as a malignant proliferation of cells exhibiting morphological and immunophenotypic features of mature histiocytes. To our knowledge, the present case is the first to be described in the bladder of a patient without a history of lymphoma. Only one case has been reported so far regarding a secondary bladder presentation in the setting of a previous diffuse large B-cell lymphoma. We discuss the case of a 68-year-old male who presented with hematuria and dysuria. CT scan revealed a 4-cm intravesical mass that histological examination defined as HS. Our objective was to describe the clinical, histological, immunophenotypical, molecular characteristics and discuss the differential diagnoses of this first case of primary bladder HS. Our research was based on a review of selected articles obtained via the PubMed database. This extremely rare experience provided us with the opportunity to depict an interesting case, highlight its uniqueness, and build up new pathological evidence.

Spectrum of histiocytic neoplasms associated with diverse haematological malignancies bearing the same oncogenic mutation

Histiocytic disorders are a spectrum of rare diseases characterised by the accumulation of macrophage-, dendritic cell-, or monocyte-differentiated cells in various tissues and organs. The discovery of recurrent genetic alterations in many of these histiocytoses has led to their recognition as clonal neoplastic diseases. Moreover, the identification of the same somatic mutation in histiocytic lesions and peripheral blood and/or bone marrow cells from histiocytosis patients has provided evidence for systemic histiocytic neoplasms to originate from haematopoietic stem/progenitor cells (HSPCs). Here, we investigated associations between histiocytic disorders and additional haematological malignancies bearing the same genetic alteration(s) using the nationwide Dutch Pathology Registry. By searching on pathologist-assigned diagnostic terms for the various histiocytic disorders, we identified 4602 patients with a putative histopathological diagnosis of a histiocytic disorder between 1971 and 2019. Histiocytosis-affected tissue samples of 187 patients had been analysed for genetic alterations as part of routine molecular diagnostics, including from nine patients with an additional haematological malignancy. Among these patients, we discovered three cases with different histiocytic neoplasms and additional haematological malignancies bearing identical oncogenic mutations, including one patient with concomitant KRAS p.A59E mutated histiocytic sarcoma and chronic myelomonocytic leukaemia (CMML), one patient with synchronous NRAS p.G12V mutated indeterminate cell histiocytosis and CMML, and one patient with subsequent NRAS p.Q61R mutated Erdheim-Chester disease and acute myeloid leukaemia. These cases support the existence of a common haematopoietic cell-of-origin in at least a proportion of patients with a histiocytic neoplasm and additional haematological malignancy. In addition, they suggest that driver mutations in particular genes (e.g. N/KRAS) may specifically predispose to the development of an additional clonally related haematological malignancy or secondary histiocytic neoplasm. Finally, the putative existence of derailed multipotent HSPCs in these patients emphasises the importance of adequate (bone marrow) staging, molecular analysis and long-term follow-up of all histiocytosis patients.

Immunophenotyping of Nonneoplastic and Neoplastic Histiocytes in Cats and Characterization of a Novel Cell Line Derived From Feline Progressive Histiocytosis

Histiocytic proliferative diseases are rare in cats, and their pathogenesis is poorly understood. In the present study, 25 cases of histiocytic sarcoma (HS) and 6 of feline progressive histiocytosis (FPH) were examined, and survival times were recorded in 19 cases. The immunophenotypes of tumor cells in these cases as well as of nonneoplastic feline histiocytes were characterized using formalin-fixed, paraffin-embedded tissues. An FPH cell line (AS-FPH01) and xenotransplant mouse model of FPH were also established. The median survival time of HS (150 days) was significantly shorter than that of FPH (470 days). Immunohistochemically, nonneoplastic histiocytes were immunopositive for various combinations of Iba-1, HLA-DR, E-cadherin, CD204, CD163, CD208, and MAC387. By immunohistochemistry, dermal interstitial dendritic cells (iDCs) and macrophages were CD204+/E-cadherin-, while epidermal Langerhans cells (LCs) were CD204-/E-cadherin+. Neoplastic cells of 4 FPH and 18 HS were CD204+/E-cadherin- (iDC/macrophage immunophenotype), while 2 FPH and 2 HS were CD204-/E-cadherin+ (LC immunophenotype), and 5 HS were CD204+/E-cadherin+ (LC-like cell immunophenotype). Furthermore, immunohistochemical and western blot analyses of AS-FPH01 cells derived from E-cadherin-negative FPH revealed that cultured cells were immunopositive for both CD204 and E-cadherin in vitro and in vivo. These results indicate that the neoplastic cells of feline HS and FPH were variably positive for iDC/macrophage and LC markers, and their immunophenotype changed in different microenvironments. The novel cell line established in the present study may serve as an experimental model of FPH that will enable further molecular and therapeutic studies on this disease.

ALK-Positive Histiocytosis with Peripheral Blood Histiocytes: A Case Report

Histiocytoses are a diverse group of rare, clinically heterogeneous disorders characterised by tissue infiltration of histiocytes, which may result in organ dysfunction and failure. Over 100 different subtypes of histiocytoses have been recognised, including rare cases of ALK-positive histiocytosis. We report a case of histiocytosis in a neonate who presented with refractory thrombocytopenia, anaemia, and intermittent neutropenia. Histiocytes were present in both peripheral blood smears and bone marrow; ALK positivity was demonstrated by immunohistochemistry. Given the scarce reports of this condition, the variable organ involvement, and the different approaches to management in the cases described, we seek to expand the literature by providing a report of our patient whose condition improved without chemotherapy. The presence of histiocytes in peripheral blood smears of patients with this condition has not previously been reported, and it underscores the importance of routine careful evaluation of blood smears.

Imprint cytology of histiocytic sarcoma manifesting as a retroperitoneal mass: A valuable aid for frozen section diagnosis

Histiocytic sarcoma (HS) is a comparatively rare hematolymphoid neoplasm. Differential diagnosis of this entity has generally been proven to be so difficult that extensive immunohistochemical staining is required to confirm its presence. We herein present a case of HS manifesting as a retroperitoneal mass in which imprint cytology (IC) contributed to the diagnosis. A 77-year-old man was referred for the evaluation of a colonic tubulovillous polypoid lesion. Preoperative imaging revealed a 3 cm-sized polypoid lesion in the transverse colon. Incidentally, a retroperitoneal mass of 8 cm in the left para-aortic region was also detected. Diff-Quik stained IC slides were prepared as an adjunct to intraoperative pathology consultation. The smear exhibited moderate cellularity involving loosely cohesive cell clusters with intervening delicate vasculature. Most of the tumor cells consisted of large single cells with abundant eosinophilic cytoplasm that was vacuolated and foamy. The nuclei were predominantly large and eccentrically located. Mitotic figures were minimal in number and finely granular brown pigments were occasionally observed. A frozen section diagnosis was reported as a malignant neoplasm, and a histologic diagnosis of HS was confirmed by conducting a battery of immunohistochemical stains: negative for pan-cytokeratin, EMA, CD117, HMB45, Melan A, CD30, PAX5, CD20, S-100, and CD23 but strong positive for vimentin and CD68. This entity is rarely described in terms of cytopathologic features. This case study highlights how cytopathologists should be aware of HS as a retroperitoneal lesion and should become familiar with cytomorphologic features.

Langerhans cell histiocytosis of an intra-mammary lymph node in an 18-year-old woman

We describe an 18-year-old woman with several month's history of a 12 x 7 mm palpable mammary nodule, that was hypoechoic, with regular margins and vascularization areas by ultrasound. A fibroadenoma was hypothesized (American College of Radiology BI-RADS 3). A 14 G needle biopsy was performed, showing a LC proliferation suspected for LCH of a lymph node, with florid dermatopathic lymphadenopathy in differential diagnosis. The multidisciplinary team of the breast clinic decided to perform a lumpectomy and a diagnosis of LCH involving an intra-mammary lymph node was made. Langerhans cells (LC) are dendritic cells characterized by grooved nuclei, irregular nuclear contours, and abundant cytoplasm, that normally reside in the skin and mucosal surfaces. They were positive for CD1a, langerin/CD207, and S100 by immunohistochemistry. Langerhans cell histiocytosis (LCH) is a clonal proliferation of histiocytes that is thought to be neoplastic in most cases. Reactive LC can be distinguished from LCH by cyclin D1 immunostaining, which is positive only in LCH. About 50% of cases have BRAF V600E mutations. The revised classification of histiocytes divides LCH in subtypes: LCH SS (single system), LCH lung positive, LCH Multiple System/Risk Organ negative and LCH Multiple System/Risk Organ positive. Localized disease can progress to multisystem involvement. The diagnosis of LCH is based on clinical and radiological findings in combination with histopathological, immunophenotypic or ultrastructural analyses identifying tissue infiltration by LC. It is recommended that biopsy confirmation of suspected LCH be performed in all cases. Lymph nodes may be the only site of disease or a part of multisystem involvement by LCH. The histologic differential diagnosis is discussed.

Primary Histiocytic Sarcoma Presenting as a Breast Mass: A Case Report

Histiocytic sarcoma is a rare hematologic malignancy, with very few cases of primary histiocytic sarcoma of the breast described in English scientific literature. Herein, we describe a case of primary histiocytic sarcoma of the breast in a 75-year-old woman, with no clinical history of malignant tumors, who presented with a palpable solitary breast mass. Microscopically, the resected breast mass showed large pleomorphic cells, some multinucleated giant cells, and admixed inflammatory components. The pleomorphic tumor cells further showed a diffuse, noncohesive growth pattern, an abundant eosinophilic cytoplasm, and strong and diffuse immunoreactivity for cluster of differentiation (CD) 68 and CD163. Furthermore, a whole-body positron-emission tomography/computed tomography using deoxy-2-[¹⁸F]fluoro-D-glucose performed after surgery showed no other masses or lesions. After surgical excision, the patient was followed up, and no evidence of tumor recurrence or metastasis was noted.

Combined Tumors in Hematolymphoid Neoplasms: Case Series of Histiocytic and Langerhans Cell Sarcomas Arising From Low-Grade B-Cell Lymphoma

We report an index case of histiocytic sarcoma arising in a 70-year-old patient with long-standing chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL). The patient presented in 2017 with painful, enlarging swelling of the left neck. He had remote history of cutaneous squamous cell carcinoma with no sign of recurrence, and his CLL/SLL was thought to be in remission. Computed tomography showed mild splenomegaly and multifocal lymphadenopathy including a 3-cm left neck mass. Biopsy of the left neck mass showed CLL/SLL with associated histiocytic sarcoma. Flow cytometry demonstrated a B cell neoplasm with CLL/SLL phenotype. Despite radiation therapy, he expired 3 months after presentation. Two similar cases (CLL/SLL and histiocytic sarcoma, follicular lymphoma and Langerhans cell sarcoma) from another institution are also illustrated. The pathological features of combined tumors in lymphoid neoplasms, a general framework to the work-up to determine interrelatedness of tumor components, and the clinical relevance are discussed.

Genomic profiling of primary histiocytic sarcoma reveals two molecular subgroups

Histiocytic sarcoma is a rare malignant neoplasm that may occur de novo or in the context of a previous hematologic malignancy or mediastinal germ cell tumor. Here, we performed whole exome sequencing and RNA-sequencing (RNA-Seq) on 21 archival cases of primary histiocytic sarcoma. We identified a high number of genetic alterations within the RAS/RAF/MAPK pathway in 21 of 21 cases, with alterations in NF1 (6 of 21), MAP2K1 (5 of 21), PTPN11 (4 of 21), BRAF (4 of 21), KRAS (4 of 21), NRAS (1 of 21), and LZTR1 (1 of 21), including single cases with homozygous deletion of NF1, high-level amplification of PTPN11, and a novel TTYH3-BRAF fusion. Concurrent NF1 and PTPN11 mutations were present in 3 of 21 cases, and 5 of 7 cases with alterations in NF1 and/or PTPN11 had disease involving the gastrointestinal tract. Following unsupervised clustering of gene expression data, cases with NF1 and/or PTPN11 abnormalities formed a distinct tumor subgroup. A subset of NF1/PTPN11 wild-type cases had frequent mutations in B-cell lymphoma associated genes and/or clonal IG gene rearrangements. Our findings expand the current understanding of the molecular pathogenesis of this rare tumor and suggest the existence of a distinct subtype of primary histiocytic sarcoma characterized by NF1/PTPN11 alterations with predilection for the gastrointestinal tract.

Nodal follicular dendritic cell sarcoma with four atypical histomorphologic features: an unusual case report

BACKGROUND

Follicular dendritic cell sarcoma (FDCS) is a rare malignancy. In addition to the classical histopathologic features, it has also some special morphological variants that can present a challenge in the diagnosis of this disease.

CASE PRESENTATION

A 45-year-old male who presented with a left supraclavicular mass was given a final diagnosis of FDCS after lymph node biopsy. The specimen obtained during radical resection revealed five different morphologies, including the classical histological appearance and atypical areas resembling desmoplastic infiltrative carcinoma, anaplastic large cell lymphoma (ALCL), hemangiopericytoma and classical Hodgkin's lymphoma (CHL). Immunohistochemistry was notable for positive CD21 and CD23 expression across all morphologies. Given the atypical appearance and location, the specimen was initially misdiagnosed as a metastatic carcinoma based on histology alone at an outside institution. The patient eventually underwent surgical resection followed by adjuvant chemotherapy and radiation. Despite treatment, the disease progressed, and the patient passed away 36 months after surgery.

CONCLUSIONS

This unusual case of FDCS contains four types of atypical histomorphologies within a single tumor specimen, including those resembling ALCL and hemangiopericytoma which are described here for the first time. Our report further expands the histopathologic spectrum of FDCS and may help assist in the diagnosis of other such challenging cases.

Identification of diverse activating mutations of the RAS-MAPK pathway in histiocytic sarcoma

Recent studies have demonstrated recurrent activating mutations involving the classical MAPK and PI3K signaling pathways in a large proportion of histiocytic neoplasms, such as Langerhans cell histiocytosis. However, very little is known about the molecular genetics of histiocytic sarcoma, a rare aggressive malignant neoplasm that shows pathologic characteristics of mature macrophages. Here we report the genomic characteristics of a large cohort of histiocytic sarcomas (n = 28) using a targeted next-generation sequencing approach to identify driver alterations. We identified recurrent mutations involving the RAS-MAPK signaling pathway (MAP2K1, KRAS, NRAS, BRAF, PTPN11, NF1, CBL) in a majority (57%) of histiocytic sarcoma cases and report a clinical response to a MEK inhibitor (Cobimetinib) in a patient with a NF1-mutated histiocytic sarcoma. A smaller subset of cases (21%) also showed mutations resulting in activation of the PI3K signaling pathway (PTEN, MTOR, PIK3R1, PIK3CA). In addition, the tumor-suppressor gene CDKN2A was the most frequently altered gene (46%). Further, a subset of histiocytic sarcoma cases shows striking molecular genetic similarities to B cell lymphomas, supporting a clonal relationship between B cell neoplasms and a subset of histiocytic sarcomas. These findings support a cooperative role for MAPK, PI3K, and cyclin-CDK4/6-INK4 signaling in the pathogenesis of histiocytic sarcoma and provide a rational basis for targeting these pathways.

Atypical Histiocytic Lesion Preceding a Peripheral T-Cell Lymphoma Involving the Skin Exhibiting the Same Molecular Alterations

Peripheral T-cell lymphoma (PTCL), not otherwise specified (NOS) is a diagnosis of exclusion, showing extreme cytological and phenotypic heterogeneity. Skin involvement of PTCL may be primary or secondary. Diagnosis of histiocytosis may be difficult, requiring clinical-pathological correlation. We describe a laryngeal atypical histiocytic lesion (AHL) and a nasal PTCL, NOS with cutaneous involvement in the same patient presenting with peculiar histopathologic and immunophenotypic features. The laryngeal neoplasm showed morphological and immunophenotypic evidence of histiocytic differentiation and does not fit any other category of the WHO classification nor the revised classification of histiocytosis. The nasal and cutaneous lesions presented features close to natural killer/T-cell lymphoma and gamma-delta T-cell lymphoma but did not meet accurately the WHO criteria. A somatic activating Q61K mutation was found on exon 3 of the NRAS gene in both AHL and PTCL, NOS. The mutation on NRAS gene in both AHL and PTCL, NOS may suggest a common origin from a precursor cell.

Mediastinal follicular dendritic cell sarcoma: a rare, potentially under-recognized, and often misdiagnosed disease

Background

Mediastinal follicular dendritic cell sarcoma (FDCS) is extremely rare. Due to potential under-recognization of this disease, it happens to be misdiagnosed, especially on core needle biopsy. We report 3 cases of mediastinal FDCS and provide a literature review to improve better understanding of the tumor and to reduce misdiagnosis.

Methods

Three cases of mediastinal FDCS in our clinic practice were studied, including their core needle biopsy and resected specimens, and those cases reported previously in English literature were retrieved and analyzed.

Results

The core needle biopsy of case 1 showed a tumor reminiscent of classical Hodgkin’s lymphoma (CHL), while the resected mass was finally diagnosed with FDCS combined with hyaline-vascular Castleman’s disease. Both the biopsy and resected tissue of case 2 were constitutive of the clear epithelioid cells with marked atypia. In both cases, definitive diagnoses were not made on core needle biopsy. In case 3, there were some areas morphologically similar to CHL, and some areas contained ovoid to spindle-shaped tumor cells with fascicular pattern. The analysis of 43 cases of mediastinal FDCS showed the age of patients were from 16 to 76 years old, the male to female ratio was 1.5:1, the maximal tumor diameters were 3–17 cm. 18 cases were underwent preoperative biopsy, whereas 15 (83.3%) of which were misdiagnosed initially, often as lymphoma. 32 patients had available follow-up data, the rates of recurrence, metastasis, and mortality were 12.5, 18.8 and 28.1%, respectively. Current limited data suggested no statistical differences between adverse prognosis and gender, age, tumor size, necrosis, or different therapeutics, respectively.

Conclusions

Mediastinal FDCS is a rare malignancy that has yet not been fully understood and been often misdiagnosed, particularly when making a diagnosis on core needle biopsy. Increased awareness of this enigmatic tumor is crucial to avoid diagnostic pitfalls.

Indeterminate Cell Histiocytosis – Case Report and Review of Literature

Introduction. Indeterminate cell histiocytosis is a rare proliferative disorder of indeterminate dendritic cells, reported in only 50 case reports so far. It is clinically presented as yellow, red or brown papules and nodules that appear in otherwise healthy adult individuals. Indeterminate cells are distinct dendritic cells of the skin that have ultrastructural similarities to the epidermal Langerhans cells but do not contain the characteristic Birbeck’s granules and they are also langerin-negative, unlike LCH. Indeterminate cell histiocytosis is an exceptional entity with variable clinical, histopathologic or immunohistochemical findings, sharing morphologic and immunophenotypic features with both Langerhans- and non-Langerhans cell histiocytoses. Case Report. We present a case of indeterminate histiocytosis in a 77-year-old man with 3-year history of asymptomatic, multiple reddish and brown papules and nodules over the entire body, including the oral mucosa. Skin biopsy was done, and histopathological analysis with immunohistochemistry was performed. The positive ICH staining of cells for CD68, CD1a, and S-100 enabled us to diagnose ICH in our patient. Also, BRAF V600E mutation was detected in tumor tissue. The treatment was started with methotrexate that was effective for 6 months, but due to the disease recurrence, further therapy with thalidomide was advised, without effect. Conclusion. Indeterminate histiocytosis is a rare disease, therefore no standardized treatment has been established and the treatment options are limited.

Histiocytic Sarcoma: Review, Discussion of Transformation From B-Cell Lymphoma, and Differential Diagnosis

Context.—

Histiocytic sarcoma is a rare neoplasm of mature histiocytes with an aggressive clinical course that can arise de novo or from a low-grade B-cell lymphoma. In particular, chronic lymphocytic leukemia/small lymphocytic lymphoma is a very common malignancy in the Western hemisphere, and most cases of chronic lymphocytic leukemia/small lymphocytic lymphoma have an indolent course and behavior. However, 2% to 8% of chronic lymphocytic leukemia/small lymphocytic lymphoma cases transform. Histiocytic sarcomatous transformation is rare and portends poor prognosis.

Objective.—

To review the clinical features, morphology, and key points related to the differential diagnosis for histiocytic sarcoma. We discuss recent understanding of the biology underlying transformation.

Data Sources.—

University of Michigan case and review of pertinent literature about histiocytic sarcoma and morphologic differential diagnosis.

Conclusions.—

Histiocytic sarcoma is a rare histiocytic neoplasm that can arise as a result of transdifferentiation from low-grade B-cell lymphomas, and has a wide differential diagnosis including other histiocytic/dendritic cell neoplasms, myeloid neoplasms, lymphomas, melanoma, and carcinoma. However, some key morphologic and immunohistochemical features allow for accurate classification.

A case of recurrent histiocytic sarcoma with MAP2K1 pathogenic variant treated with the MEK inhibitor trametinib

Histiocytic sarcoma in advanced clinical stages is typically an aggressive neoplasm, with poor response to conventional chemotherapy. An 18-year-old male with refractory histiocytic sarcoma that had transformed from Rosai-Dorfman disease was admitted to our hospital. A pathogenic variant of MAP2K1 was detected by next-generation sequencing of tumor specimens. Affected regions showed excellent responses to the MEK inhibitor trametinib. It has been reported that RAS/MEK/ERK pathway is activated in many cases of histiocytic sarcoma. MEK inhibition may represent a useful treatment option in histiocytic sarcoma.

Case of primary central nervous system histiocytic sarcoma with prominent proliferation of histiocytic cells between the trabeculae of reactive glial cells

Histiocytic sarcoma (HS) is an extremely rare malignant neoplasm that exhibits morphologic and immune-phenotype evidence of histiocytic differentiation. The disease most commonly involves the lymph nodes, gastrointestinal tract, skin, and soft tissue, as well as in the central nervous system (CNS) being relatively rare. Here we report a case of primary CNS HS with unusual histopathological characteristics. A 65-year-old woman presented with CNS HS in the left frontal lobe region, showing two distinct histological patterns. Approximately half of the lesion displayed histological characteristics typical of HS, including diffuse invasion of large round-to-ovoid pleomorphic cells, with mitotic figures (Ki-67 index: 30%) and coagulative necrotic foci. The other half exhibited prominent proliferation of histiocytic cells between the trabeculae of reactive glial cells, with rare mitotic figures (Ki-67 index: < 1%) and no necrotic foci. There were transitions between two morphologies. The HS tumor cells and the histiocytic cells between the trabeculae of reactive glial cells possessed nearly identical histomorphologic and immunophenotypic features, although the HS tumor cells showed a more pronounced degree of cytologic atypia and mitotic activity. To our knowledge, this is the first reported case of HS with prominent proliferation of the histiocytic cells between the trabeculae of reactive glial cells. Here we present the detailed histological, immunohistochemical, and molecular findings. Investigating cases of HS may provide insight into the pathogenesis of this disease.

Targeting MEK in a Translational Model of Histiocytic Sarcoma

Histiocytic sarcoma in humans is an aggressive orphan disease with a poor prognosis as treatment options are limited. Dogs are the only species that spontaneously develops histiocytic sarcoma with an appreciable frequency, and may have value as a translational model system. In the current study, high-throughput drug screening utilizing histiocytic sarcoma cells isolated from canine neoplasms identified these cells as particularly sensitive to a MEK inhibitor, trametinib. One of the canine cell lines carries a mutation in PTPN11 (E76K), and another one in KRAS (Q61H), which are associated with the activation of oncogenic MAPK signaling. Both mutations were previously reported in human histiocytic sarcoma. Trametinib inhibited sensitive cell lines by promoting cell apoptosis, indicated by a significant increase in caspase 3/7. Furthermore, in vitro findings were successfully recapitulated in an intrasplenic orthotopic xenograft mouse model, which represents a disseminated aggressive form of histiocytic sarcoma. Mice with histiocytic sarcoma xenograft neoplasms that were treated with trametinib had significantly longer survival times. Target engagement was validated as activity of ERK, downstream of MEK, was significantly downregulated in neoplasms of treated mice. Additionally, trametinib was found in plasma and neoplastic tissues within projected therapeutic levels. These findings demonstrate that in dogs, histiocytic sarcoma may be associated with a dysfunctional MAPK pathway, at least in some cases, and may be effectively targeted through MEK inhibition. Clinical trials to test safety and efficacy of trametinib in dogs with histiocytic sarcoma are warranted, and may provide valuable translational information to similar diseases in humans. Mol Cancer Ther; 17(11); 2439-50. ©2018 AACR.

Histiocytic cell neoplasms involving the bone marrow: summary of the workshop cases submitted to the 18th Meeting of the European Association for Haematopathology (EAHP) organized by the European Bone Marrow Working Group, Basel 2016

The bone marrow is a preferential site for both reactive and neoplastic histiocytic proliferations. The differential diagnosis ranges from reactive histiocyte hyperplasia in systemic infections, vaccinations, storage diseases, post myeloablative therapy, due to increased cell turnover, and in hemophagocytic lymphohistiocytosis, through extranodal Rosai-Dorfman disease to neoplasms derived from histiocytes, including histiocytic sarcomas (HS), Langerhans cell histiocytoses (LCH), Erdheim-Chester disease (ECD), and disseminated juvenile xanthogranuloma (JXG). One of the most important recent developments in understanding the biology of histiocytic neoplasms and in contributing to diagnosis was the detection of recurrent mutations of genes of the Ras/Raf/MEK/ERK signaling pathway, in particular the BRAF^V600E mutation, in LCH and ECD. Here, we summarize clinical and pathological findings of 17 histiocytic neoplasms that were presented during the bone marrow symposium and workshop of the 18th European Association for Haematopathology (EAHP) meeting held in Basel, Switzerland, in 2016. A substantial proportion of these histiocytic neoplasms was combined with clonally related lymphoid (n = 2) or myeloid diseases (n = 5, all ECD). Based on the latter observation, we suggest excluding co-existent myeloid neoplasms at initial staging of elderly ECD patients. The recurrent nature of Ras/Raf/MEK/ERK signaling pathway mutations in histiocytic neoplasms was confirmed in 6 of the 17 workshop cases, illustrating their diagnostic significance and suggesting apotential target for tailored treatments.

2018 ESMO Sarcoma and GIST Symposium: 'take-home messages' in soft tissue sarcoma

The 7th edition of the ‘ESMO Sarcoma and GIST Symposium’ was held in Milan in February 2018. For the first time, the Symposium brought together representatives from the European Reference Network on rare adult solid cancer (EURACAN) joined by sarcoma experts from the USA, Japan and patient advocacy groups, to share insights and discuss future directions in this rare condition. This commentary will summarise the highlights in soft tissue sarcomas.