Cytokine Storm Syndrome.

Cytokine storm syndrome (CSS), which is frequently fatal, has garnered increased attention with the ongoing coronavirus pandemic. A variety of hyperinflammatory conditions associated with multiorgan system failure can be lumped under the CSS umbrella, including familial hemophagocytic lymphohistiocytosis (HLH) and secondary HLH associated with infections, hematologic malignancies, and autoimmune and autoinflammatory disorders, in which case CSS is termed macrophage activation syndrome (MAS). Various classification and diagnostic CSS criteria exist and include clinical, laboratory, pathologic, and genetic features. Familial HLH results from cytolytic homozygous genetic defects in the perforin pathway employed by cytotoxic CD8 T lymphocytes and natural killer (NK) cells. Similarly, NK cell dysfunction is often present in secondary HLH and MAS, and heterozygous mutations in familial HLH genes are frequently present. Targeting overly active lymphocytes and macrophages with etoposide and glucocorticoids is the standard for treating HLH; however, more targeted and safer anticytokine (e.g., anti-interleukin-1, -6) approaches are gaining traction as effective alternatives.

BRAF V600E is associated with higher incidence of second cancers in adults with Langerhans cell histiocytosis.

In this retrospective study, BRAF mutation status did not correlate with disease extent or (event-free) survival in 156 adults with Langerhans cell histiocytosis. BRAFV600E was associated with an increased incidence of second malignancies, often comprising hematological cancers, which may be clonally related.

Langerhans cell histiocytosis: NACHO update on progress, chaos, and opportunity on the path to rational cures.

Langerhans cell histiocytosis (LCH) is a myeloid neoplastic disorder characterized by lesions with CD1a-positive/Langerin (CD207)-positive histiocytes and inflammatory infiltrate that can cause local tissue damage and systemic inflammation. Clinical presentations range from single lesions with minimal impact to life-threatening disseminated disease. Therapy for systemic LCH has been established through serial trials empirically testing different chemotherapy agents and durations of therapy. However, fewer than 50% of patients who have disseminated disease are cured with the current standard-of-care vinblastine/prednisone/(mercaptopurine), and treatment failure is associated with long-term morbidity, including the risk of LCH-associated neurodegeneration. Historically, the nature of LCH-whether a reactive condition versus a neoplastic/malignant condition-was uncertain. Over the past 15 years, seminal discoveries have broadly defined LCH pathogenesis; specifically, activating mitogen-activated protein kinase pathway mutations (most frequently, BRAFV600E) in myeloid precursors drive lesion formation. LCH therefore is a clonal neoplastic disorder, although secondary inflammatory features contribute to the disease. These paradigm-changing insights offer a promise of rational cures for patients based on individual mutations, clonal reservoirs, and extent of disease. However, the pace of clinical trial development behind lags the kinetics of translational discovery. In this review, the authors discuss the current understanding of LCH biology, clinical characteristics, therapeutic strategies, and opportunities to improve outcomes for every patient through coordinated agent prioritization and clinical trial efforts.

Mixed histiocytic neoplasms: A multicentre series revealing diverse somatic mutations and responses to targeted therapy.

Histiocytic neoplasms are diverse clonal haematopoietic disorders, and clinical disease is mediated by tumorous infiltration as well as uncontrolled systemic inflammation. Individual subtypes include Langerhans cell histiocytosis (LCH), Rosai-Dorfman-Destombes disease (RDD) and Erdheim-Chester disease (ECD), and these have been characterized with respect to clinical phenotypes, driver mutations and treatment paradigms. Less is known about patients with mixed histiocytic neoplasms (MXH), that is two or more coexisting disorders. This international collaboration examined patients with biopsy-proven MXH with respect to component disease subtypes, oncogenic driver mutations and responses to conventional (chemotherapeutic or immunosuppressive) versus targeted (BRAF or MEK inhibitor) therapies. Twenty-seven patients were studied with ECD/LCH (19/27), ECD/RDD (6/27), RDD/LCH (1/27) and ECD/RDD/LCH (1/27). Mutations previously undescribed in MXH were identified, including KRAS, MAP2K2, MAPK3, non-V600-BRAF, RAF1 and a BICD2-BRAF fusion. A repeated-measure generalized estimating equation demonstrated that targeted treatment was statistically significantly (1) more likely to result in a complete response (CR), partial response (PR) or stable disease (SD) (odds ratio [OR]: 17.34, 95% CI: 2.19-137.00, p = 0.007), and (2) less likely to result in progression (OR: 0.08, 95% CI: 0.03-0.23, p < 0.0001). Histiocytic neoplasms represent an entity with underappreciated clinical and molecular diversity, poor responsiveness to conventional therapy and exquisite sensitivity to targeted therapy.

Liver involvement with Langerhans cell histiocytosis in adults.

BACKGROUND AND AIMS: Liver involvement portends poor prognosis in adults. We aimed to characterize the clinical features, liver function tests, radiologic findings, molecular profiles, therapeutic approaches and outcomes of adults patients with Langerhans cell histiocytosis (LCH) with liver involvement. METHODS: We conducted a retrospective analysis of all adults with LCH (≥ 18 years) seen at Peking Union Medical College Hospital (Beijing, China) between January 2001 and December 2022. RESULTS: Among the 445 newly diagnosed adults with LCH, 90 patients had liver involvement at diagnosis and 22 patients at relapse. The median age was 32 years (range, 18-66 years). Of 112 evaluable patients, 108 had full liver function testing, including alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase (ALP), γ-glutamyl transpeptidase (GGT), and total bilirubin and albumin. Elevated ALP was seen in 63.0% and GGT in 86.1%; 14.8% had elevated bilirubin. Next-generation sequencing of 54 patients revealed frequent BRAFN486_P490 (29.6%), BRAFV600E (18.5%), and MAP2K1 (14.8%). OUTCOMES: After a median 40 months' follow-up (range 1-168 months), 3-year progression-free survival (PFS) and overall survival were 49.7% and 86.6% respectively. In multivariable analyses, ≥3 abnormal liver function tests (HR 3.384, 95% CI 1.550-7.388, P = .002) associated with inferior PFS; immunomodulatory drug therapy (HR 0.073, 95% CI, 0.010-0.541, P = .010) correlated with superior PFS versus chemotherapy. CONCLUSIONS: In summary, elevated GGT and ALP were common in adults with LCH liver involvement. Greater than equal to 3 abnormal liver function tests predicted poor outcomes. Immunomodulatory drug therapy was associated with favorable progression-free survival compared to chemotherapy.

International expert consensus recommendations for the diagnosis and treatment of Langerhans cell histiocytosis in adults.

Langerhans cell histiocytosis (LCH) can affect children and adults with a wide variety of clinical manifestations, including unifocal, single-system multifocal, single-system pulmonary (smoking-associated), or multisystem disease. The existing paradigms in the management of LCH in adults are mostly derived from the pediatric literature. Over the last decade, the discovery of clonality and MAPK-ERK pathway mutations in most cases led to the recognition of LCH as a hematopoietic neoplasm, opening the doors for treatment with targeted therapies. These advances have necessitated an update of the existing recommendations for the diagnosis and treatment of LCH in adults. This document presents consensus recommendations that resulted from the discussions at the annual Histiocyte Society meeting in 2019, encompassing clinical features, classification, diagnostic criteria, treatment algorithm, and response assessment for adults with LCH. The recommendations favor the use of 18F-Fluorodeoxyglucose positron emission tomography-based imaging for staging and response assessment in the majority of cases. Most adults with unifocal disease may be cured by local therapies, while the first-line treatment for single-system pulmonary LCH remains smoking cessation. Among patients not amenable or unresponsive to these treatments and/or have multifocal and multisystem disease, systemic treatments are recommended. Preferred systemic treatments in adults with LCH include cladribine or cytarabine, with the emerging role of targeted (BRAF and MEK inhibitor) therapies. Despite documented responses to treatments, many patients struggle with a high symptom burden from pain, fatigue, and mood disorders that should be acknowledged and managed appropriately.

Clinical characteristics, molecular aberrations, treatments, and outcomes of malignant histiocytosis.

Malignant histiocytosis (MH) is an extremely rare neoplasm of the macrophage-dendritic cell lineage. We report the clinical characteristics, molecular aberrations, treatments, and outcomes of patients with MH seen at two referral centers from January 2000 to May 2023. We identified 43 patients with MH, of which 26 had histiocytic sarcoma (MH-H), 9 interdigitating dendritic cell sarcoma (MH-IDC), and 8 Langerhans cell sarcoma (MH-LC). The median age at diagnosis was 61 years (range, 3-83). Thirty-three patients (77%) had multifocal disease, while 10 had unifocal involvement. Tumor specimens from 22 patients (51%) underwent targeted next generation sequencing, and 19 of 22 (86%) had at least one pathogenic mutation, including mutations in MAPK pathway genes (73%). The median overall survival (OS) among the entire cohort was 16 months (95% CI: 8-50). The outcomes of those with multifocal disease were significantly shorter than their unifocal counterpart: median OS of 10 months versus 50 months (p = .07). Patients with risk organ involvement (bone marrow, spleen, or liver) had significantly inferior outcomes. Chemotherapy and surgery were the most common first-line treatments for multifocal and unifocal disease, respectively. While the outcome for patients with multifocal disease was poor, there was a subset of patients who had durable responses to treatment. Our study highlights that MH has heterogeneous clinical presentation, frequent oncogenic mutations, and prognosis, which is strongly tied to disease extent and type of organ involvement.

Malignant Histiocytosis Comprises a Phenotypic Spectrum That Parallels the Lineage Differentiation of Monocytes, Macrophages, Dendritic Cells, and Langerhans Cells.

Malignant histiocytoses (MHs), or the 'M group' of the Histiocyte Society classification, are characterized by neoplastic histiocytes with large pleomorphic nuclei. MH encompasses the diagnoses of histiocytic sarcoma, interdigitating dendritic cell sarcoma, and Langerhans cell sarcoma. We aimed to define the phenotypic spectrum of MH and examine the genotypic features across this spectrum. Using immunohistochemistry, we arranged the 22 cases into 4 subtypes that correspond to the lines of differentiation from monocytic and dendritic cell precursors as follows: (1) macrophage (n = 5): CD68+, CD163+, CD14+, and Factor 13a+; (2) monocyte-macrophage (n = 5): CD68+, CD163+, CD14+, S100+, and OCT2+; (3) dendritic cell (n = 6): CD68+, CD11c+, S100+, lysozyme+, ZBTB46+, and CD1a/langerin < 5%; and (4) Langerhans cell (n = 6): CD68+, CD11c+, S100+, ZBTB46+, CD1a+, and langerin+. The phenotypic subtypes align with those seen in low-grade histiocytic neoplasms as follows: MH-macrophage type correlates with Erdheim-Chester disease phenotype; MH-monocyte-macrophage type with Rosai-Dorfman disease phenotype, and MH-Langerhans cell type with Langerhans cell histiocytosis. Activating mutations in MAPK-pathway genes were identified in 80% of MH cases; 29% had mutations in the PI3k-AKT-mTOR pathway and 59% had mutations in epigenetic modulating genes. Strong expression of cyclin D1 was present in all cases, whereas p-ERK and p-AKT were not uniformly expressed. Eight of 22 (36%) MH cases were proven to be clonally related to a prior B-cell lymphoma. Defining the phenotypic spectrum of MH provides a guide to diagnosis and allows further exploration into the potential biological and clinical significance.

Ophthalmologic Involvement in Adults with Histiocytic Disorders: Clinical Presentation and Treatment Outcomes.

PURPOSE: To evaluate the clinical presentation, treatment, and outcomes in adult patients with histiocytic disorders with ocular, orbital, optic nerve, or cavernous sinus involvement. DESIGN: Observational, retrospective chart review. PARTICIPANTS: Adult patients (age ≥ 18 years) at Mayo Clinic from January 1, 1996, to July 1, 2021, with histiocytic disorders. Inclusion criteria were (1) histiocytic disorder by biopsy and appropriate clinical phenotype; (2) available medical records; and (3) ocular, orbital, optic nerve, or cavernous sinus involvement. METHODS: Retrospective chart review. MAIN OUTCOME MEASURES: Response to therapy, measured in clinical and radiographic impact. RESULTS: Thirty-two patients were identified: 7 with Langerhans cell histiocytosis (LCH); 15 with Erdheim-Chester disease (ECD); 1 with mixed LCH/ECD phenotype; 8 with Rosai-Dorfman disease (RDD); and 1 with mixed RDD/ECD phenotype. Ophthalmologic involvement was part of the initial presentation in 69% of patients (22/32). Eyelid edema (13/32, 41%) and proptosis (12/32, 38%) were the most frequent presentations. Isolated orbital or cavernous sinus involvement was present in 3 of 7 patients with LCH and 1 of 8 patients with RDD. Optic nerve sheath involvement was present in 2 of 7 LCH patients, 14 of 15 ECD patients, and 1 RDD/ECD patient. Diffuse (> 75%) orbital involvement was seen in 12 of 15 ECD patients and 1 of 7 LCH patients. Ocular involvement was seen in 1 of 15 ECD patients, 6 of 8 RDD patients, and 1 of 1 mixed RDD/ECD patient. The cavernous sinuses were involved in 1 of 7 LCH patients, 5 of 15 ECD patients, and both mixed phenotype patients. Visual acuity was affected in 14 patients (14/24, 58%) with a median logarithm of the minimum angle of resolution visual acuity of 0.1 (range, -0.12 to 3). BRAF V600E mutations were found in 75% (3/4) of LCH patients and 91% (10/11) of ECD patients. Patients received a variety of treatment, and response was variable across disease types. CONCLUSIONS: Orbital involvement was more commonly seen in LCH and ECD, whereas ocular involvement was more common in RDD. Visual acuity may be impacted from ocular involvement or compression of the optic nerve with diffuse orbital involvement.

Outcomes of limited stage primary bone diffuse large B-cell lymphoma in the rituximab era: a multicenter, retrospective study.

Primary bone diffuse large B cell lymphoma (DLBCL) is a rare variant of extranodal non-Hodgkin lymphoma (NHL) historically treated with induction chemotherapy followed by consolidative radiation therapy (RT). It remains unknown whether RT confers additional benefit following rituximab-based chemoimmunotherapy (CIT) induction in patients with limited-stage disease. We conducted a multicenter retrospective analysis of patients treated between 2005 and 2019 using rituximab-based CIT regimens with or without consolidative RT to discern whether consolidative RT adds benefit in patients with stage I-II disease that could be encompassed in one radiation field. A total of 112 patients were included: 78 received CIT and radiation (RT group), and 34 received CIT alone (no RT group). The OS at 10 years was 77.9% in the RT group and 89.0% in the no RT group (p = 0.42). The RFS at 10 years was 73.5% in the RT group and 80.3% in the no RT group (p = 0.88). Neither improved OS nor RFS was associated with the addition of consolidative RT. Subgroup analysis of patients only achieving a partial response after CIT suggests that these patients may benefit from consolidative RT.

Effect of cumulative dose of brentuximab vedotin maintenance in relapsed/refractory classical Hodgkin lymphoma after autologous stem cell transplant: an analysis of real-world outcomes.

Sixteen cycles of Brentuximab vedotin (BV) after autologous stem cell transplant (ASCT) in high-risk relapsed/refractory classical Hodgkin lymphoma demonstrated an improved 2-year progression-free survival (PFS) over placebo. However, most patients are unable to complete all 16 cycles at full dose due to toxicity. This retrospective, multicenter study investigated the effect of cumulative maintenance BV dose on 2-year PFS. Data were collected from patients who received at least one cycle of BV maintenance after ASCT with one of the following high-risk features: primary refractory disease (PRD), extra-nodal disease (END), or relapse <12 months (RL<12) from the end of frontline therapy. Cohort 1 had patients with >75% of the planned total cumulative dose, cohort 2 with 51-75% of dose, and cohort 3 with ≤50% of dose. The primary outcome was 2-year PFS. A total of 118 patients were included. Fifty percent had PRD, 29% had RL<12, and 39% had END. Forty-four percent of patients had prior exposure to BV and 65% were in complete remission before ASCT. Only 14% of patients received the full planned BV dose. Sixty-one percent of patients discontinued maintenance early and majority of those (72%) were due to toxicity. The 2-year PFS for the entire population was 80.7%. The 2-year PFS was 89.2% for cohort 1 (n=39), 86.2% for cohort 2 (n=33), and 77.9% for cohort 3 (n=46) (P=0.70). These data are reassuring for patients who require dose reductions or discontinuation to manage toxicity.

Checkpoint inhibitor-based salvage regimens prior to autologous stem cell transplant improve event-free survival in relapsed/refractory classic Hodgkin lymphoma.

Clinical trials of novel salvage therapies have encouraging outcomes for relapsed/refractory transplant-eligible classic Hodgkin lymphoma (R/R cHL) but comparison with conventional chemotherapy is lacking. Herein, we report the final analysis of a multicenter retrospective cohort of R/R cHL assessing outcomes by type of salvage therapy before autologous stem cell transplant (ASCT). R/R cHL patients who underwent ASCT at 14 institutions across the United States were included. Outcomes were compared among patients receiving conventional chemotherapy, brentuximab vedotin (BV) + chemotherapy, BV alone, and a checkpoint inhibitor (CPI)-based regimens before ASCT. Study endpoints included event-free survival (EFS), progression-free survival (PFS), and overall survival (OS). All endpoints are defined from relapse. Of 936 patients, 728 received conventional chemotherapy, 73 received BV + chemotherapy, 70 received BV alone, and 65 received CPI-based regimens prior to ASCT. When adjusted for time to relapse, pre-ASCT response and use of BV maintenance, patients receiving CPI-based regimens had superior 2-year EFS compared to conventional chemotherapy, BV + chemotherapy, and BV alone (79.7, 49.6, 62.3, and 36.9%, respectively, p < .0001). Among 649 patients transplanted after 1 line of salvage therapy, CPI-based regimens were associated with superior 2-year PFS compared to conventional chemotherapy (98% vs. 68.8%, hazard ratio: 0.1, 95% confidence interval: 0.03-0.5, p < .0001). OS did not differ by pre-ASCT salvage regimen. In this large multicenter retrospective study, CPI-based regimens improved EFS and PFS compared to other salvage regimens independent of pre-ASCT response. These data support earlier sequencing of CPI-based regimens in R/R cHL in the pre-ASCT setting.

Neurological Manifestations of Histiocytic Disorders.

PURPOSE OF REVIEW: Histiocytic disorders, including Erdheim-Chester disease (ECD), Langerhans cell histiocytosis (LCH), and Rosai-Dorfman disease (RDD), are rare neoplasms that may present with a spectrum of neurologic involvement. Diagnostic delay is common due to heterogeneity in presentation and challenging pathology. RECENT FINDINGS: Recent advances in the treatment of these diseases targeted towards mutations in the MAP kinase pathway have led to an improved prognosis in these patients with neurologic involvement. It is critical for clinicians to have a high index of suspicion to allow for early targeted treatment and optimize neurologic outcomes. A systematic approach to diagnosis is presented in this article to allow for accurate diagnosis of these rare diseases.

Dynamics of Ku and bacterial non-homologous end-joining characterized using single DNA molecule analysis.

We use single-molecule techniques to characterize the dynamics of prokaryotic DNA repair by non-homologous end-joining (NHEJ), a system comprised only of the dimeric Ku and Ligase D (LigD). The Ku homodimer alone forms a ∼2 s synapsis between blunt DNA ends that is increased to ∼18 s upon addition of LigD, in a manner dependent on the C-terminal arms of Ku. The synapsis lifetime increases drastically for 4 nt complementary DNA overhangs, independently of the C-terminal arms of Ku. These observations are in contrast to human Ku, which is unable to bridge either of the two DNA substrates. We also demonstrate that bacterial Ku binds the DNA ends in a cooperative manner for synapsis initiation and remains stably bound at DNA junctions for several hours after ligation is completed, indicating that a system for removal of the proteins is active in vivo. Together these experiments shed light on the dynamics of bacterial NHEJ in DNA end recognition and processing. We speculate on the evolutionary similarities between bacterial and eukaryotic NHEJ and discuss how an increased understanding of bacterial NHEJ can open the door for future antibiotic therapies targeting this mechanism.

Mimics of Erdheim-Chester disease.

Erdheim-Chester disease (ECD) is a rare histiocytic neoplasm with frequent multiorgan involvement. An accurate diagnosis of ECD requires the correlation of clinical features, histopathologic and radiologic findings. We describe a case series of patients with a referral diagnosis of ECD, whereby the diagnosis was changed to non-histiocytic diseases after comprehensive review at a tertiary care center. This accurate revision of the referral diagnosis of ECD enabled initiation of proper disease-directed therapy in a timely manner for these patients and avoided unnecessary exposure to systemic cytotoxic chemotherapy or targeted agents. Our study highlights the value of a multidisciplinary team of histiocytosis experts in confirming the diagnosis of ECD and also brings attention to other conditions to consider that can mimic ECD, including osteopoikilosis, tenosynovial giant cell tumour, IgG4-related disease, fibrous dysplasia and chronic recurrent multifocal osteomyelitis.

Impact of a Multidisciplinary Tumor Board on the Care of Patients with Histiocytic Disorders: The Histiocytosis Working Group experience.

INTRODUCTION: Histiocytic disorders pose significant diagnostic and management challenges for the clinicians due to diverse clinical manifestations and often non-specific histopathologic findings. Herein, we report the tumor board experience from the first-of-its-kind Histiocytosis Working Group (HWG). MATERIALS AND METHODS: The HWG was established in June 2017 and consists of experts from 10 subspecialties that discuss cases in a multidisciplinary format. We present the outcome of tumor board case discussions during the first 2 years since its inception (June 2017-June 2019). RESULTS: Forty cases with a suspected histiocytic disorder were reviewed at HWG during this time period. Average number of subspecialties involved in HWG case discussion was 5 (range, 2-9). Histiocytosis Working Group tumor board recommendations led to significant changes in the care of 24 (60%) patients. These included change in diagnosis (n = 11, 27%) and change in treatment (n = 13, 33%). CONCLUSION: Our report highlights the feasibility of a multidisciplinary tumor board and its impact on outcomes of patients with histiocytic disorders.

Erdheim-Chester disease: consensus recommendations for evaluation, diagnosis, and treatment in the molecular era.

Erdheim-Chester disease (ECD) is a rare histiocytosis that was recently recognized as a neoplastic disorder owing to the discovery of recurrent activating MAPK (RAS-RAF-MEK-ERK) pathway mutations. Typical findings of ECD include central diabetes insipidus, restrictive pericarditis, perinephric fibrosis, and sclerotic bone lesions. The histopathologic diagnosis of ECD is often challenging due to nonspecific inflammatory and fibrotic findings on histopathologic review of tissue specimens. Additionally, the association of ECD with unusual tissue tropism and an insidious onset often results in diagnostic errors and delays. Most patients with ECD require treatment, except for a minority of patients with minimally symptomatic single-organ disease. The first ECD consensus guidelines were published in 2014 on behalf of the physicians and researchers within the Erdheim-Chester Disease Global Alliance. With the recent molecular discoveries and the approval of the first targeted therapy (vemurafenib) for BRAF-V600-mutant ECD, there is a need for updated clinical practice guidelines to optimize the diagnosis and treatment of this disease. This document presents consensus recommendations that resulted from the International Medical Symposia on ECD in 2017 and 2019. Herein, we include the guidelines for the clinical, laboratory, histologic, and radiographic evaluation of ECD patients along with treatment recommendations based on our clinical experience and review of literature in the molecular era.

Near Complete Response to Trametinib Treatment in Histiocytic Sarcoma Harboring a Somatic KRAS Mutation.

Survival outcomes of patients with histiocytic neoplasms are poor, with no standard-of-care treatments available for these malignancies. Recent characterization of the genomic landscape of various histiocytic neoplasms have shown a predominance of activating driver mutations within the MAPK/ERK pathway (ie, BRAF, MEK, KRAS, MAPK, and NRAS). Subsequently, successful treatment of these malignancies with BRAF and MEK inhibitors has been reported. This report presents the first patient with histiocytic sarcoma harboring a somatic KRAS Q61H mutation who was subsequently treated to a near complete response with the MEK inhibitor trametinib. Due to patient preference, lack of standard of care treatments, and associated morbidity from head and neck dissection, initial disease reduction provided by trametinib therapy allowed for a less morbid resection. This case report highlights the utility of up-front next-generation sequencing and the efficacy of MEK inhibition in patients with histiocytic sarcoma harboring activating KRAS mutations.

Sustained, complete response to pexidartinib in a patient with CSF1R-mutated Erdheim-Chester disease.

Erdheim-Chester disease (ECD) is a histiocytic neoplasm that predominantly harbors mitogen-activated protein kinase (MAPK) pathway variants. MAPK inhibitors typically are effective treatments, but mutations outside the MAPK pathway, such as CSF1R variants, may cause refractory ECD. We describe a patient with a novel somatic mutation in CSF1R (CSF1RR549_E554delinsQ ) that resulted in refractory ECD affecting the central nervous system. Cell model studies, RNA sequencing analysis, and in silico protein modeling suggested that she had a gain-of-function mutation occurring in a region critical for autoinhibition. The patient was treated with pexidartinib, a CSF1R inhibitor, and has had a complete clinical and metabolic response lasting more than 1.5 years to date. To our knowledge, this is the first report to describe successful treatment of a patient with ECD by using an agent that specifically targets CSF1R. This case also highlights the critical role of individualized molecular profiling to identify novel therapeutic targets in ECD.

Histiocytic Neoplasms, Version 2.2021, NCCN Clinical Practice Guidelines in Oncology.

Histiocytic neoplasms are rare hematologic disorders accounting for less than 1% of cancers of the soft tissue and lymph nodes. Clinical presentation and prognosis of these disorders can be highly variable, leading to challenges for diagnosis and optimal management of these patients. Treatment often consists of systemic therapy, and recent studies support use of targeted therapies for patients with these disorders. Observation ("watch and wait") may be sufficient for select patients with mild disease. These NCCN Guidelines for Histiocytic Neoplasms include recommendations for diagnosis and treatment of adults with the most common histiocytic disorders: Langerhans cell histiocytosis, Erdheim-Chester disease, and Rosai-Dorfman disease.