Loss of function of ENT3 drives histiocytosis and inflammation through TLR-MAPK signaling.

Histiocytoses are inflammatory myeloid neoplasms often driven by somatic activating mutations in mitogen-activated protein kinase (MAPK) cascade genes. H syndrome is an inflammatory genetic disorder caused by germ line loss-of-function mutations in SLC29A3, encoding the lysosomal equilibrative nucleoside transporter 3 (ENT3). Patients with H syndrome are predisposed to develop histiocytosis, yet the mechanism is unclear. Here, through phenotypic, molecular, and functional analysis of primary cells from a cohort of patients with H syndrome, we reveal the molecular pathway leading to histiocytosis and inflammation in this genetic disorder. We show that loss of function of ENT3 activates nucleoside-sensing toll-like receptors (TLR) and downstream MAPK signaling, inducing cytokine secretion and inflammation. Importantly, MEK inhibitor therapy led to resolution of histiocytosis and inflammation in a patient with H syndrome. These results demonstrate a yet-unrecognized link between a defect in a lysosomal transporter and pathological activation of MAPK signaling, establishing a novel pathway leading to histiocytosis and inflammation.

Efficacy of MEK inhibition in patients with histiocytic neoplasms.

Histiocytic neoplasms are a heterogeneous group of clonal haematopoietic disorders that are marked by diverse mutations in the mitogen-activated protein kinase (MAPK) pathway1,2. For the 50% of patients with histiocytosis who have BRAFV600 mutations3-5, RAF inhibition is highly efficacious and has markedly altered the natural history of the disease6,7. However, no standard therapy exists for the remaining 50% of patients who lack BRAFV600 mutations. Although ERK dependence has been hypothesized to be a consistent feature across histiocytic neoplasms, this remains clinically unproven and many of the kinase mutations that are found in patients who lack BRAFV600 mutations have not previously been biologically characterized. Here we show ERK dependency in histiocytoses through a proof-of-concept clinical trial of cobimetinib, an oral inhibitor of MEK1 and MEK2, in patients with histiocytoses. Patients were enrolled regardless of their tumour genotype. In parallel, MAPK alterations that were identified in treated patients were characterized for their ability to activate ERK. In the 18 patients that we treated, the overall response rate was 89% (90% confidence interval of 73-100). Responses were durable, with no acquired resistance to date. At one year, 100% of responses were ongoing and 94% of patients remained progression-free. Cobimetinib treatment was efficacious regardless of genotype, and responses were observed in patients with ARAF, BRAF, RAF1, NRAS, KRAS, MEK1 (also known as MAP2K1) and MEK2 (also known as MAP2K2) mutations. Consistent with the observed responses, the characterization of the mutations that we identified in these patients confirmed that the MAPK-pathway mutations were activating. Collectively, these data demonstrate that histiocytic neoplasms are characterized by a notable dependence on MAPK signalling-and that they are consequently responsive to MEK inhibition. These results extend the benefits of molecularly targeted therapy to the entire spectrum of patients with histiocytosis.

Cutaneous crystal storing histiocytosis: A case series with review of literature.

Crystal-storing histiocytosis (CSH) is a rare condition in which crystals accumulate in the cytoplasm of histiocytes and is usually associated with a lymphoplasmacytic neoplasm. Cutaneous CSH is extraordinarily rare and limited to case reports in the literature. We report two cases of this disease with cutaneous involvement. Case 1 was a 65-year-old male with a 4-month history of a pruritic eruption that started as a solitary pink to skin-colored indurated plaque on the anterior neck before progressing to involve the whole neck, chest wall, and face. Case 2 was a 54-year-old woman with a history of unspecified "lymphoma" who presented with a soft nodule on the forearm. Biopsies from both cases had similar findings and showed a proliferation of epithelioid cells with pink cytoplasm and intracellular crystalline structures infiltrating the dermis and subcutaneous fat. In the first case, the cells were positive for CD43, CD45, CD68, and IgG kappa, and in the second case, the crystals were positive for IgG lambda. Based on these findings, the patients were diagnosed with cutaneous CSH. We highlight this rare diagnosis and the importance of investigating an underlying lymphoplasmacytic neoplasm.

Anaplastic lymphoma kinase-positive histiocytosis with multisystem involvement: A case report.

Anaplastic lymphoma kinase (ALK)-positive histiocytosis is a rare disease that usually occurs in infants and young children and is characterized by ALK-positive histiocytes infiltrating organs. We present a case of multisystem involvement of ALK-positive histiocytosis in a female infant with skin nodules as the initial presentation. Despite multiorgan involvement, most tumors had spontaneously regressed, and all bones were partially healed after 40 months of regular follow-up without treatment. However, gait abnormalities persisted, indicating that early treatment may have greater impact in maintaining a child's quality of life when the disease involves the brain or the critical period of bone development.

PU.1 is a useful nuclear marker to distinguish between histiocytosis and histiocyte-rich tumours.

AIMS: The aim was to test the expression of PU.1 on different types of histiocytoses and to test the utility of PU.1 in confirming or excluding a histiocytic origin in tumour samples with suspicion of histiocytosis. METHODS AND RESULTS: We analysed 66 biopsies of nonmalignant histiocytoses represented by Langerhans-cell histiocytosis (n = 13), Erdheim-Chester disease (ECD) (n = 19), Rosai-Dorfman disease (RDD) (n = 14), mixed ECD-RDD (n = 3), ALK-positive histiocytosis (n = 6), and juvenile xanthogranuloma (n = 11). All cases were positive for PU.1 in reactive and neoplastic histiocytes. In addition, 39 cases of tumours with high-grade cytological atypia were referred to our center as suspicion of malignant histiocytosis/histiocytic sarcoma and only 18 were confirmed. Indeed, more than half of these tumours (21/39) were either undifferentiated malignant tumours with a stroma rich in histiocytes, diffuse large B-cell lymphoma, or high-grade dedifferentiated liposarcoma. PU.1 was useful to distinguish between the negativity of large atypical nuclei and the positivity of stromal reactive histiocytes. CONCLUSION: PU.1 is expressed by all types of histiocytosis. It distinguishes histiocytosis from histiocyte-rich tumours with an easy interpretation due to its sharp nuclear staining. Its negativity in lesional/tumour cells in histiocyte-like lesions is useful to eliminate a histiocytosis.

Intralymphatic Histiocytosis Associated With Tubo-ovarian High-grade Serous Carcinoma: Case Report and Review of the Literature.

Intralymphatic histiocytosis is a condition characterized by the accumulation of mononuclear phagocytes within lymphatic vessels and lymph nodes that may be isolated or secondary to autoimmune or neoplastic diseases. Secondary intralymphatic histiocytosis frequently involves the skin and is associated with malignancies in up to a tenth of cases. We describe a case of intralymphatic histiocytosis associated with high-grade serous carcinoma and reviewed the literature on neoplasia associated with the broader category of histiocytoses with raisinoid nuclei. Moreover, we try to elucidate the pathogenesis of these rare and intriguing disorders.

Histiocytoid melanoma: Diagnostic pitfall and mimicker of non-Langerhans cell histiocytoses including reticulohistiocytoma.

Melanoma and benign histiocytic proliferations can sometimes show considerable clinical and histopathologic overlap. Recently, cases of melanomas resembling xanthogranuloma and Rosai-Dorfman disease have been reported, and herein we report a case of melanoma closely mimicking reticulohistiocytoma. An 84-year-old man presented with a 1 cm purple-red nodule on his arm concerning for squamous cell carcinoma. While the biopsy findings resembled reticulohistiocytoma, the clinical context and regression changes at the lesion perimeter raised stronger concern for melanoma, which was confirmed with immunohistochemistry. We review prior rare reports of melanomas resembling non-Langerhans cell histiocytic proliferations and summarize helpful clinical and histopathologic clues to avoid a diagnostic pitfall when confronted with this unusual quandary.

A somatic mutation in erythro-myeloid progenitors causes neurodegenerative disease.

The pathophysiology of neurodegenerative diseases is poorly understood and there are few therapeutic options. Neurodegenerative diseases are characterized by progressive neuronal dysfunction and loss, and chronic glial activation. Whether microglial activation, which is generally viewed as a secondary process, is harmful or protective in neurodegeneration remains unclear. Late-onset neurodegenerative disease observed in patients with histiocytoses, which are clonal myeloid diseases associated with somatic mutations in the RAS-MEK-ERK pathway such as BRAF(V600E), suggests a possible role of somatic mutations in myeloid cells in neurodegeneration. Yet the expression of BRAF(V600E) in the haematopoietic stem cell lineage causes leukaemic and tumoural diseases but not neurodegenerative disease. Microglia belong to a lineage of adult tissue-resident myeloid cells that develop during organogenesis from yolk-sac erythro-myeloid progenitors (EMPs) distinct from haematopoietic stem cells. We therefore hypothesized that a somatic BRAF(V600E) mutation in the EMP lineage may cause neurodegeneration. Here we show that mosaic expression of BRAF(V600E) in mouse EMPs results in clonal expansion of tissue-resident macrophages and a severe late-onset neurodegenerative disorder. This is associated with accumulation of ERK-activated amoeboid microglia in mice, and is also observed in human patients with histiocytoses. In the mouse model, neurobehavioural signs, astrogliosis, deposition of amyloid precursor protein, synaptic loss and neuronal death were driven by ERK-activated microglia and were preventable by BRAF inhibition. These results identify the fetal precursors of tissue-resident macrophages as a potential cell-of-origin for histiocytoses and demonstrate that a somatic mutation in the EMP lineage in mice can drive late-onset neurodegeneration. Moreover, these data identify activation of the MAP kinase pathway in microglia as a cause of neurodegeneration and this offers opportunities for therapeutic intervention aimed at the prevention of neuronal death in neurodegenerative diseases.

Combined crystal-storing histiocytosis, light chain proximal tubulopathy, and light chain crystalline podocytopathy in a patient with multiple myeloma: a case report and literature review.

BACKGROUND: Crystal-storing histiocytosis (CSH), light chain proximal tubulopathy (LCPT), and light chain crystalline podocytopathy (LCCP) are rare complications of multiple myeloma (MM) or monoclonal gammopathy of renal significance, and their diagnoses are challenging. CASE PRESENTATION: In this case, a 69-year-old Chinese woman presented with suspicious Fanconi syndrome with renal insufficiency. Immunofixation electrophoresis of both serum and urine revealed elevated immunoglobulin G kappa (IgGkappa) and kappa light chain. Bone marrow aspirate revealed 15% plasma cells with considerable cytoplasmic granular inclusions and needle-shaped crystals. Renal biopsy confirmed the final pathologic diagnosis of kappa-restricted CSH, combined LCPT and LCCP by immunoelectron microscopy. A number of special casts were present which could easily be misdiagnosed as light chain cast nephropathy. Immunofluorescence on frozen tissue presented false negative for kappa light chain, as ultimately proven by paraffin-embedded tissue after pronase digestion. MM and CSH were diagnosed, and two cycles of chemotherapy were given. The patient subsequently refused further chemotherapy, and her renal function remained relatively stable during a 2.5-year follow-up period. CONCLUSIONS: In conclusion, we report a rare case of generalized kappa-restricted CSH involving bone marrow and kidney, combined with LCPT and LCCP, provide a comprehensive summary of renal CSH, and propose a new nomenclature of monoclonal immunoglobulin-induced crystalline nephrology. The presentation of monoclonal immunoglobulin and Fanconi syndrome should suggest the presence of monoclonal immunoglobulin-induced crystalline nephrology. Use of paraffin-embedded tissue after pronase digestion and immunoelectron microscopy is beneficial to improve the sensitivity of diagnosis.

[Transient thickening of the pituitary stalk followed by a supra-pituitary Langerhansian histiocytosis : consequence or mere coincidence ?] / Cas clinique. Épaississement transitoire de la tige pituitaire suivi d'une histiocytose langerhansienne supra-hypophysaire : conséquence ou simple coïncidence ?

One of the difficult challenges in endocrinology is the etiological diagnosis of isolated thickened pituitary stalk (PS). We report the case of a woman in whom a thickened PS was diagnosed following the onset of central diabetes insipidus revealed by polyuria-polydypsia syndrome of late pregnancy and postpartum. The pituitary exploration showed panhypopituitarism with disconnecting hyperprolactinemia. An etiological investigation for an inflammatory, granulomatous or tumour cause was carried out, but was negative. Postpartum lymphocytic hypophysitis was then retained. However, the course was puzzling with a control pituitary MRI showing disappearance of the PS thickening with paradoxical appearance of a supra-pituitary tumour, the biopsy of which concluded of being a Langerhansian histiocytosis. This paradoxical sequence is unusual and has not been reported before. It called into question the autoimmune lymphocytic origin of the thickened PS, initially considered, and raised the likelihood of a causal relationship between this PS thickening and Langerhansian histiocytosis.

Le diagnostic étiologique d'un épaississement isolé de la tige pituitaire (TP) constitue l'un des grands défis en endocrinologie. Nous rapportons le cas d'une patiente chez qui un épaississement de la TP a été diagnostiqué suite à la survenue d'un diabète insipide central révélé par un syndrome polyuro-polydypsique de fin de grossesse et du post-partum. Le bilan hypophysaire a montré un panhypopituitarisme avec une hyperprolactinémie de déconnexion. Une enquête étiologique à la recherche d'une cause inflammatoire, granulomateuse ou tumorale a été menée et s'est avérée négative. Une hypophysite lymphocytaire du post-partum a alors été retenue. Cependant, l'évolution a été déroutante avec, à l'IRM hypophysaire de contrôle, la disparition de l'épaississement de la TP et l'apparition paradoxale d'une tumeur suprahypophysaire dont la biopsie a conclu à une histiocytose langerhansienne. Cette évolution paradoxale est inhabituelle et n'a pas été rapportée auparavant. Elle a remis en question l'origine lymphocytaire auto-immune de l'épaississement de la TP, retenue initialement, et a soulevé la possibilité d'une relation de cause à effet entre cet épaississement de la TP et l'histiocytose langerhansienne.

[Pulmonary crystal-storing histiocytosis: a case report].

A 45-year-old female patient was found to have a nodule in the right lower lobe on physical examination. Chest CT showed the nodule was lobulated measuring 24 mm×23 mm, with obvious enhancement and adjacent pleural traction. As the PET-CT showed increased 18F-FDG uptake suggesting malignancy, the wedge resection of the right lower lobe was performed. Grossly, the mass was adjacent to the pleural area with indistinct boundary. On cut sections, the lesion was solid and tough, with a greyish-pink colour. Microscopically, the lesion had an ill-defined margin, and was composed of spindle and polygonoid histiocytes with rich eosinophilic cytoplasm similar to rhabdoid muscle cells. The cytoplasm of histiocytes was filled with diamond-shaped or club-shaped crystals. Immunohistochemistry (IHC) showed the histiocytes were positive for CD68, κ, λ, IgG, IgM and IgA. The patient had been followed up for 41 months and had shown neither recurrences nor new diseases. CSH is a rare non-neoplastic histiocytic proliferative disease. Pulmonary CSH should be differentiated from multiple diseases. Accurate pathological diagnosis depends on its morphology and immunophenotype. This disease is often related to potential lymphoproliferative or plasma cell disorder. After diagnosis, a systemic examination is required and long-term follow-up is recommended.

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.

Cutaneous histiocytoses in children.

Cutaneous histiocytoses constitute a heterogeneous group of diseases characterised by the cutaneous accumulation of cells with the cytological and phenotypic features of macrophages or dendritic cells. The clinical spectrum ranges from self-resolving, skin-limited conditions to severe, multiorgan disease with a high morbidity rate. Until recently, cutaneous histiocytoses were classified according to the immunophenotype of the pathological cells, with differentiation between Langerhans cell histiocytosis (LCH) [CD1a+, CD207 (langerin)+] and non-Langerhans cell histiocytosis (CD68+, CD163+, CD1a-, CD207-). Over the last 12 years, a number of new pathophysiological findings (in particular, molecular pathology results) regarding histiocytoses have contributed to a new classification based on molecular alterations, as well as on clinical and imaging characteristics and the phenotype. The most frequent entities in children are juvenile xanthogranuloma and LCH.

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

Histiocytes and dendritic cells may display cytological atypia and an aberrant immunophenotype even in reactive processes. Herein, we describe two cases of "Hodgkinoid histiocytosis" that show distinctive clinicopathological features, mimicking 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 haematopoietic 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.

Failure of crizotinib based systemic treatment in ALK positive histiocytosis involving the central nervous system: a case report and literature review.

BACKGROUND: Among the histiocytic disorders, anaplastic lymphoma kinase (ALK)-positive histiocytosis emerged in 2008. As more and more cases of the novel entity are reported, our understanding of it is deepened. However, only a few cases with central nervous system (CNS) involvement have been reported. Furthermore, the lesion in the suprasellar region has not been documented.  CASE PRESENTATION: We presented a case of ALK-positive histiocytosis involving the suprasellar region of a one-year-and-four-month-old boy. Through clinical, neuropathological, and genomic analyses, the patient was diagnosed with ALK-positive histiocytosis. After lesions were resected he started treatment with a combination of the three compounds vincristine, prednisolone, and crizotinib, but they did not work. Cytarabine was then added as an additional chemotherapy drug for him, and the lesions in the brain and lungs were shrunk by combining treatment of crizotinib, dexamethasone, vincristine, and cytarabine according to the RECIST (esponse Evaluation Criteria In Solid Tumours). CONCLUSIONS: Additional adjuvant chemotherapy drugs are needed when ALK-inhibitor treatment is ineffective.

Pathological and Immunohistochemical Characteristics of Granuloma and Lymphatics in Cheilitis Granulomatosa.

ABSTRACT: Cheilitis granulomatosa (CG) is an idiopathic, rare, and chronic granulomatous disorder involving the lips. We characterized the pathological and immunohistopathological findings of these granulomas and their relationship with the lymphatic vessels. Pathologically confirmed cases of primary CG from 2001 to 2016 were collected. Cases of inflammatory cheilitis without the presence of granuloma were included in the control group. Demographic data, clinical presentation, response to therapy, and pathological differences were compared. Periodic acid-Schiff and acid-fast stains excluded patients having infections. CD68, CD163, and D2-40 stains demonstrated features of granuloma, macrophage polarization, and the relationship between granuloma and lymphatic vessels. Thirteen patients diagnosed with CG were enrolled. Thirteen people were enrolled in the control group. The granulomas were either mononuclear or sarcoidal. They were predominantly positive for CD68 but negative for CD163. Perilymphatic granulomas were found in all patients. Intralymphatic histiocytosis and lymphatic dilatation were more commonly observed in patients diagnosed with CG than those in controls (54% vs. 15%, P = 0.03 and 92% vs. 23%, P < 0.01). TH1 immune response due to CD68+ M1 macrophages results in CG. Perilymphatic aggregation of macrophages and intralymphatic histiocytosis were important pathological clues for diagnosis.

An Atypical Myelomonocytic Cell Infiltrate: Use of Next-Generation Sequencing to Diagnose Indeterminate Cell Histiocytosis.

ABSTRACT: Indeterminant cell histiocytosis (ICH) is a rare lymphoproliferative disorder that demonstrates features of Langerhans and non-Langerhans cell histiocytoses and diagnosis can be challenging. We present a case of a 62 year old woman with a generalized eruption of erythematous papules on the face, trunk and extremities. Skin biopsies demonstrated a dermal mononuclear cell infiltrate with monocytic (CD4, CD33), histiocytic (CD68, CD163), and dendritic cell (CD1a) immunophenotype but negative for Langerhans' cell marker (CD207). The differential diagnosis included leukemia cutis and ICH, and further workup revealed a normal bone marrow biopsy. To confirm the diagnosis of ICH, next generation sequencing with ETV3-NCOA2 gene fusion was performed and was positive. The patient's condition improved with methotrexate and narrow band UVB phototherapy. Our case adds to the existing literature supporting the use of next-generation sequencing to test for ETV3-NCOA2 gene fusion in suspected cases of ICH.

Solitary Extramedullary Plasmacytoma of the Lacrimal Sac With Associated Crystal-Storing Histiocytosis.

PURPOSE: To report a rare case of crystal-storing histiocytosis associated with solitary extramedullary plasmacytoma of the lacrimal sac and to review literature on the 2 entities to summarize important diagnostic, management, and prognostic considerations. METHODS: A case report of the ophthalmologic presentation, pathology workup, and oncologic management is presented. Literature search with focus on lesions occurring in ophthalmic sites and management guidelines from expert panels and working groups. RESULTS: Crystal-storing histiocytosis associated with solitary extramedullary plasmacytoma arose within the lacrimal sac of a previously healthy middle-aged woman and presented as a painless nodule with epiphora. The biopsy tissue showed sheets of crystal-filled histiocytes, interspersed with monoclonal plasma cells and rarely demonstrated plasma cell phagocytosis. Imaging and laboratory studies confirmed the localized nature. CONCLUSIONS: Crystal-storing histiocytosis is an uncommon entity in which crystals, most commonly arising from altered immunoglobulins, aggregate within histiocytes and form symptomatic mass lesions. It has been reported in ophthalmic regions in patients with a concurrent lymphoproliferative or plasma cell disorder and can rarely predate a malignancy. The current case is notable because crystal-storing histiocytosis occurs with a localized process, solitary extramedullary plasmacytoma, and presents in an unusual site, the lacrimal sac. Tissue biopsy with multimodal pathological evaluation is necessary to make the diagnosis. Ophthalmologists should recognize that crystal-storing histiocytosis is commonly associated with a hematologic malignancy and, when appropriate, refer the patient for oncologic management. Surveillance may be indicated in cases with no established etiology. Solitary extramedullary plasmacytoma should also be monitored, as a proportion of cases progress to multiple myeloma.

[Histiocytosis of Rosai-Dorfman-Destombes extranodal of the jawbone: Report of a case and review of the literature]. / Histiocytose de Rosai-Dorfman-Destombes extra-ganglionnaire du maxillaire : à propos d'un cas et revue de la littérature.

Rosai-Dorfman-Destombes histiocytosis is a rare histiocytosis characterized by an accumulation of histiocytes within the tissues. It is a heterogeneous entity with various clinical phenotypes: isolated lymph nodes, in association with extranodal involvement, autoimmune disease or neoplasm. These extra nodal lesions mainly affect the skin, the nasal cavity, orofacial sinuses, or are lytic bone lesions or even damage to the central nervous system. The diagnosis is histopathological. We present here the case of a histiocytosis of Rosai-Dorfman-Destombes in a 46-year-old patient with a lesion of the left palate. Our observation discuss the diagnostic hypotheses in front of a lytic lesion of the ENT sphere predominantly histiocytic.

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.