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.

Classic Hodgkin lymphoma, nodular lymphocyte predominant Hodgkin lymphoma, and T cell/histiocyte-rich large B cell lymphoma form a unique set of lymphomas with similar morphologic growth patterns (occasional neoplastic cells within a prominent cellular cell background) that are pathobiologically related. Distinguishing these entities has been historically difficult by flow cytometry; however, our laboratory has developed antibody-fluorochrome combinations capable of immunophenotyping these lymphomas. Additionally, characterization of the background reactive lymphocytes can aid in narrowing the differential diagnosis. This review summarizes the immunophenotypic features and insights of the neoplastic and reactive populations found in this unique group of lymphomas.

BACKGROUND: Rosai-Dorfman disease (RDD), a rare form of non-Langerhans cell histiocytosis with heterogenous clinical features, arises from precursor cells that give rise to cells of the histiocytic and monocytic lineages. An association with hematological neoplasms has been reported. Testicular RDD is rarely described, with only 9 reported cases in the literature. Genetic data to assess clonal relationships between RDD and other hematological neoplasms remain scarce. We describe an instance of testicular RDD against a background of chronic myelomonocytic leukemia (CMML), with genetic studies in both neoplasms.
METHODS: A 72-year-old patient with a history of CMML sought evaluation of growing bilateral testicular nodules. Solitary testicular lymphoma was suspected; orchidectomy was performed. The diagnosis of testicular RDD was established morphologically and confirmed immunohistochemically. Molecular analysis of testicular lesions and of archived patient bone marrow revealed the KRAS variant c 0.35 G>A / p.G12D in both, suggesting a clonal relationship.
CONCLUSIONS: These observations support classifying RDD as a neoplasm that can be clonally related to myeloid neoplasms.

Anaplastic lymphoma kinase (ALK)-positive histiocytosis, a novel rare histiocytic proliferation, was first described in 2008; it occurs in early infancy with liver and hematopoietic involvement. The spectrum was subsequently broadened to include localized diseases in older children and young adults. However, its full clinicopathological features and molecular lineage have not been fully elucidated.
Here, we report four cases of multisystem ALK-positive histiocytosis without hematopoietic involvement. Clinically, three patients were adults aged between 32 and 51 years. Two patients\', whose main manifestations were intracranial mass and numerous micronodules in the thoracoabdominal cavity organs and skin papules respectively, had a partial response to ALK inhibitors after surgery. One patient presented with mediastinal neoplasm without surgical treatment, and progressive disease occurred after two years of ALK inhibitor therapy. The fourth patient was a 17-month-old male with a large intracranial mass and presented with a poor response to ALK inhibitor and chemoradiotherapy; he died eight months after surgery. Pathologically, the histiocytes were large, with abundant eosinophilic cytoplasm, and mixed with variable numbers of foamy cells and Touton giant cells. Interstitial fibrosis was also observed. Histiocytes were positive for macrophage markers (CD68 and CD163) and ALK. KIF5B-ALK fusions were detected in two cases, EML4-ALK in one, and both DCTN1-ALK and VRK2-ALK fusions were detected in one case.
We observed that ALK inhibitors present robust and durable responses in adult patients but a poor response in young children with central nervous system involvement. There is no consensus on the optimal treatment regimen and long-term prognosis requires further observation. Moreover, every unusual histiocytic proliferative lesion, especially unresectable and multisystem involvement, should be routinely tested for ALK immunohistochemical staining to identify this rare disease.

Rosai Dorfman Disease (RDD) is a benign histiocytic lymphoproliferative disease that has variable presentations. The concurrent presentation of RDD in the spinal cord and brain parenchyma is an extremely rare entity. Here, we report another case of a 24-year-old gentleman who presented with a tuberculum sellae and bilateral cavernous extra-axial tumors extending to the subtemporal lobe and was found to have craniocervical lesions. Axillary lymph node biopsy was done showing markedly dilated sinuses filled with large histiocytes and emperipolesis of numerous lymphocytes and plasma cells confirming the diagnosis of RDD. Because the definitive diagnosis of RDD is always pathological, the clinical presentation plays a major role in widening the margin of differential diagnosis. Finally, surgical intervention is the first option to treat RDD with relatively satisfactory follow-up outcomes, and other adjuvant therapies optimize the prognosis.

UNASSIGNED: Adenodermatofibromas are an extremely rare subtype of dermatofibroma (DF) characterized by a dermal proliferation of spindle-shaped fibroblasts and histocytes, dilated glandular structures with apocrine secretion, and prominent vascular proliferation, with or without hemosiderotic features. We describe a recent extraordinary case of a hemosiderotic adenodermatofibroma in a 25-year-old female. We review histologic findings and theories behind etiology, as well as review the spectrum of clinical presentations for this lesion. We also discuss imaging findings that may make identification of these entities challenging.

BACKGROUND: Tumours of dendritic or histiocytic lineage are amongst the rarest tumours and probably account for < 1% of tumours affecting the lymph nodes or soft tissue. Because several of these entities were poorly recognised until recently, the true incidence is not determined.
METHODS: We present what we believe is the first reported case report of a fibroblastic reticular cell tumour arising in the oral cavity as well as reviewing the current literature regarding this rare subset of tumours.
RESULTS: We discuss the clinical and histopathological findings of our reported case and examine the literature regarding this entity. We discuss the key differential diagnoses to consider when making this diagnosis.
CONCLUSIONS: Histiocytic and dendritic cell derived tumours are exceptionally rare within the head and neck region although a number of these tumours have been reported within the oral cavity. We present what we believe is the first reported case of a fibroblastic reticular cell tumour arising within the oral cavity.

BACKGROUND: The association between KFD and autoimmune diseases, not only with systemic lupus erythematosus, has been repeatedly described.
OBJECTIVE: The aim of this review is to evaluate whether an overlap syndrome is present between KFD and autoimmune diseases, whether there is a chronological and a casual relationship between the pathologies.
METHODS: The databases used for the overlap case search were Medline and Embase from which we extrapolated the studies of interest. The search queries used were: Kikuchi-Fujimoto Syndrome and juvenile idiopathic arthritis or systemic lupus erythematosus or Systemic Sclerosis or Antiphospholipid Syndrome or Sjogren\'s Syndrome. All study types were considered (n = 103).
RESULTS: Total number of included studies are 43. We have shown that there is an \"overlap\" syndrome between KFD and other autoimmune diseases. The chronology of disease onset was variable; autoimmune disease may be \"preceding\" (n = 11 cases) or \"simultaneous\" (n = 20 cases) or \"post\" (n = 8 cases). Kikuchi-Fujimoto Syndrome. Also, the autoimmune disease can present with a complete clinical picture or only with the presence of autoantibodies.
CONCLUSIONS: the different pathologies associated with KFD with different chronologies would suggest that there is an alteration of the immune system that allows the pathologies to occur in different temporal relationships.

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BACKGROUND: Langerhans cell histiocytosis (LCH) and Erdheim-Chester disease (ECD) are rare histiocytic disorders in the L (Langerhans) group diseases. They range from self-limited benign diseases to lethal disseminated forms.
METHODS: This retrospective study was conducted in 3 tertiary hospitals in Saudi Arabia and Brazil. Histopathological records were searched for all patients diagnosed with ocular and periocular histiocytic disorders from January 1993 to December 2018. Histopathological slides and medical files were reviewed for data collection and simple analysis of demographics, clinical manifestations, and management. The relevant literature is reviewed.
RESULTS: Twenty-two eyes of 18 patients with biopsy-proven histiocytic disorders in the L group were included. Female-to-male ratio was 1.25:1. Average age at presentation was 14 years (range, 1-54). LCH was diagnosed in 14 eyes, while eight eyes had ECD. All LCH cases were unilateral and confined to the bone as cases of eosinophilic granuloma (EG), while patients with ECD were bilateral. Commonest presentations in EG and ECD were eyelid swelling (85.7%) and periocular xanthomas (75%), respectively. Orbit was involved in 100% of EG cases, with bony erosion in 54.5%. Relevant systemic involvement was found in 100% of ECD and 21% of EG cases. Surgical intervention was needed in 16 of the 22 eyes (72.7%). All EG and 25% of patients with ECD required surgical excision.
CONCLUSIONS: Histiocytic disorders are a rare group of diseases, including the L group. Relevant systemic associations require specific and selective therapy. A high clinical index and multidisciplinary collaboration are essential for the proper evaluation and management of these patients.