Erdheim-Chester disease with concomitant Rosai-Dorfman like lesions: a distinct entity mainly driven by MAP2K1

Current Concepts in Histiocytic Neoplasms

Histiocytic neoplasms are a diverse group of disorders arising from macrophages, dendritic cells, and monocytes of the mononuclear phagocyte system. These neoplasms encompass a clinical spectrum from indolent, self-limited, and localized conditions to highly aggressive malignancies. Since the publication of the Revised Fourth Edition of the World Health Organization (WHO) classification, advances in molecular diagnostics have improved our understanding of the pathogenesis and classification of these disorders. In contrast to the Revised Fourth Edition, the International Consensus Classification (ICC) now recognizes Rosai-Dorfman-Destombes disease as a neoplastic disorder and introduces ALK-positive histiocytosis as a distinct entity. This manuscript reviews the current concepts regarding histiocytic neoplasms, focusing on the diagnostic criteria recommended by the ICC based on histopathology, immunophenotype, molecular alterations, as well as clinical and imaging characteristics.

IgG4-related disease and other fibro-inflammatory conditions

IgG4-related disease (IgG4-RD) is a fibro-inflammatory disorder usually characterized by multi-organ involvement. Its pathogenesis is complex and involves genetic and environmental factors, while immune responses usually mediate organ damage and promote fibrosis, which is a key feature of the disease. IgG4 responses, however, are not exclusive to IgG4-RD and can be encountered in other diseases with phenotypes that partially overlap that of IgG4-RD. Although IgG4-RD has clinical and histological hallmarks, the lack of validated diagnostic criteria often makes the diagnosis challenging, requiring a multi-dimensional approach that integrates clinical, radiological and serological data. The present Review covers recent advances in the understanding of disease drivers and its clinical phenotypes, mainly focusing on the differential diagnosis with potential IgG4-RD mimickers, namely histiocytoses, lymphoproliferative disorders, systemic vasculitides and other immune-mediated conditions. The Review also provides a schematic approach to IgG4-RD treatment, including a brief overview of glucocorticoid-sparing agents and emerging therapies, from B cell-depleting monoclonal antibodies to cytokine-targeting drugs, the majority of which are currently under investigation in randomized clinical trials.

Histiocytosis Advancements Parallel Ophthalmic Innovations: The LXXXI Edward Jackson Memorial Lecture

PURPOSE

To highlight innovations in ophthalmic oncology through histiocytosis advancements.

DESIGN

Perspective and retrospective review.

METHODS

The literature outlining the recent advancements in histiocytosis and ocular oncology was reviewed and combined with trial data and personal recollection. Intersections between these two fields were discussed.

RESULTS

The understanding of genetic mutations in disease-both in which cells they occur and the timing of mutation development-has expanded in tandem for the fields of ophthalmic oncology and histiocytosis. Similarly, advancements in diagnostic and treatment technology in one field can help patients in the other. For example, in one study, cell-free DNA testing reliably detected mutations in 14 of 18 (78%) patients with suspected histiocytosis. This technique has also been used in ophthalmic oncology as an alternative to invasive biopsy to avoid the risk of tumor externalization, vision impairment, and other side effects. These and other advancements have allowed both fields to utilize targeted agents to successfully treat diseases with an actionable mutation; or deliver more targeted chemotherapy via the intraarterial technique.

CONCLUSIONS

The explosion of molecular genetics technology and targeted therapies has revolutionized cancer treatment, including histiocytosis and ophthalmic oncology. Recent progress in both fields has shown how these seemingly disparate areas have many intersections, and this speaks to the collaborative spirit that is inherent in clinical research.

Recent advances in therapeutic strategies of Erdheim-Chester disease

Erdheim-Chester disease (ECD) is a rare form of non-LCH characterized by excessive accumulation of histiocytes in various tissues, leading to significant morbidity. The estimated prevalence of ECD is low, with fewer than 1000 cases reported globally, yet it presents considerable clinical challenges due to its heterogeneous manifestations, which include bone pain, cardiovascular complications, and neurological symptoms. Traditional treatment approaches, primarily involving corticosteroids and chemotherapy, have limitations, including inconsistent responses and significant side effects. Recent advances in understanding the pathogenesis of ECD, particularly the role of the BRAF V600E mutation, have led to the exploration of novel therapeutic strategies, such as targeted BRAF inhibitors, MEK and mTOR inhibitors, and other immunotherapies, which offer promise in improving patient outcomes. The review further explores clinical manifestations, and radiographic features of Erdheim-Chester disease, and discusses treatment strategies, current clinical studies in the field of ECD. By integrating these aspects, this review aims to provide a thorough understanding of ECD and its evolving treatment landscape, ultimately contributing to improved patient outcomes.

Updates on Langerhans cell histiocytosis and other histiocytosis in children: invited review-challenges and novelties in paediatric tumours

Langerhans cell histiocytosis (LCH), juvenile xanthogranuloma (JXG) family lesions, and Rosai-Dorfman-Destombes disease (RDD) are now classified by the World Health Organization (WHO) under the heading of histiocytic/dendritic cell neoplasms. Each disease may manifest as a focal lesion, as multiple lesions, or as a widespread aggressive systemic disease with visceral organ involvement. Erdheim-Chester disease (ECD) is a rare systemic disease process of adults with limited cases in children. Challenges in diagnosis and novel disease presentations, including ALK-positive histiocytosis (a newly recognized WHO entity), mixed histiocytosis, and secondary histiocytic lesions following a prior leukemia/lymphoma are also discussed. Malignant histiocytic neoplasms (MHN) are distinct high-grade histiocytosis, which while rare in childhood occur both as primary disease and as secondarily after a prior hematologic malignancy. Of note, despite its name, hemophagocytic lymphohistiocytosis (HLH) is not considered a histiocytic neoplasm and does not define one specific disease "entity." HLH is a spectrum of hyperinflammation with various triggers and is not covered for the purposes of this targeted review.

Approach to the Patient: From Endocrinopathy to the Diagnosis of a Histiocytic Disorder

Endocrinopathies are frequently the initial presentation of histiocytic neoplasms, which are rare hematologic disorders affecting multiple organ systems. Langerhans cell histiocytosis and Erdheim-Chester disease are 2 such disorders known to infiltrate the hypothalamus and/or pituitary gland, leading to arginine vasopressin deficiency (AVP-D) and anterior pituitary dysfunction (APD) in 20% to 30% of cases, often as the first manifestation. Conversely, histiocytic disorders account for a notable proportion (10-15%) of all pituitary stalk lesions. The diagnosis of histiocytoses is often delayed in such cases due to the nonspecific presentation of endocrinopathies and pituitary involvement. Consequently, endocrinologists are at the frontline and uniquely positioned to achieve early diagnosis by recognizing the varied nonendocrine features of these disorders. This article provides an overview of the endocrine manifestations of histiocytic disorders and presents a simplified algorithm to guide the diagnostic workup in cases presenting with "idiopathic" AVP-D or APD. Such cases should be evaluated for histiocytic neoplasms with additional imaging studies and biopsies of suspected disease sites. If no disease site beyond the pituitary is identified, the risks and benefits of a pituitary stalk lesion biopsy must be carefully considered. While treatments of histiocytic neoplasms are highly efficacious, endocrinopathies are considered permanent and require long-term hormone replacement. It remains unclear whether early diagnosis and novel targeted therapies can reverse these endocrine disorders. Therefore, the role of the endocrinologist role is critical in the diagnosis and management of these rare diseases.

BRAF V600E mutation and high expression of PD-L1 in Rosai-Dorfman disease: case report and review of the literature

BRAF V600E mutations are frequently found in histiocytic/dendritic cell neoplasms such as Erdheim-Chester disease (ECD) and Langerhans cell histiocytosis (LCH), but few reports have also described BRAF mutations in Rosai-Dorfman disease (RDD), and even these cases may predominantly represent mixed histiocytosis. BRAF mutations have been studied in histiocytic/dendritic cell neoplasms and described to be associated with increased risk of relapse and long-term consequences, but few studies have examined BRAF V600E mutation in RDD, which is recognized as a neoplasm given the high frequency of MAPK pathway alterations. Here, we report a case of BRAF V600E-mutated RDD in a patient who presented with generalized lymphadenopathy. During our evaluation of this patient, we also found expression of PD-L1 in neoplastic histiocytes. During our review period, only few cases of RDD reported to harbor BRAF mutation or were evaluated for the expression of PDL1 by neoplastic cells. Given the potential challenges in distinguishing RDD from other histiocytic/dendritic cell neoplasms, including mixed histiocytosis with similar clinicopathological manifestations, we will discuss the current state of knowledge regarding the frequency and clinical impact of BRAF V600E in RDD, as well as the role of BRAF mutations in RDD pathogenesis. Distinction of BRAF V600E mutated histiocytic/dendritic cell neoplasms requires consideration of distinctive histopathological and immunophenotypic findings in appropriate clinical and radiologic setting. Given the increasing use of BRAF inhibitors as well as checkpoint blockade inhibitors to treat a number of cancers, we will discuss the clinical implications of the presence of BRAF V600E mutation and PD-L1 expression in RDD.

Orbital masses as a rare presentation of Rosai-Dorfman disease: Clinicopathologic characterization of five cases

Rosai-Dorfman disease (RDD) is a rare, non-Langerhans cell histiocytosis. Most cases present with marked, non-tender lymphadenopathy due to the proliferation of atypical histiocytes. A minority of cases involves extranodal sites and can present as bone lesions, skin rashes, pulmonary nodules, and rarely orbital masses. Orbital involvement in RDD is rare and may infrequently present as an isolated tumor mass without lymphadenopathy. This study aims to better characterize this uncommon presentation of this rare disease. Five cases of orbital RDD were identified from the last 18 years and the clinical characteristics of each case were compared with histopathological findings. Three men and two women ages 12-36 presented with complaints of eye swelling and/or vision changes. One patient had a history of neurofibromatosis type I and inflammatory pseudotumors while the other four had no signs of systemic disease or other sites of extranodal involvement at the time of presentation. Masses ranged in size from 1.0 cm to 3.5 cm and primarily involved the superior orbit. Resected lesions all displayed characteristic findings of admixed atypical histiocytes, lymphocytes, and plasma cells with a fibrotic background. Emperipolesis was seen in all cases. Immunostaining for S100 and CD68 was diffusely positive in the histiocyte population. Clinical follow-up was obtained for 4 of 5 patients: all four were disease-free at 1 to 15 years after resection. RDD should be considered in the differential for patients with orbital masses, even in the absence of lymphadenopathy or signs of systemic disease.

Indeterminate DC histiocytosis is distinct from LCH and often associated with other hematopoietic neoplasms

ABSTRACT

Indeterminate dendritic cell histiocytosis (IDCH) is a rare and poorly understood entity characterized by accumulation of CD1a+/S100+ histiocytes (as Langerhans cell histiocytosis [LCH]) but with reduced-absent expression of Langerin/CD207. We assembled 43 cases of IDCH (defined by CD1a+/CD207<20% immunophenotypic profile) examining the clinical, pathologic, and molecular landscape. Median age at presentation was 70 years (interquartile range, 44-80) with cutaneous (31/43; 72%) and nodal (11/43; 26%) involvement predominating. Eighteen (42%) individuals had an associated nonhistiocytic hematopoietic neoplasm ("secondary" IDCH) whereas 7 of 43 (16%) had a concurrent non-IDCH histiocytosis ("mixed" histiocytosis). Most cases exhibited morphology indistinguishable from LCH but with a CD1c+/CSF1R(CD115)- phenotype, mirroring the signature of normal indeterminate cells and conventional DC type 2. Mutational analysis revealed frequent KRAS (13/32; 41%) and BRAF p.V600E (11/36, 31%) mutations that were nearly mutually exclusive. RNA-sequencing analysis uncovered ETV3::NCOA2 fusion in 6 other patients presenting as a sole genetic alteration without any other concurrent histiocytic or hematopoietic neoplasm. BRAF and MAP2K1 alterations were significantly associated with partial/retained (1%-20%) Langerin expression (P = .005) and mixed histiocytosis (P = .002). Remarkably, myeloid alterations (DNMT3A, TET2, and SRSF2) co-occurred in IDCH tissues of several individuals. Paired sequencing of IDCH and concurrent non-IDCH hematopoietic neoplasm in 4 individuals revealed shared mutations. Age at diagnosis and any nodal involvement at diagnosis predicted inferior overall survival, but BRAF/RAS pathway alterations did not affect outcome. These data have implications for the diagnostic evaluation, classification, and therapeutic management of IDCH.

Rosai-Dorfman-Destombes disease in adults: a single center experience

Recent advances in Rosai-Dorfman-Destombes disease (RDD), notably molecular testing, targeted therapy, and PET-CT imaging, hold promise for better recognition and improved outcomes. This study presents patients diagnosed and treated in a "real world" setting, where navigating limited resources must be considered. This retrospective single-center review includes 15 adult patients diagnosed with RDD at Vancouver General Hospital between November 2015 and October 2023. The cohort comprised five males and ten females with a median age 53 years (range 19-80 years). All 15 patients had extra-nodal disease; 11 patients exclusively had extra-nodal disease, and four patients also had lymph node involvement. Seven patients had tissue next-generation sequencing, identifying MAP2K1 mutations in four cases and a KRAS p.K117N mutation in one case that was treated with targeted therapy using trametinib. PET-CT was used for disease staging in four cases. Six patients with refractory disease tolerated lenalidomide and dexamethasone without significant toxicity; three patients achieved complete response, and three had partial response. This study highlights RDD's diverse extra-nodal manifestations. Lenalidomide combined with dexamethasone is an effective and well-tolerated treatment option for select patients, especially those with refractory disease. Broad utilization of NGS and PET-CT can positively influence management decisions.

Imaging in Erdheim-Chester Disease

Erdheim-Chester disease (ECD) is a rare, multisystemic, inflammatory, non-Langerhans cell histiocytic neoplasm. The discovery of recurrent and somatic mutations in the mitogen-activated protein kinase signaling pathway, most commonly BRAFV600E, has led to a reclassification of ECD from an inflammatory disorder to a neoplastic process. It is now included in the revised 2016 World Health Organization classification of hematopoietic tumors and in the Langerhans group in the revised 2016 Histiocytosis Classification of the Histiocyte Society. When symptomatic, ECD most commonly manifests with bone pain and fatigue. Also, neurologic manifestations, central diabetes insipidus, exophthalmos, and periorbital xanthelasma-like lesions are frequently encountered. Pathologic findings may vary depending on the site of biopsy and may display a spectrum of features. Thus, due to the diverse clinical presentation and variable histologic findings, imaging can often show the first sign of the disease. Radiologic findings are, however, interpreted in conjunction with clinical and histologic findings to establish the diagnosis of ECD. From providing classic findings that facilitate diagnosis to helping radiologists determine the extent of disease and predicting a prognosis, the role of radiology in ECD has evolved with the understanding of the disease itself. Insights into the molecular pathogenesis and the development of targeted therapeutic agents along with approval of vemurafenib and cobimetinib have necessitated revision of the guidelines for the management of ECD. The authors discuss various radiologic findings of ECD and differential diagnoses by using an organ system-based approach and briefly describe the revised consensus recommendations for evaluation, diagnosis, and treatment based on the International Medical Symposia on ECD from a radiologist's perspective. ©RSNA, 2024 Supplemental material is available for this article. The full digital presentation is available online.

Clinical phenotype of adult-onset systemic histiocytosis harboring BRAF in-frame deletions

Not available.

Advances in Understanding and Management of Erdheim-Chester Disease

Erdheim Chester Disease (ECD) is a rare histiocytic disorder marked by infiltration of organs with CD68+ histiocytes. ECD stems from mutations of BRAF and MAP2K1 in hematopoietic stem and progenitor cells (HSPCs), which further differentiate into monocytes and histiocytes. Histopathology reveals lipid-containing histiocytes, which test positive for CD68 and CD133 in immunohistochemistry. Signs and symptoms vary and depend on the organ/s of manifestation. Definitive radiological results associated with ECD include hairy kidney, coated aorta, and cardiac pseudotumor. Treatment options primarily include anti-cytokine therapy and inhibitors of BRAF and MEK signaling.

Selected Case From the Arkadi M. Rywlin International Pathology Slide Seminar: Involvement of Skin and Soft Tissue by Erdheim-Chester Disease

Erdheim-Chester disease is a rare form of non-Langerhans cell histiocytosis that preferentially involves long bones but can affect a variety of other organs. Initial presentation with extraskeletal involvement is not unusual and is most commonly observed in the central nervous system, heart, retroperitoneum, lungs, and skin. Initial presentation of the disease as a subcutaneous soft tissue mass is exceedingly rare and may pose difficulties for diagnosis. We describe a case of Erdheim-Chester disease that initially presented as a cutaneous and subcutaneous soft tissue mass in the right posterior shoulder of a 52-year-old man.

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.

Lenalidomide and dexamethasone for Rosai-Dorfman disease: a single arm, single center, prospective phase 2 study

Background

Rosai-Dorfman disease (RDD) is a rare heterogeneous histiocytic disorder lacking standardized first-line treatment.

Methods

This single-center, phase 2 prospective study enrolled 13 newly diagnosed and 10 recurrent RDD patients from June 2021 to March 2023 at Peking Union Medical College Hospital (Beijing, China). Lenalidomide 25 mg days 1-21 plus dexamethasone 40 mg days 1, 8, 15, 22 was administered in 28-day cycles, totaling 12 cycles. The primary endpoint was progression-free survival (PFS). Secondary endpoints were overall response rate (ORR) to lenalidomide and dexamethasone (RD) regimen, toxicity, and overall survival (OS) measured from RD start to death or last follow-up. OS and PFS were estimated according to Kaplan-Meier survival analysis and compared with the log-rank test. For OS and OR rate, 95% confidence limits were obtained using the Clopper-Pearson method, with standard methods used for PFS. p < 0.05 was considered statistically significant. The trial was registered with ClinicalTrials.gov (NCT04924647).

Findings

The median age was 44 years (IQR 35-54). All patients had extranodal RDD. MAPK pathway alterations occurred in 6/18 (33%). Elevated IL-6 and TNF-α were found in 39% (n = 9) and 70% (n = 16), respectively. All patients received ≥6 cycles (median 12, range 6-12, IQR 10-12). The ORR was 87% (20/23, 95% CI 66%-97%), 30% (n = 7) complete remission, 57% (n = 13) partial remission). Treatment with RD significantly decreased median serum levels of both IL-6 (from 5.9 (IQR 4.2-8.7) to 2.9 (IQR 2.1-5.9) pg/mL, p = 0.031) and TNF-α (from 12.2 (IQR 8.6-17.9) to 8.3 (IQR 6.1-10.5) pg/mL, p = 0.0012). With a median 26 months follow-up (range 6-28, IQR 16-28), 4 patients relapsed and none died. Two-year OS and PFS were 100.0% (95% CI 85%-100%) and 69.0% (95% CI 51%-94%), respectively. No grade 3-4 adverse events or discontinuations due to adverse events occurred. Twelve patients (n = 12, 52%) had grade 1-2 hematological toxicity. Other toxicities included constipation (n = 2, 9%), glucose intolerance (n = 2, 9%), edema (n = 2, 9%), insomnia (n = 1, 4%), and tremor (n = 1, 4%).

Interpretation

Lenalidomide and dexamethasone regimen is an effective and safe regimen for newly diagnosed and recurrent RDD.

Funding

National Natural Science Foundation of China, Beijing Natural Science Haidian frontier Foundation Funding, and the National High Level Hospital Clinical Research Funding.

Characterizing Rosai-Dorfman disease with [18F]FDG PET/CT: a retrospective analysis of a single-center study

OBJECTIVES

Rosai-Dorfman disease (RDD) is a rare non-Langerhans cell histiocytic proliferative disorder. We performed a retrospective study to characterize the clinical and [18F]FDG PET/CT features of RDD.

METHODS

We retrospectively recruited 38 RDD patients with [18F]FDG PET/CT scan in our center. [18F]FDG PET/CT features were assessed, and clinical and follow-up data were recorded.

RESULTS

In the recruited patients, 20/38 (52.6%) patients had single-system disease, while others (18/38, 47.4%) had disease affecting multiple system. RDD most commonly involved the upper respiratory tract (47.4%), followed by cutaneous/subcutaneous lesion (39.5%), lymph node (36.8%), bone (31.6%), central nervous system (28.9%), and cardiovascular system (13.2%) in the recruited patients. In PET/CT, the RDD lesions were FDG-avid, and the SUVmax of the hottest lesion in an individual patient was positively correlated with C-reactive protein levels (r = 0.418, p = 0.014), and negatively correlated with hemoglobin levels (r =  -0.359, p = 0.036). The overall response rate of the first-line treatment was 80.8% in newly diagnosed RDD patients, and for patients with relapsed/progressive RDD, the overall response rate was 72.7%.

CONCLUSION

[18F]FDG PET/CT could be a useful tool for evaluating RDD.

KEY POINTS

• About half of the patients with Rosai-Dorfman disease had single-system disease, while others had disease affecting multiple system. Rosai-Dorfman disease most commonly involved the upper respiratory tract, followed by cutaneous/subcutaneous lesion, lymph node, bone, central nervous system, and cardiovascular system. • In [18F]FDG PET/CT, Rosai-Dorfman disease was usually hypermetabolic, and the SUVmax of the hottest lesion in an individual patient was positively correlated with C-reactive protein levels. • Rosai-Dorfman disease usually has a high overall response rate after treatment.

The value of 18F-FDG PET/CT in the systemic evaluation of patients with Rosai-Dorfman disease: a retrospective study and literature review

BACKGROUND

Rosai-Dorfman disease (RDD) is a rare form of non-Langerhans cell histiocytic disease. The aim of this study was to review the characteristics of RDD using 18F-FDG PET/CT and determine its efficacy in the disease management.

RESULTS

A total of 28 RDD patients underwent 33 18F-FDG PET/CT scans for systematic assessment and follow-up. The common involved sites included the lymph nodes (17, 60.7%), upper respiratory tract (11, 39.3%), and skin (9, 32.1%). Five patients had more lesions detected in PET/CT images than in CT and/or MRI, including inapparent nodules (n = 5) and bone destruction (n = 3). After thorough treatment evaluation using PET/CT, the treatment strategies of 14 patients (14/16, 87.5%) were changed. Five patients underwent PET/CT twice during follow-up and the SUVs were significantly decreased (15.3 ± 3.4 vs. 4.4 ± 1.0, p = 0.02), which demonstrated disease improvement.

CONCLUSIONS

18F-FDG PET/CT contributed to displaying the holistic characteristics of RDD, in particular during initial assessment, treatment strategy adjustment, or efficacy evaluation, and could compensate for some disadvantages of CT and MRI images.

How I Diagnose Rosai-Dorfman Disease

OBJECTIVES

Rosai-Dorfman disease (RDD) is one of 3 major types of histiocytosis, along with Erdheim-Chester disease and Langerhans cell histiocytosis. While historically, RDD was considered a benign self-limited condition, current data show MAPK/ERK pathway mutations in 30% to 50% of cases, indicative of a clonal process. Rosai-Dorfman disease was incorporated as a histiocytic neoplasm in the fifth edition of the World Health Organization classification of hematopoietic tumors and the International Consensus Classification.

METHODS

We discuss the diagnosis of RDD using 2 illustrative cases, interpretative challenges, and a diagnostic algorithm.

RESULTS

Rosai-Dorfman disease involves nodal and extranodal sites, including skin, sinuses, salivary gland, orbit, central nervous system, kidney, and bone. In a subset, RDD can coexist with other neoplasms (lymphomas, other histiocytosis) or autoimmune disease. Morphologically, RDD histiocytes are characterized by enlarged round to oval nuclei, distinct nucleoli, and voluminous cytoplasm with engulfment of inflammatory cells (emperipolesis). By immunohistochemistry, they express CD68, CD163 (majority), S100, OCT2, and cyclin D1. Appropriate use of ancillary studies is important to support the diagnosis of RDD while excluding other histiocytic neoplasms and reactive histiocytic proliferations.

CONCLUSIONS

Management of RDD is dependent on the extent of organ involvement and clinical symptoms. In patients who require therapy, next-generation sequencing is recommended to identify MAPK/ERK pathway mutations for targeted therapy.

OCT2 expression in histiocytoses

Diagnosis of histiocytosis can be difficult and one of the biggest challenges is to distinguish between reactive and neoplastic histiocytes on histology alone. Recently, OCT2 nuclear expression was reported in Rosai-Dorfman disease (RDD). Our purpose was to expand the testing of OCT2 on a broader variety of sporadic or H syndrome-related histiocytoses. Cases of histiocytoses were retrieved from the files of Ambroise Paré Pathology Department. All slides and molecular analyses were reviewed, and staining was completed with immunohistochemistry for OCT2. A total of 156 samples from different localizations were tested. Among sporadic cases, 52 patients had RDD, and 10 patients had mixed histiocytosis combining RDD with Erdheim Chester disease (ECD, n = 8), Langerhans cell histiocytosis (LCH, n = 2) or juvenile xanthogranuloma (JXG, n = 1). All these patients were positive for OCT2 in RDD characteristic histiocytes. Twenty-three patients had ECD and all but two (91% - 21/23) were negative for OCT2. By contrast, OCT2 was positive in 11/27 (41%) LCH and 6/16 (38%) JXG. Among the 10 samples of H syndrome-associated histiocytosis, 3 had typical RDD histology, 6 had unclassified histiocytosis, and one had mixed RDD-LCH; all were positive for OCT2. On 16 samples of granulomatous lymphadenitis, OCT2 was negative in epithelioid histiocytes. Our study shows that OCT2 has a sensitivity of 100% for RDD cases and mixed histiocytoses with an RDD component. It is negative in 92% of ECD but expressed in at least 38% of LCH, JXG, and C group histiocytoses. Finally, OCT2 is positive in all H syndrome-related histiocytoses, independent of their histology.

Erdheim-Chester disease with Rosai-Dorfman-like lesions: treatment with methotrexate, anakinra and upadacitinib

Erdheim-Chester disease (ECD) is a non-Langerhans cell histiocytosis characterised by clonal expansion of histiocytes in various organs. These induce an inflammatory environment, which leads to damage of the affected areas. Recently, a new disease entity was proposed encompassing key features of ECD but also of Rosai-Dorfman-Destombes disease, another histiocytosis. Mitogen-activated protein kinase kinase 1 (MAP2K1) mutations seem to present a specific genetic lesion for this subtype.Here, we describe a case of this new disease entity with clinical, radiological and genetic findings compatible with ECD but histological findings compatible with Rosai-Dorfman-Destombes disease. In particular, there were intraabdominal and retroperitoneal lesions, which tested positive for a (c.167A>C; p.Q56P) mutation of the MAP2K1 gene. On histological examination, S100-positive, giant histiocytes with focal emperipolesis of haematological cells in addition to infiltration by lymphocytes and granulocytes were seen.As described for this rare variant of ECD, there was also bilateral testicular infiltration. We also describe a manifestation of oligoarthritis in this patient with ECD.The patient was treated with methotrexate and prednisolone. While radiological response to this regime was excellent, arthritis persisted. We added anakinra, which induced a response of the arthritis for more than a year. Due to treatment failure therapy was switched to upadacitinib, which induced a remission of the arthritis as well.This case adds a rare phenotype to an already rare presentation of ECD. The patient responded to immunosuppressive therapy.

Rosai-Dorfman Disease between Proliferation and Neoplasia

Rosai-Dorfman disease (RDD) is a rare myeloproliferative disorder of histiocytes with a broad spectrum of clinical manifestations and peculiar morphologic features (accumulation of histiocytes with emperipolesis). Typically, the patient with RDD shows bilateral painless, massive cervical lymphadenopathy associated with B symptoms. Approximately 43% of patients presented with extranodal involvement. According to the 2016 revised histiocytosis classification, RDD belongs to the R group, including familial and sporadic form (classical nodal, extranodal, unclassified, or RDD associated with neoplasia or immune disease). Sporadic RDD is often self-limited. Most RDD needs only local therapies. Nevertheless, a small subpopulation of patients may be refractory to conventional therapy and die of the disease. Recent studies consider RDD a clonal neoplastic process, as approximately 1/3 of these patients harbor gene mutations involving the MAPK/ERK pathway, e.g., NRAS, KRAS, MAP2K1, and, rarely, the BRAF mutation. In addition to typical histiocytic markers (S100/fascin/CD68/CD163, etc.), recent studies show that the histiocytes in RDD also express BCL-1 and OCT2, which might be important in pathogenesis. Additionally, the heterozygous germline mutation involving the FAS gene TNFRSF6 is identified in some RDD patients with an autoimmune lymphoproliferative syndrome type Ia. SLC29A3 germline mutation is associated with familial or Faisalabad histiocytosis and H syndrome.

[Pelvic mass in 70 years old patient: Difficult diagnosis of Erdheim-Chester disease]

Erdheim-Chester disease (EC) is a rare disease that is included in Group L in the 2016 revised classification of Langheransian histiocytoses. This disease may be clinically asymptomatic or manifest as a multi-systemic and life-threatening condition. All organs can be affected but typically there is bone involvement, retroperitoneal fibrosis, pituitary involvement, involvement of large vessels, lung, pleura or central nervous system. We are reporting a 70-year-old patient who, as of 2014, had a pelvic mass with retroperitoneal fibrosis and large vessel vasculitis without a definite diagnosis. Histological and molecular examination of the surgical specimen of the pelvic mass with the discovery of the BRAF V600E mutation provided new elements for the definitive diagnosis of Erdheim-Chester disease. We will describe the clinical, histological and molecular features to be known in EC disease.

Rosai-Dorfman disease. A legacy of Professor Rosai that is still not exploited completely

Rosai-Dorfman disease (RDD) is a rare form of non-Langerhans cell histiocytosis described by Rosai and Dorfman in 1969. It is a fascinating disease characterized by accumulation of large, pale histiocytes, frequently showing the emperipolesis phenomenon. The variety of pathological aspects and the spectrum of different clinical forms were deeply investigated by Prof. Rosai. Despite recent advancements in the dissection of pathogenetic mechanisms of RDD, with the identification of gene mutations in the MAP kinase pathway, several biological and clinical aspects of this disease remains to be elucidated: this is one of the Prof. Rosai's legacies.

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.

Histiocytosis and the nervous system: from diagnosis to targeted therapies

Histiocytoses are heterogeneous hematopoietic diseases characterized by the accumulation of CD68(+) cells with various admixed inflammatory infiltrates. The identification of the pivotal role of the mitogen-activated protein kinase (MAPK) pathway has opened new avenues of research and therapeutic approaches. We review the neurologic manifestations of 3 histiocytic disorders with frequent involvement of the brain and spine: Langerhans cell histiocytosis (LCH), Erdheim-Chester disease (ECD), and Rosai-Dorfman-Destombes disease (RDD). Central nervous system (CNS) manifestations occur in 10%-25% of LCH cases, with both tumorous or neurodegenerative forms. These subtypes differ by clinical and radiological presentation, pathogenesis, and prognosis. Tumorous or degenerative neurologic involvement occurs in 30%-40% of ECD patients and affects the hypothalamic-pituitary axis, meninges, and brain parenchyma. RDD lesions are typically tumorous with meningeal or parenchymal masses with strong contrast enhancement. Unlike LCH and ECD, neurodegenerative lesions or syndromes have not been described with RDD. Familiarity with principles of evaluation and treatment both shared among and distinct to each of these 3 diseases is critical for effective management. Refractory or disabling neurohistiocytic involvement should prompt the consideration for use of targeted kinase inhibitor therapies.

Histiocytosis

Histiocytoses constitute a heterogeneous group of rare disorders, characterised by infiltration of almost any organ by myeloid cells with diverse macrophage or dendritic cell phenotypes. Histiocytoses can start at any age. Diagnosis is based on histology in combination with appropriate clinical and radiological findings. The low incidence and broad spectrum of clinical manifestations often leads to diagnostic delay, especially for adults. In most cases, biopsy specimens infiltrated by histiocytes have somatic mutations in genes activating the MAP kinase cell-signalling pathway. These mutations might also be present in blood cells and haematopoietic progenitors of patients with multisystem disease. A comprehensive range of investigations and molecular typing are essential to accurately predict prognosis, which can vary from spontaneous resolution to life-threatening disseminated disease. Targeted therapies with BRAF or MEK inhibitors have revolutionised salvage treatment. However, the type and duration of treatment are still debated, and the prevention of neurological sequelae remains a crucial issue.

Immune Thrombocytopenia Revealing Enriched IgG-4 Peri-Renal Rosai-Dorfman Disease Successfully Treated with Rituximab: A Case Report and Literature Review

Immune thrombocytopenia (ITP) is a rare autoimmune-mediated condition characterized by isolated thrombocytopenia (<100 G/L) after exclusion of other causes. Mostly primary, it is associated with hematological malignancy, autoimmune disorders, or infection in 20% of patients. It is exceptionally described in patients with histiocytosis, mostly in children (seven patients in literature). We report a case of a 69-year-old man with ITP leading to the diagnosis of histiocytosis. At ITP's diagnosis, the patient had elevated gamma-globulins leading to computed tomography showing bilateral peri-renal infiltration. The biopsy showed enriched IgG-4 peri-renal Rosai Dorfman disease with MAP2K1 mutation, although peri-renal infiltration is highly suggestive of Erdheim-Chester disease. This overlapping association was described in men with mutation in MAP2K1 gene. Macrophages are implicated in the pathophysiology of ITP in multiple ways, notably by the phagocytosis of opsonized platelets and their function of antigen-presenting cells able to stimulate autoreactive T cells. Histiocytic cells derivate from monocyte-macrophage lineage. Activation of macrophages in active histiocytosis is responsible for consequential platelet destruction in ITP associated histiocytosis. Finally, this case highlights a rare presentation of ITP revealing histiocytosis, both being efficiently treated with rituximab.

Erdheim-Chester disease: a comprehensive review from the ophthalmologic perspective

Erdheim-Chester disease (ECD) is a rare clonal histiocytic neoplasm with less than 1200 documented cases to date. The disease is life-threatening and difficult to recognize, although increasing awareness as well as the integration of clinical, imaging, pathology information , and genetic studies have led to a recent exponential increase in new reported cases. ECD affects multiple organs and systems, including skeletal, neurologic, and cardiovascular. Pulmonary, retroperitoneal, and cutaneous lesions have also been reported in various combinations. Until the discovery that more than half of ECD patients harbor the BRAF-V600E mutation or other mutations in the mitogen-activated protein kinase (MAPK) and RAS pathways, Interferon-a was the first-line treatment. Nowadays BRAF and MEK-inhibitors targeted therapies are the mainstay of treatment. Ophthalmologic involvement occurs in 25% -30% of ECD cases, usually in the form of orbital involvement presenting with exophthalmos and ophthalmoplegia. Other ophthalmologic manifestations include palpebral xanthelasmas, anterior uveitis and vitritis, optic disk edema, choroidal infiltration, recurrent serous retinal detachment, retinal drusen-like deposits and retinal pigment epithelial changes. ECD patients can also present with ocular symptoms as a result of adverse effects of the treatment regimens. In some cases with smoldering or protean symptoms, the emergence of eye manifestations triggered the diagnosis. Ophthalmologists have to be aware of the disease, recognize the constellation of ECD symptoms, and contribute to the diagnosis, treatment, and follow-up of ECD patients.

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.

Rosai-Dorfman Disease Displays a Unique Monocyte-Macrophage Phenotype Characterized by Expression of OCT2

Rosai-Dorfman disease (RDD) is a rare histiocytosis with heterogenous clinical features. In this study, we characterized the histologic and phenotypic features in 33 RDD patients to better define the pathologic diagnosis. Cases included 24 patients with extracutaneous disease ("R" group), and 9 patients with lesions limited to the skin or subcutaneous tissue ("C" group). We identified OCT2 as a novel marker for the monocyte-macrophage phenotype of RDD, expressed in 97% of RDD cases. In contrast, OCT2 expression was seen in 0% of Erdheim-Chester disease cases and 6.7% of Langerhans cell histiocytosis cases. Other markers useful in the diagnosis of RDD included S100 (100%), CD163 (88%), and cyclin D1 (97%). In a subset of cases, RDD showed moderate to strong expression of factor 13a (30%), p16 (64%), and phosphorylated extracellular signal-regulated kinase (45%); RDD was uniformly negative for ZBTB46, CD1a, and langerin. Within the "R group" of RDD, increased expression of factor 13a or phosphorylated extracellular signal-regulated kinase showed a statistically significant association with multifocal disease (P<0.05). Identification of the unique monocyte-macrophage phenotype of RDD with OCT2 expression furthers our understanding of this complex disease and allows for more uniform classification.

Cerebellar ataxia and exercise intolerance in Erdheim-Chester disease

Background

Erdheim-Chester disease (ECD), a rare disorder of monocyte/macrophage lineage, has been related to cerebellar dysfunction. To increase the awareness of this rare, protean disease, an unusual, myasthenia-like onset of ECD is reported.

Case presentation

A 42-year-old man presented with a 6-year history of mild evening fatigability in his four limbs followed by motor and cognitive symptoms associated with cerebellar atrophy, dentate nuclei and dentato-thalamic pathway degeneration. Magnetic resonance imaging revealed hyperintense signals in T2 and fluid-attenuated inversion recovery sequences within the pons, cerebellar white matter, dentate nuclei and globi pallidi in the absence of any contrast enhancement. Whole-body bone scintigraphy with ⁹⁹Technetium - methylene diphosphonate and fluorodeoxyglucose-positron emission tomography both revealed symmetric uptake in the lower extremities a finding suggestive of a diagnosis of ECD. Histological examination revealed diffuse infiltration of CD 68⁺ histiocytes with foamy cytoplasms in the presence of B-type of Rapidly Accelerated Fibrosarcoma protein kinase (BRAF)^V600E activating mutation in tumor cells.

Conclusion

In patients with myasthenia-like symptoms who test negatively for myasthenia gravis, neurodegenerative diseases, and disorders of the hypothalamus, a diagnosis of ECD should be taken into consideration.

MAP2K1-driven mixed Langerhans cell histiocytosis, Rosai-Dorfman-Destombes disease and Erdheim-Chester disease, clonally related to acute myeloid leukemia

Mixed histiocytoses are a rare and recently recognized subset of histiocytic disorders that may involve the skin, characterized by the synchronous or metachronous development of lesions with Langerhans and/or non-Langerhans cell histiocytosis histopathological features. Around 10% of patients diagnosed with histiocytosis may develop a hematological malignancy, often with dramatic prognostic consequences. We hereby describe the exceptional case of a patient developing a MAP2K1-driven mixed histiocytosis with Langerhans cell histiocytosis, Rosai-Dorfman-Destombes disease, and Erdheim-Chester disease features and cutaneous involvement, progressing to a fatal and clonally-related acute myeloid leukemia. We reviewed the literature on similar cases and discussed the histopathological difficulties in their diagnosis and their clinical-pathological features.

Erdheim-Chester disease

Erdheim-Chester disease (ECD) is characterized by the infiltration of tissues by foamy CD68+CD1a- histiocytes, with 1500 known cases since 1930. Mutations activating the MAPK pathway are found in more than 80% of patients with ECD, mainly the BRAFV600E activating mutation in 57% to 70% of cases, followed by MAP2K1 in close to 20%. The discovery of BRAF mutations and of other MAP kinase pathway alterations, as well as the co-occurrence of ECD with LCH in 15% of patients with ECD, led to the 2016 revision of the classification of histiocytoses in which LCH and ECD belong to the "L" group. Both conditions are considered inflammatory myeloid neoplasms. Ten percent of ECD cases are associated with myeloproliferative neoplasms and/or myelodysplastic syndromes. Some of the most striking signs of ECD are the long bone involvement (80%-95%), as well as the hairy kidney appearance on computed tomography scan (63%), the coated aorta (40%), and the right atrium pseudo-tumoral infiltration (36%). Central nervous system involvement is a strong prognostic factor and independent predictor of death. Interferon-α seems to be the best initial treatment of ECD. Since 2012, more than 200 patients worldwide with multisystem or refractory ECD have benefitted from highly effective therapy with BRAF and MEK inhibitors. Targeted therapies have an overall, robust, and reproducible efficacy in ECD, with no acquired resistance to date, but their use may be best reserved for the most severe manifestations of the disease, as they may be associated with serious adverse effects and as-yet-unknown long-term consequences.

The triptych of mixed histiocytosis: a systematic review of 105 cases and proposed clinical classification

Histiocytoses are one of the ultimate diagnostic challenges that every physician face at least once in his/her life. Giving their protean manifestation and differentiated therapeutic needs, histiocytosis requires extensive characterization and multidisciplinary management. Mixed histiocytosis is an emerging group of syndromes defined by the overlap of Langerhans cell histiocytosis and another histiocytic disorder of different type. Despite rare, it may account for up to a fifth of systemic histiocytosis patients in some series. In this work, we comprehensively review for the first time the clinical, radiological, histopathological and molecular features of mixed histiocytosis in children and adults. Moreover, we propose a clinical classification in three groups that differentiate patients with systemic involvement and worse overall survival to other groups with more localized manifestations and indolent behavior, wanting to ease their recognition and treatment. Interestingly we also found that mixed histiocytosis harbor BRAFV600E mutations with a higher frequency comparing to all other histiocytoses, and may therefore benefit of specific inhibitory drugs.

Rosai-Dorfman Disease-Utility of 18F-FDG PET/CT for Initial Evaluation and Follow-up

BACKGROUND

The role of imaging in the management of Rosai-Dorfman disease (RDD), a rare non-Langerhans cell histiocytosis, is not clearly defined. We present an analysis of FDG PET/CT findings obtained for initial disease characterization, follow-up evaluation, and treatment planning for this disease.

METHODS

From an institutional pathology database (2001-2018), we identified RDD patients who underwent FDG PET/CT scans either as part of clinical care or when done as part of clinical trials. For all scans, sites of abnormal FDG uptake were assessed, and SUVmax was measured. Comparison of PET/CT findings was made with anatomic (CT/MRI-based) imaging, where available. Instances of changing treatment based on PET/CT were recorded.

RESULTS

We reviewed 109 FDG PET/CT scans in 27 patients with RDD. Five of 27 patients had only nodal/cutaneous disease, whereas 22 patients had extranodal disease, most commonly in bone (n = 9) and central nervous system (n = 7). PET/CT identified sites of active disease in 24 of 27 patients. All identified bone and extraskeletal lesions, except for a brain lesion in 1 patient, were FDG-avid. In 6 of 20 patients (30%) with available prior CT or MRI, PET/CT demonstrated additional RDD lesions (bones: n = 5, pleura: n = 1) that were not apparent on anatomic imaging; 3 of these lesions were outside the CT field of view, and 3 were not recognized on CT. Overall, 13 of 109 PET/CT scans led to a change in management, affecting 41% (11/27) of patients.

CONCLUSION

FDG PET/CT was valuable in defining disease extent and optimizing treatment strategy in patients with RDD.

Discovery through clinical sequencing in oncology

The molecular characterization of tumors now informs clinical cancer care for many patients. This advent of molecular oncology is driven by the expanding number of therapeutic biomarkers that can predict sensitivity to both approved and investigational agents. Beyond its role in driving clinical trial enrollments and guiding therapy in individual patients, large-scale clinical genomics in oncology also represents a rapidly expanding research resource for translational scientific discovery. Here, we review the progress, opportunities, and challenges of scientific and translational discovery from prospective clinical genomic screening programs now routinely conducted in cancer patients.

Rosai-Dorfman disease: an overview

CONTEXT

Rosai-Dorfman disease is an uncommon histiocytic disorder most frequently presenting as bilateral cervical lymphadenopathy in children and young adults. Extranodal disease occurs in a significant proportion of patients. It has been recently classified as part of the 'R group' of histiocytoses by the Histiocyte Society in 2016. Cutaneous Rosai-Dorfman disease is regarded as a separate disease entity that falls into the 'C group' of histiocytoses according to this classification system. The pathogenesis was previously poorly understood; however, recent evidence demonstrating clonality in a subset of cases raises the possibility of a neoplastic process. A possible association with IgG4-related disease remains controversial.

OBJECTIVES

To provide a comprehensive review of Rosai-Dorfman disease, including nodal, extranodal and cutaneous forms, with a particular emphasis on new insights into the possible clonal nature of the disease; to discuss the recently revised classification of the histiocytoses by the Histiocyte Society; and to summarise the findings from the literature regarding the controversial association with IgG4-related disease.

DATA SOURCES

This review is based on published peer-reviewed English literature.

CONCLUSIONS

Classic Rosai-Dorfman disease, which may be sporadic or familial, is considered a separate entity from cutaneous disease, which is reflected in the revised classification of histiocytoses. An increase in IgG4-positive plasma cells may be seen in Rosai-Dorfman disease. This finding in isolation is of limited significance and should be interpreted with caution. Studies investigating the molecular profile of the disease show that in at least a subset of cases the disease is a clonal process. The classification of Rosai-Dorfman disease is therefore likely to change as our understanding of the aetiopathogenesis evolves.

Reticulohistiocytoses: a revision of the full spectrum

Reticulohistiocytoses (RH) are rare and clinically heterogeneous histiocytic disorders of dermatological interest. Three clinical entities with superimposable histopathological features are currently considered, namely solitary reticulohistiocytoma, diffuse/generalized reticulohistiocytosis and multicentric reticulohistiocytosis. Although in the last decade, RH studies have only minimally progressed, histiocytosis research has advanced considerably: the prognostic and therapeutic importance of the clinical subclassification of histiocytosis patients as well as of the detection of genetic alterations in the genes of the ERK pathway has been highlighted. According to these insights, we previously reported the presence of molecular alteration RH and described a subset of patients with disseminated multisystem involvement lacking arthritis. In the present review, we aim to update and revise the knowledge regarding RH. We first reviewed their histopathological, immunophenotypical and ultrastructural features, discussed their histopathological differential diagnosis with other conditions characterized by infiltrates made of oncocytic or epithelioid cells (with special regard to Destombes-Rosai-Dorfman disease) and finally summarized the molecular landscape of RH. We therefore tried to adjust the clinical subclassification of Langerhans cell histiocytosis to the clinical phenotypes of RH, outlining five clinically different groups of patients. Finally, we reconsidered the clinical workflow to the evaluation of RH patients, in light of the 5 different clinical groups and discussed the different therapeutic approaches and the possible role of target inhibitors.