Primary histiocytic sarcoma of the spleen associated with erythrophagocytic histiocytosis

Undifferentiated pleomorphic sarcoma of the spleen: a case report and literature review

BACKGROUND

Undifferentiated pleomorphic sarcoma is an uncommon sarcoma and its presence in the spleen is even rarer, with only a handful of cases reported in English literature. It is typically only diagnosed following histological analysis. Its rarity also means that there is little consensus over ideal management.

CASE PRESENTATION

This report presents a case of a 40-year-old Caucasian male who was found to have a splenic mass after presenting with non-specific abdominal pain and generalized malaise. Numerous imaging modalities were used which demonstrated a large partially solid and partially cystic lesion in spleen with no evidence of metastasis. As core biopsies were undiagnostic, he was planned for a diagnostic and therapeutic splenectomy. However, despite magnetic resonance imaging 11 days prior to his operation showed no evidence of liver metastasis, a massive splenic tumour with hepatic metastases was identified intraoperatively. An open splenectomy, distal pancreatectomy and liver metastasectomy was hence carried out. Histological analysis confirmed liver metastasis secondary to a splenic undifferentiated pleomorphic sarcoma. The patient recovered well and was discharged home. He presented again three weeks after his operation with lower back pain, abdominal pain and fever. Computed tomography demonstrated extensive recurrent disease burden in the peritoneum and liver. The patient passed away a month after surgery.

CONCLUSION

Splenic undifferentiated pleomorphic sarcoma is a rare tumour which may pose a significant diagnostic challenge on both clinical and histopathological grounds. Following diagnosis and treatment, its aggressive nature often results in a poor prognosis. Current literature fails to delineate any superior management strategy to increase survival.

Primary splenic histiocytic sarcoma successfully treated with splenectomy: a case report and literature review

A 61-year-old woman was referred to our hospital with refractory thrombocytopenia and splenomegaly. She was diagnosed with immune thrombocytopenia 3 years prior to admission and received steroid therapy. However, her platelet count started decreasing six months prior to admission. A diagnostic and therapeutic splenectomy was performed, which led to the diagnosis of histiocytic sarcoma. The patient's platelet count recovered promptly after splenectomy, and she was in complete remission for over a year.

Primary splenic histiocytic sarcoma associated with hemophagocytic lymphohistiocytosis: A case report and review of literature of next-generation sequencing involving FLT3, NOTCH2, and KMT2A mutations

Background

Histiocytic sarcoma is a very rare monocyte/macrophage‐derived hematopoietic system tumor with a poor prognosis whose diagnosis is pathologically challenging due to its extreme rarity and histological overlap with various mimicking entities in which histiocytes also predominate.

Case

We report the case of a 33‐year‐old male patient with hemophagocytic lymphohistiocytosis, purpuric syndrome, and significant splenomegaly. The patient underwent splenectomy; subsequent macroscopic examination revealed a spleen weighing 2065 grams with hyperemic red pulp and multiple infarcts at the periphery. The histological and immunohistochemical study established a diagnosis of primary splenic histiocytic sarcoma with frequent hemophagocytosis. Next‐generation sequencing demonstrated mutations in FLT3, NOTCH2, and KMT2A, microsatellite stability, and a tumor mutational burden of 2 mut/Mb. The patient's condition deteriorated clinically from the appearance of the first symptoms and he died 6 months later from multi‐organ failure.

Conclusion

Primary splenic histiocytic sarcoma is one of the rarest tumors of the hematopoietic system. We report the first case with mutations in FLT3, NOTCH2, and KMT2A, and associated hemophagocytic lymphohistiocytosis.

Myeloid, mast cell, histiocytic and dendritic cell neoplasms and proliferations involving the spleen

Splenic involvement and consequent splenomegaly are usually seen as part of systemic involvement by myeloid neoplasms as well as mast cell and histiocytic neoplasms. Primary splenic involvement by these neoplasms is rare. Splenectomy is usually not performed for establishing a diagnosis of these entities. However, in rare instances, the pathologist may need to evaluate the spleen secondary to splenic rupture or palliative splenectomy to alleviate symptoms related to splenomegaly. This review article describes the clinicopathologic features of a broad group of myeloid, mastocytic, and histiocytic proliferative and neoplastic disorders.

Thrombocytopenia with multiple splenic lesions - histiocytic sarcoma of the spleen without splenomegaly: A case report

BACKGROUND

Histiocytic sarcoma (HS) of the spleen is reported to be a rare and lethal disease. The clinicopathological features of splenic HS have not been well described. The objective of this paper is to describe the diagnosis and treatment of a case of this rare disease and provide a review of the literature.

CASE SUMMARY

In this article, we discuss the case of a 40-year-old Hispanic female who presented with progressive thrombocytopenia and multiple hypoechoic lesions in the spleen without splenomegaly. Positron emission tomography-computed tomography showed increased activity in cervical lymph nodes, as well as multiple bone and splenic lesions with positive uptake. Two bone marrow biopsies and fine-needle aspiration of the cervical lymph node were inconclusive. Laparoscopic splenectomy was performed, and gross examination showed a 110.1 g spleen with multiple rubbery, nodular lesions within the subcapsular sinus and splenic parenchyma. The microscopic findings showed multinodular histiocyte proliferation with atypia and multilobulated nuclei, which were positive for CD163, CD4, and CD68 by immunohistochemical analysis. The final pathologic diagnosis was difficult and was found to be low-grade HS of the spleen, after consultations with two renowned hematopathology institutions. At the patient’s five-month follow-up visit, her bone marrow metastasis had progressed. She is waiting to be enrolled in a clinical trial.

CONCLUSION

Pathologic diagnosis of splenic HS can be challenging. Low-grade differentiation may be associated with a slow progressive disease.

Potential application and prevalence of the CD30 (Ki-1) antigen among solid tumors: A focus review of the literature

BACKGROUND

CD30 (Ki-1) is a cell membrane protein derived from the tumor necrosis factor (TNF) receptor family. The CD30 antigen has been associated primarily with Hodgkin lymphoma (HL) and systemic anaplastic large cell lymphoma (sALCL). Brentuximab vedotin (BV) is an antibody-drug conjugate targeting the CD30 antigen. FDA approval for BV includes relapsed and refractory HL and sALCL. The CD30 antigen also has been identified in many solid tumors, predominantly of germ cell origins and early clinical data is promising.

OBJECTIVE

Perform a focus literature review evaluating the prevalence of the CD30 antigen among nonlymphomatous tumors with a potential correlate for CD30 targeted therapy.

ELIGIBILITY CRITERIA

Inclusion criteria: all retrospective reviews and case reports citing CD30 positivity or negativity in non-lymphomatous malignancies in which data were presented based on location.

EXCLUSION CRITERIA

studies with hematopoetic malignancies, cutaneous malignancies, non-human populations, and non-english publications.

INCLUDED STUDIES

A total of 119 articles met these criteria and are summarized in this manuscript.

CONCLUSION

The CD30 antigen has shown variable prevalence among non-hematopoetic tumors, most notably among germ cell tumors and mesothelioma. With additional, preclinical and properly powered clinical studies, CD30 targeted therapy such as that of BV, alone or in combination with other agents may prove to be a strong candidate in the treatment of various CD30⁺ malignancies.

Hepatic Involvement of Histiocytic Sarcoma: CT and MRI Findings

Histiocytic sarcoma in the liver is an extremely rare hematological malignancy. Herein, we reported the case of a 68-year-old woman who presented with characteristic wedge-shaped abnormality bounded by hepatic veins on computed tomography and magnetic resonance imaging of the liver. In the wedge-shaped area, decreased portal flow and the deposition of iron were observed. These imaging findings are consistent with intrasinusoidal tumor cell infiltration. A liver biopsy was performed, and histiocytic sarcoma was confirmed histopathologically.

Flow cytometry used to identify histiocytic sarcoma: A case report

Histiocytic sarcoma (HS) is an extremely rare but aggressive malignancy of hematopoietic origin. The diagnosis of HS mainly relies on pathological morphology and immunohistochemical staining combined with clinical symptoms. However, whether histiocytic sarcoma can be detected using flow cytometry (FCM) is not clear. We report the case of a 61-year-old female patient with a group of abnormal cells identified primarily by FCM. Largeabnormal cells -CD68⁺ CD11c⁺ CD14⁺ HLA-DR⁺ CD123⁺ CD45⁺ presented in upper forward scatter and side scatter, indicating that a malignancy originally derived from monocyte/macrophage could be histocytic sarcoma. This patient with histiocytic sarcoma was finally confirmed with pathology and immunohistochemistry. In conclusion, HS can be detected by assessment of CD68⁺ CD11c⁺ CD14⁺ HLA-DR⁺ CD123⁺ CD45⁺ cells using FCM, suggesting that FCM can be an effective tool for early finding of HS. © 2015 International Clinical Cytometry Society.

Histiocytic sarcoma : an updated literature review based on the 2008 WHO classification

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.

Diagnosis and treatment of cancer-related anemia

Cancer-related anemia (CRA) is due to multiple etiologies, including chemotherapy-induced myelosuppression, blood loss, functional iron deficiency, erythropoietin deficiency due to renal disease, marrow involvement with tumor as well as other factors. The most common treatment options for CRA include iron therapy, erythropoietic-stimulating agents (ESAs), and red cell transfusion. Safety concerns as well as restrictions and reimbursement issues surrounding ESA therapy for CRA have resulted in suboptimal treatment. Similarly, many clinicians are not familiar or comfortable using intravenous iron products to treat functional iron deficiency associated with CRA. This article summarizes our approach to treating CRA and discusses commonly encountered clinical scenarios for which current clinical guidelines do not apply.

Laparoscopic splenectomy for histiocytic sarcoma of the spleen

Primary histiocytic sarcoma of the spleen is a rare but potentially lethal condition. It can remain asymptomatic or only mildly symptomatic for a long time. An 81-year-old woman presented with an extremely enlarged spleen. She suffered from progressive anemia and required a red blood cell transfusion once a month. Although computed tomography, ultrasonography, and magnetic resonance imaging were performed for diagnosis, a confirmed diagnosis was not obtained. Her enlarged spleen compressed her stomach, and she suffered from gastritis and a sense of gastric fullness just after meals. She underwent laparoscopic splenectomy for therapeutic and diagnostic purposes. Her post-operative course was uneventful. After surgery, her red blood cell and platelet counts increased markedly. The tumor was diagnosed as splenic histiocytic sarcoma. Post-surgical chemotherapy was not performed, and the patient died of liver failure due to liver metastasis 5 mo after surgery. Laparoscopic splenectomy is minimally invasive and useful for the relief of symptoms related to hematological disorders. However, in cases of an enlarged spleen, optimal views and working space are limited. In such cases, splenic artery ligation can markedly reduce the size of the spleen, thus facilitating the procedure. The case reported herein suggests that laparoscopic splenectomy may be useful for the treatment of splenic malignancy.

Primary splenic histiocytic sarcoma complicated with prolonged idiopathic thrombocytopenia and secondary bone marrow involvement: a unique surgical case presenting with splenomegaly but non-nodular lesions

Abstract

A 67-year-old Japanese female was followed up due to prolonged idiopathic thrombocytopenia with non-response to steroid therapy for 4 years, but recent progressive pancytopenia, hypo-albuminemia, and hypo-γ-globulinemia were presented. An abdominal CT scan revealed heterogeneously enhanced splenomegaly without any nodular lesions. A splenectomy was performed, and gross examination showed markedly hyperemic red pulp, weighing 760 g, accompanied by multiple foci of peripheral anemic infarction. Surprisingly, microscopic findings exhibited a diffuse proliferation of medium-sized to large tumor cells having pleomorphic nuclei, prominent nucleoli, and abundant eosinophilic cytoplasm, predominantly within the sinuses and cords of the red pulp, which occasionally displayed conspicuous hemophagocytosis and vascular permeation. In immunohistochemistry, these atypical cells were specifically positive for CD68 (KP-1), CD163, and lysozyme, which was consistent with histiocytic sarcoma (HS) of the spleen. Subsequently, section from the aspiration of bone marrow showed infiltration of the neoplastic cells associated with erythrophagocytosis 2 months after the operation, but never before it. Therefore, primary splenic HS presenting with secondary bone marrow involvement was conclusively diagnosed. Since early diagnosis and treatment are necessary for the HS patients with poor outcomes, splenic HS should be considered as a differential diagnosis in cases with chronic thrombocytopenia and splenomegaly.

Virtual slides

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/1009474924812827

Complete response of a patient with advanced primary splenic histiocytic sarcoma by treatment with chemotherapeutic drugs selected using the collagen gel droplet-embedded culture drug sensitivity test

A 69-year-old man presented with multiple nodules on the spleen and liver that had been detected by computed tomography (CT). A liver tumor biopsy was performed, and the patient was diagnosed to have histiocytic sarcoma (HS). Splenectomy was performed, and the chemosensitivity of the spleen tumor was measured using the collagen gel droplet-embedded culture drug sensitivity test (CD-DST). Hepatic arterial infusion chemotherapy was administered, based on the results of the CD-DST. The patient achieved complete remission (CR) after this therapy and remained alive without recurrence at the final follow-up. This is the first known case of CR in response to chemotherapy for advanced splenic HS.

Paraneoplastic syndrome in haemophagocytic histiocytic sarcoma in a dog

A case of metastatic splenic haemophagocytic histiocytic sarcoma (HHS) in a 6-year-old neutered male flat-coated retriever is described. The main clinical findings were hypoalbuminaemia and regenerative anaemia. The diagnosis was based on histological features and expression of CD11d by the neoplastic cells. Tumour cells were shown to produce interleukin (IL)-6, to phagocytose erythrocytes and to take up albumin, as demonstrated by immunohistochemistry and ultrastructural examination. Quantitative polymerase chain reaction identified increased IL-6 gene expression in affected organs. These findings suggest that neoplastic cells are responsible for the clinical features of HHS, by removing erythrocytes and albumin from the blood and releasing cytokines, such as IL-6.

[A case of extranodal histiocytic sarcoma of stomach mimicking gastric adenocarcinoma]

Histiocytic sarcoma is a rare malignant neoplasm that originates from a histiocytic hematopoietic lineage characterized by histiocytic differentiation and its corresponding immunophenotypic features. Patients with histiocytic sarcoma usually have a poor prognosis due to its aggressive clinical behavior. Here we report a rare case of extranodal histiocytic sarcoma of the stomach which was confirmed through immunohistochemical staining. A 71-year- old man was presented with epigastric pain. Gastroscopy, abdominal CT, and EUS revealed a mass located on the posterior wall of upper body and fundus of the stomach. Grossly, grayish white solid masses were seen extending down to the submucosal layer. Microscopically, the tumor cells had eosinophilic cytoplasm, abundant vacuole, and mitosis. Immunohistochemical staining revealed that the tumor cells were positive for LCA, CD68, and lysozyme. Early detection and accurate diagnosis of this rare neoplasm is important because it can make a great difference in prognostic outcomes. To make an accurate and definitive diagnosis, immunohistochemical staining is essential in the confirmation of histiocytic origin.

Primary histiocytic sarcoma of the spleen associated with hemophagocytosis

We report a patient with primary histiocytic sarcoma of the spleen associated with prominent hemophagocytosis. Although thrombocytopenia, probably due to hemophagocytosis, was refractory to corticosteroid therapy, the transfusion of platelets, and splenic irradiation, partial splenic embolization was effective and facilitated splenectomy for a diagnosis. The majority of the spleen showed necrosis, but viable neoplastic cells with pleomorphic nuclei and abundant cytoplasm, showing occasional erythrocytes or leukocytes, were still discernible. The neoplastic cells expressed CD68, lysozyme, and S-100 protein, and were negative for lymphoid, myeloid, and epithelial cell markers. CD163, a monocyte/macrophage-specific molecule, was positive in only some of them. Despite multiagent chemotherapy, the patient died of the disease, showing a rapidly progressive clinical course. Although the preoperative diagnosis of primary splenic histiocytic sarcoma is difficult, it has been confirmed in patients with splenomegaly of unknown etiology that clinicolaboratory features suggestive of hemophagocytosis may be important clues suggestive to the disease. CD163 expression by neoplastic cells could be confirmed only after careful observation, because the molecule may only be seen in some of the neoplastic cells.

Histiocytic sarcoma of the spleen: case report of asymptomatic onset of thrombocytopenia and complex imaging features

Histiocytic sarcoma of the spleen, in which the malignant cells display morphologic and immunophenotypic features similar to those of mature tissue histiocytes, is a rare but potentially lethal condition that can remain asymptomatic or only mildly symptomatic for a long period of time. We studied a case of histiocytic sarcoma of the spleen in an 82-year-old woman with prolonged chronic thrombocytopenia that was non-responsive to steroid therapy. Ultrasonography, computed tomography, and magnetic resonance imaging showed a characteristically enlarged spleen and liver. Palliative irradiation therapy was clinically effective; however, disease progression proved lethal. Autopsy revealed the proliferation of tumor cells within the splenic sinus and the liver sinusoids, which displayed extreme hemophagocytosis and strong expression of the histiocytic markers CD68 (KP1 and PG-M1) and CD163. The postmortem diagnosis showed histiocytic sarcoma of the spleen with liver infiltration. This and previous reports indicate that early detection (facilitated by imaging and clinical features) and management may improve patient prognosis and survival. Histiocytic sarcoma of the spleen should be considered as a differential diagnosis in therapeutically unresponsive patients with chronic thrombocytopenia.

High incidence, early onset of histiocytic sarcomas in mice with Hertwig's anemia

OBJECTIVE

Histiocytic sarcoma (HS) is a rare, rapidly disseminated, usually lethal tumor in humans. Treatment specific for HS has not been developed primarily due to deficiencies of appropriate animal models with high incidence/early onset. Mice with Hertwig's anemia (an/an) provide a potential model.

METHODS

Here, we compare HS susceptibility in an/an and unaffected control mice maintained on three genetic backgrounds. As a potential therapeutic measure, genetically marked bone marrow is transplanted between high and low susceptibility animals.

RESULTS

HS is detected earlier and the overall incidence is 15-fold higher in WBB6F1(F1)-an/an than in F1-+/?, B6-an/an and -+/? mice. Neither WB-an/an nor their normal WB-+/? littermates present with HS. Liver myelopoiesis and aneuploidy coexist with HS but the former is also rampant (33.7% incidence) in HS-free +/? and an/an mice. Marrow transplantation experiments provide evidence that (1) myelopoiesis is associated with HS and (2) early-onset/high-incidence HS is blocked by using late-onset F1-+/+ mice, as either donor or recipient.

CONCLUSIONS

Homozygosity for an on an F1 genetic background is essential for high-incidence/early-onset HS; myelopoiesis and HS coexist; and therapeutic transplantation may be feasible.

Mediastinal germ cell tumor associated with histiocytic sarcoma of spleen: case report of an unusual association

We present an unusual association of mediastinal germ cell tumor containing seminoma and angiosarcoma components and splenic histiocytic sarcoma. A 15-year-old boy presented with chest pain. Histopathologically, an anterior mediastinal mass contained typical seminoma, immature teratoma, embryonal carcinoma, angiosarcoma, yolk sac tumor, and polyembryoma. An abdominal ultrasonogram revealed a huge splenomegaly with multiple ill-defined low echogenic nodules, 1 month after the second cycle of chemotherapy. Histopathologically, large, round-to-oval tumor cells with abundant eosinophilic cytoplasm often contained eccentrically placed nuclei with vesicular chromatin and an irregular nuclear membrane. The tumor cells were immunoreactive for CD68, CD31, and CD4. The cytogenetic results showed deletion of the long arm of chromosome 5 and trisomy 8. This lesion might have been on the pathway of multistep tumorigenesis toward a final leukemia.

Histiocytic sarcoma with fatal duodenal ulcers

Histiocytic sarcoma is an uncommon neoplasm of mature histiocytes with very poor outcome. We report an autopsy case of a true histiocytic sarcoma with characteristic symptoms of so-called "malignant histiocytosis of the intestine". The liver and spleen were enlarged, with remarkable tumor cell infiltration in the hepatic sinusoids and splenic sinuses. Tumor cells aggregated to form sporadic nodular lesions in the liver, which often showed coagulative necrosis. Infarcted lesions also occurred at the splenic subcapsular area. In addition, tumor cell infiltration was noted in the sinuses of bone marrow and lymph node. Tumor cells often demonstrated moderate pleomorphism with multinucleated giant cells. They were positive for CD68 and negative for T- and B-cell lineage markers, megakaryocytic markers, and CD30. Various examinations were done to rule out infection-associated hemophagocytic syndrome, and the absence of infectious diseases was revealed. Thus, the diagnosis of histiocytic sarcoma was made. Apart from these lesions, multiple ulcerations, some with fatal perforation, were found in the esophagus and duodenum. They showed only non-specific inflammatory changes without tumor cell involvement. The ulcers probably derived from ischemic condition through an embolic process caused by tumor cell infiltration elsewhere in the blood vessels at the periphery of the ulcers.

Extranodal histiocytic sarcoma: clinicopathologic analysis of 14 cases of a rare epithelioid malignancy

Histiocytic sarcoma is a rare malignant neoplasm that occurs in lymph nodes, skin, and the gastrointestinal tract. Many previously published cases were likely misdiagnosed examples of non-Hodgkin lymphoma. Only small numbers of bona fide examples exist in the world literature; cases arising primarily at extranodal sites are not well described and often seem to go unrecognized. To characterize these tumors further, 14 extranodal histiocytic sarcomas were analyzed. Hematoxylin and eosin sections were reexamined, immunohistochemistry was performed, and clinical details were obtained from referring hospitals. Eight patients were female and 6 male (median age, 55 years; range, 15-89 years). All patients presented with a solitary mass, ranging in size from 1.8 to 12 cm (median 6.8 cm). Seven tumors arose in soft tissue (6 lower limb; 1 upper limb), 5 in the gastrointestinal tract (1 involving both stomach and colon, 1 ileum, 2 rectum, 1 anus), 1 in the nasal cavity, and 1 in the lung. Three gastrointestinal tract tumors also involved regional lymph nodes, and 1 involved the liver. Most cases had infiltrative margins. The tumors were generally composed of sheets of large epithelioid cells with abundant eosinophilic cytoplasm, oval to irregular nuclei, vesicular chromatin, and large nucleoli. Binucleated cells were common, and 6 cases contained tumor giant cells. Mitoses ranged from 1 to 64 per 10 HPF (median 11 per 10 HPF). Necrosis was present in 8 cases. Nearly all tumors showed a striking inflammatory infiltrate, most often of neutrophils or lymphocytes. All cases were reactive for LCA, CD45RO, and CD68 (KP1 and PG-M1); 13 of 14 (93%) expressed CD4, 12 of 14 (86%) lysozyme, 8 of 10 (80%) CD31, 7 of 14 (50%) S-100 protein, and 5 of 14 (36%) focal CD1a. Two tumors showed weak, focal cytoplasmic positivity for CD30, and 1 for epithelial membrane antigen. The tumors were negative for ALK-1, CD21, CD35, CD3, CD20, CD34, myeloperoxidase, HMB-45, and keratins. Gastrointestinal tract cases were negative for c-kit and desmin. Six patients were treated with postoperative radiation and 7 with chemotherapy (CHOP or ProMACE-MOPP). Follow-up was available for 10 patients (median, 24 months; range, 4 months to 11 years). Two tumors recurred locally, and 5 patients developed distant spread: 3 to lymph nodes, 1 to lung, and 1 to bone. At the last follow-up, 2 patients have died of disseminated disease, 4 and 5 months following initial diagnosis. The patients who died thus far had the largest primary tumors. Histiocytic sarcoma may arise primarily in soft tissue and shows reproducible histologic features, including abundant eosinophilic cytoplasm and a prominent inflammatory infiltrate. Metastatic carcinoma, metastatic melanoma, and large cell non-Hodgkin lymphomas should be excluded by immunohistochemistry. Histiocytic sarcoma has the potential for an aggressive clinical course, most often with lymph node involvement. However, a subset of cases presenting with clinically localized disease have a favorable long-term outcome. Tumor size may be a prognostic factor.