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Context.—: Undifferentiated pleomorphic sarcoma (UPS) of soft tissue is defined as a sarcoma with no recognizable line of differentiation. During the past few decades, advances in ancillary studies and review of prior UPS diagnoses have narrowed the category of UPS by excluding more-specific malignancies. However, few of those studies have specifically targeted pleomorphic hematolymphoid neoplasms., Objective.—: To determine what fraction of UPS cases are misclassified pleomorphic hematolymphoid neoplasms, such as anaplastic large cell lymphoma, diffuse large B-cell lymphoma, histiocytic sarcoma (HS), myeloid sarcoma, and follicular dendritic cell sarcoma., Design.—: Sixty-one UPS cases were screened by tissue microarray and an immunostain panel with subsequent analysis on whole block sections for suspicious cases., Results.—: Five of 61 tumors (8%) were suggestive of HS based on the screening panel and were further evaluated with additional immunostains (PU.1, CD45, CD163) using whole sections. The 5 candidate HS cases were only focally positive for at most one stain with most staining in smaller, less-pleomorphic cells. Ultimately, no UPS met criteria for anaplastic large cell lymphoma, diffuse large B-cell lymphoma, myeloid sarcoma, follicular dendritic cell sarcoma, or HS., Conclusions.—: Our results suggest that a UPS of somatic soft tissue is unlikely to represent a misclassified hematopoietic malignancy. Exclusion of HS is most challenging, but immunostaining for PU.1, a nuclear transcription factor, may be easier to interpret in this context., (© 2020 College of American Pathologists.)

Histiocytic sarcoma (HS) is an extremely rare malignant neoplasm showing morphologic and immunophenotypic evidence of histiocytic differentiation. The vast majority of previously reported HSs are now generally recognized to be misdiagnosed examples of non-Hodgkin lymphomas, predominantly diffuse large B-cell lymphoma or anaplastic large cell lymphoma. The recognition of such tumors parallels the development and widespread use of immunohistochemical techniques, along with the development of molecular genetic methods to detect immunoglobulin (IG) or T-cell receptor (TCR) gene rearrangement. The 2001 World Health Organization (WHO) definition of HS requires the absence of clonal B/T-cell receptor gene rearrangements. However, the 2008 WHO classification no longer strictly requires the absence of clonal immunoglobulin heavy chain (IGH) or TCR gene rearrangement for the diagnosis of HS. Recent studies demonstrated that HSs that occur subsequent to or concurrent with B- or T-lymphoblastic lymphoma/leukemia or mature B-cell neoplasms generally show clonal IgH and/or TCR gene rearrangement. These findings suggest the possibility of transdifferentiation of the two otherwise morphologically and immunohistochemically distinctive neoplasms. In addition, a recent study suggested clonal IG gene rearrangements may be detected at a high frequency in sporadic HS, indicating that a large subset of sporadic HSs may inherit the B-lymphocyte genotype. These findings provide new insights into the pathogenesis of HS, although the etiology of HS is still unknown. HS is a diagnosis of exclusion. It is necessary to rule out other diseases that could be misdiagnosed as HS with extensive immunophenotypical analysis before diagnosing HS.

The Flat Coated Retriever is a breed at risk of development of histiocytic sarcoma (HS), but in contrast to the disseminated form of disease recognized in the Bernese Mountain Dog, most reports of HS in Flat Coated Retrievers describe a localized lesion affecting the musculature or fascia of limbs. The purpose of this study was to review data and material received though an ongoing Flat Coated Retriever tumor survey to better define the presentation of HS in the breed and to determine the utility of subclassification of tumors arising at different sites by histology and immunohistologic phenotyping. Data on 180 dogs bearing HS-like tumors were available for review, which showed that although the majority (101 lesions, 57%) were primary limb lesions, 47 dogs (26%) had visceral, mainly splenic lesions with no peripheral primary tumor. A detailed histologic and immunohistologic review of 20 limb tumors and 20 splenic tumors showed that 2 distinct phenotypic subtypes could be identified: a histiocytic subtype, most prevalent in the splenic tumors, and a histiocytic-spindle-pleomorphic subtype, mainly seen in the limb tumors. Despite their variable morphology, all tumors expressed major histocompatibility complex class II and the leukocyte antigen CD18, but only those tumors in the spleen consistently expressed CD11d. The majority of tumors also contained a mild to moderate infiltrate of T lymphocytes.

Although myelomonoblastic leukemia is thought to originate from a malignant transformation of the stem cell of the mononuclear phagocyte system, malignant histiocytosis (MH) is classically assumed to represent a malignant change of the terminal and fixed elements of this system. Indeed, MH is characterized by the proliferation of large, clear, pleomorphic, "histiocytic-like" HLADR and CD30+ cells resulting in a nodal and extranodal disseminated neoplasm affecting preferentially and severely children and young adults. Although there is broad agreement on the clinicopathologic presentation of this condition, there is currently quite a controversy over the T-lymphoid or histiocytic origin of the proliferative cells that results in a nosologic discussion between the anaplastic large cell lymphoma (ALCL) advocates and the MH supporters. This article has dealt mainly with this nosologic discussion and with the contributions provided by the investigations performed on MH permanent cell lines. These in vitro studies have demonstrated that the proliferation is characterized by a unique chromosomal abnormality, the 5q35bp usually associated with a t(2;5) translocation generating a fusion gene NPM/ALK and the subsequent translation of p80 protein. Although it is known that no single chromosomal abnormality is strictly restricted to a cell lineage, this 5q35bp and associated translocations seem today to represent the hallmark for this condition. In view of these chromosomal aberrations, the CD30+ ALCLs represent a heterogeneous group because 15% to 50% express the NPM/ALK fusion gene. In addition, these in vitro investigations have shown that 5q35bp proliferative cells are glass-adherent, can develop an immunodependent phagocytosis, and are able to reduce NBT and produce TNF-alpha. More significantly, they express constitutively the c-fms (the receptor of the macrophage growth factor) and, under TPA stimulation, are able to modulate the expression of this receptor and its ligand, as well as TNF-alpha and IL-1. None of these cell lines express CD3, but several express CD68 and CD71. In contrast, genomic investigations have shown the underlying existence of monoallelic and even biallelic gene rearrangements for TCR beta and IgJH. In view of these discrepancies between the genomic and phenotypic features of these cells, the histogenetic debate should remain open but must take into account these new chromosomal and molecular data.

Malignant histiocytic disorders, other than leukemias, are extremely rare in childhood. Despite unresolved nosologic and terminologic difficulties, they should be classified according to the lineage of the aberrant cells in a given tumor. There are no common and typical clinical presentations, nor are there established treatment modalities available. For the disseminated forms, aggressive systemic treatment modalities--similar if not identical to those used for large cell anaplastic lymphomas--appear to be the best treatment option. For the localized forms, which are primarily dendritic cell sarcomas, a more localized and individualized therapy is appropriate.

Pathologists and pediatric hematologist/ oncologists of the World Health Organization's Committee on Histiocytic/Reticulum Cell Proliferations and the Reclassification Working Group of the Histiocyte Society present a classification of the histiocytic disorders that primarily affect children. Nosology, based on the lineage of lesional cells and biological behavior, is related to the ontogeny of histiocytes (macrophages and dendritic cells of the immune system). Dendritic cell-related disorders of varied biological behavior are dominated by Langerhans cell histiocytosis, but separate secondary proliferations of dendritic cells must be differentiated. Juvenile xanthogranuloma represents a disorder of dermal dendrocytes, another dendritic cell of skin. The hemophagocytic syndromes are the most common of the macrophage-related disorders of varied biological behavior. Guidelines for distinguishing the exceedingly rare malignant diseases of histiocytes from large cell lymphomas through the use of a battery of special studies are provided.

Malignant histiocytosis (MH) and true histiocytic lymphoma (THL) are hematopoietic malignancies of the mononuclear phagocytic system distinguished from each other by clinical presentation and presumed cell of origin. THL present as a localized mass derived from the fixed tissue histiocyte which may or may not disseminate. MH originates from the circulating monocyte or tissue macrophage and is characterized by a syndrome of systemic symptoms, pancytopenia, adenopathy, hepatosplenomegaly, and wasting. The distinction between MH and THL is at times arbitrary and overlap exists between these syndromes. The clinicopathologic studies that defined these entities were performed prior to the development of immunophenotyping and other molecular techniques currently used to ensure proper classification of hematopoietic malignancies. Nine patients from the University of Minnesota originally diagnosed with MH were retrospectively analyzed using a panel of antibodies reactive against T cell, B cell, and myelomonocytic antigens. Only one patient was reclassified as a possible histiocytic malignancy after reevaluation. Similar immunophenotyping studies have also shown cases previously diagnosed as MH or THL express lymphoid antigens, and would now be classified as Ki-1 positive anaplastic large cell lymphoma (ALCL) or some other hematopoietic neoplasm. These results indicate true histiocytic neoplasms are extremely rare, and previous concepts concerning clinical presentation and therapeutic outcome of the entities are inaccurate. In this paper we summarize the results of multiple retrospective analyses of cases previously diagnosed as MH or THL, including our experience at University of Minnesota, to illustrate the overall rarity of these entities. The current literature on malignant histiocytic disorders is reviewed, and the clinical presentation of patients determined to have histiocytic malignancies using contemporary analytical techniques is discussed.

Malignant histiocytosis (MH) is a rare, rapidly fatal disorder, characterized by systemic proliferation of abnormal histiocytes. Most patients present with pancytopenia. We report the case of a patient with MH in the leukaemic stage, who presented extremely pronounced general symptoms and multisystemic involvement. Determination of serum cytokines showed high levels of tumor necrosis factor (TNF), interleukin 6 (IL-6) and interleukin 1 receptor antagonist (IL-1ra). Cytogenetic studies proved the monoclonality of the histiocytic proliferation. These findings strongly suggest that we are dealing with a proliferation of activated macrophages (= histiocytes). By analogy with the monoblastic (M5a) and monocytic (M5b) acute leukaemia of the French-American-British (FAB) classification, we propose a new entity, 'M5c', designating acute histiocytic leukaemia.

A workshop of the Histiocyte Society was recently held, in order to discuss the problems and confusion of malignant histiocytosis (MH) and large cell anaplastic (Ki-1) lymphoma (LCAL). The aims of this workshop were to clarify the terminology for malignant histiocytic disorders and LCAL and to produce reliable criteria for diagnosis of both MH and LCAL. This article summarises the conclusions reached.

During the last five years increasing evidence has accumulated that many tumours classified as 'histiocytic' in the past do not originate from macrophages, but from transformed (or anaplastic) large lymphoid cells. Most of these studies have focused upon adult neoplasms. Knowledge concerning the lineage of 'histiocytic' tumours in the paediatric age group is more limited. In this study we have examined the clinical, morphological and immunophenotypical features of six childhood malignancies originally diagnosed as being of histiocytic origin. Three patients showed an aggressive course with involvement of internal organs and very short survival times. Two patients were brought into remission: one is alive without active disease after seven years; the other died after seven years due to treatment-related cardiomyopathy. The remaining patient had a protracted course for two and a half years, but subsequently deteriorated and died three years after diagnosis. The histomorphological features in five cases were those of anaplastic large cell lymphomas. The remaining case consisted of pleomorphic (rather than anaplastic) large lymphoid cells. In all cases the immunophenotypical examination showed features characteristic of activated T lymphocytes. All cases were positive for Ki-1 (CD30), and three were positive for epithelial membrane antigen (EMA). Histiocyte-associated markers were positive in residual reactive macrophages, but nowhere could unequivocal positivity for macrophage-associated markers be seen in the neoplastic cells. It is concluded that most childhood malignancies in the past classified as 'histiocytic' are examples of anaplastic large cell (Ki-1) lymphomas of T-cell type and that true histiocytic malignancies are exceedingly rare in the paediatric age group.

Malignant histiocytosis (MH) is a term that has been used to describe a syndrome in which there is a systemic proliferation of cells that have the cytologic appearance of atypical histiocytes. Biopsy materials from 15 patients with malignant lymphoma diagnosed as malignant histiocytosis in a previous study reported in 1975 were analyzed by a panel of antibodies and reclassified using current nosologic concepts of malignant lymphoma. The antibodies used comprised reagents detecting a formalin-resistant epitope on B-cells (L26), T-cells (anti-CD3, anti-leu 22 [CD43], and UCHL1 [CD45RO]), monocyte/macrophage-derived cells (KP1 [CD68]), as well as antibodies that detect leukocyte common antigen (PD7 [CD45RB]), and a formalin-resistant epitope of Ki-1 (Ber-H2 [CD30]). The authors found that nine lymphomas had a profile consistent with T-lineage, including six in which Ki-1 (CD30) was coexpressed, and two were B-lineage. Three lymphomas showed no specific lineage characteristics although two were Ki-1 (CD30) positive, and none had expression of KP1 (CD68). The 12 lymph node biopsy specimens showed a variety of patterns of involvement, including sinusoidal, paracortical, and diffuse; the spleens showed predominantly red pulp involvement. A 15th case was believed most consistent with a virus-associated hemophagocytic syndrome. These findings support previous suggestions that the majority of cases diagnosed as MH represent T-lineage-associated hematolymphoid neoplasms, and that only a rare case will be of monocyte/macrophage origin. It is suggested that the term MH be subsumed under the rubric of large cell lymphoma and unless there are compelling immunohistochemical data to support a histiocytic origin, that the term MH be abandoned in favor of a more accurate descriptive term, such as sinusoidal large cell lymphoma.

The recommended classification of the histiocytosis syndromes in children is as follows: 1. Langerhans' cell histiocytoses. 2. histiocytoses of mononuclear phagocytes. 3. neoplastic histiocytoses. The previous term, histiocytosis X, including eosinophilic granuloma of bone, the Hand-Schüller-Christian syndrome and Letterer-Siwe's disease, is now more correctly called Langerhans' cell histiocytosis (LCH) since the infiltrating cell in histiocytosis X both histologically and immunophenotypically is identified as the Langerhans' cell. Local and generalized LCH differmarkedly with respect to treatment and outcome. It is therefore necessary to perform an extensive investigation at the time of the initial evaluation of the patient. The most common mononuclear phagocytic syndromes are the familial erythrophagocytic lymphohistiocytosis and infection-associated or reactive hemophagocytic syndromes. The pathognomonic cell in familial erythrophagocytic lymphohistiocytosis is possibly a hybrid phenotype sharing characteristics of the two main types the mononuclear phagocyte system - i.e. the Langerhans' cells and phagocytic macrophages. Malignant histiocytosis is rarely seen in children and is a form of the Ki-1 positive anaplastic cell lymphoma.