Histiocytic Sarcoma

Histiocytic sarcoma (HS) is a rare and aggressive malignant neoplasm arising from dendritic cells or macrophages of the mononuclear phagocyte system. It is classified into two main forms: localized histiocytic sarcoma (LHS), which presents as a solitary mass, typically in the skin, subcutis, or lymph node, and disseminated histiocytic sarcoma (DHS), also known as malignant histiocytosis, which involves multiple organs including the spleen, liver, lungs, bone marrow, and lymph nodes. The disease is characterized by highly pleomorphic, malignant histiocytic cells with abundant cytoplasm, often exhibiting hemophagocytosis. HS is most commonly diagnosed in dogs, particularly in purebred retrievers, and is rare in cats. The clinical course is rapidly progressive, with a poor prognosis, especially in the disseminated form. Accurate diagnosis requires histopathology and immunohistochemistry, as clinical signs and imaging findings are nonspecific.

The exact etiology of histiocytic sarcoma is unknown, but it is believed to arise from neoplastic transformation of interstitial dendritic cells or macrophages. Genetic predispositions have been identified, particularly in Bernese Mountain Dogs, where a strong familial association suggests an inherited susceptibility. In this breed, a genetic mutation in the CDKN2A/B tumor suppressor genes has been implicated. Other breeds, such as Flat-Coated Retrievers and Rottweilers, also show increased risk, indicating a genetic component. Environmental factors, such as exposure to certain chemicals or chronic inflammation, have been hypothesized but not definitively proven. Viral etiologies have not been identified. The neoplastic cells often exhibit dysregulation of signaling pathways, including activation of the RAS-MAPK and PI3K-AKT pathways, and overexpression of growth factors such as CSF-1 (colony-stimulating factor 1), which promotes tumor cell proliferation and survival.

Histiocytic sarcoma is most commonly diagnosed in dogs, with a higher incidence in certain breeds. Bernese Mountain Dogs are at the highest risk, with a reported incidence of up to 15% in some lines. Flat-Coated Retrievers, Rottweilers, Golden Retrievers, and Labrador Retrievers are also overrepresented. The disease typically affects middle-aged to older dogs, with a median age of 7-9 years. No significant sex predilection has been consistently reported. In cats, histiocytic sarcoma is rare, but it has been reported in middle-aged to older cats, with no breed predilection. The disseminated form is more common than the localized form in dogs, accounting for approximately 60-70% of cases. The disease is sporadic, with no geographic or seasonal variation. In Bernese Mountain Dogs, a familial pattern suggests an autosomal recessive mode of inheritance with incomplete penetrance.

Histiocytic sarcoma arises from the malignant transformation of dendritic cells or macrophages. These cells are part of the mononuclear phagocyte system and are involved in antigen presentation and immune regulation. The neoplastic cells exhibit uncontrolled proliferation, tissue invasion, and metastasis. In localized histiocytic sarcoma, the tumor typically arises in the skin, subcutis, or a single lymph node, and may remain localized for a period before metastasizing. In disseminated histiocytic sarcoma, the disease is multicentric from the outset, with involvement of the spleen, liver, bone marrow, lungs, and lymph nodes. The tumor cells produce various cytokines and growth factors, including CSF-1, which promotes their own growth and recruitment of inflammatory cells. Hemophagocytosis by neoplastic cells leads to anemia, thrombocytopenia, and leukopenia. The infiltration of organs disrupts normal tissue architecture and function, leading to organ failure. The disease is rapidly progressive, with a median survival time of only a few months without treatment.

The primary predisposing factor for histiocytic sarcoma is genetic susceptibility, particularly in Bernese Mountain Dogs, where a strong familial association has been documented. Other breeds, such as Flat-Coated Retrievers and Rottweilers, also have a genetic predisposition. Age is a risk factor, with middle-aged to older dogs being more commonly affected. Immunosuppression, whether due to concurrent disease or drug therapy, may increase the risk of developing histiocytic sarcoma, as the immune system plays a role in tumor surveillance. Chronic inflammation has been suggested as a potential trigger, but evidence is limited. Environmental factors, such as exposure to certain chemicals or radiation, have not been conclusively linked. In cats, no specific predisposing factors have been identified, but the disease is more common in older animals.

Clinical signs of histiocytic sarcoma vary depending on the form and organs involved. In localized histiocytic sarcoma, a solitary mass may be palpable in the skin or subcutis, often on the limbs or trunk. The mass may be firm, non-painful, and slow-growing initially. If a lymph node is involved, lymphadenopathy may be noted. In disseminated histiocytic sarcoma, clinical signs are often nonspecific and include lethargy, anorexia, weight loss, and fever. Splenomegaly and hepatomegaly may be detected on abdominal palpation. Respiratory signs, such as cough or dyspnea, may occur if the lungs are involved. Bone marrow infiltration can lead to anemia, thrombocytopenia, and leukopenia, resulting in pale mucous membranes, petechiae, and increased susceptibility to infections. Neurologic signs may develop if the central nervous system is affected. The disease progresses rapidly, and many dogs are presented in an advanced state with severe systemic illness.

Differential diagnoses for histiocytic sarcoma include: 1) Lymphoma, particularly large cell lymphoma, which can present with similar lymphadenopathy and organ involvement; differentiation requires cytology or histopathology with immunohistochemistry (CD3, CD79a for B/T cells vs. CD18, CD1c for histiocytic cells). 2) Hemangiosarcoma, especially when splenic masses and hemoperitoneum are present; histopathology and immunohistochemistry (Factor VIII, CD31) are definitive. 3) Malignant fibrous histiocytoma, a soft tissue sarcoma with histiocytic-like cells; immunohistochemistry for CD18 and CD204 can help. 4) Metastatic carcinoma, which may present with multiple organ masses; cytology and histopathology with cytokeratin staining are useful. 5) Systemic histiocytosis, a benign but progressive inflammatory disease in Bernese Mountain Dogs; histopathology shows non-neoplastic histiocytic infiltration, and the disease responds to immunosuppressive therapy. 6) Fungal infections, such as histoplasmosis or blastomycosis, which can cause granulomatous lesions and organomegaly; cytology and fungal culture are diagnostic. 7) Ehrlichiosis or other tick-borne diseases, which can cause fever, cytopenias, and organomegaly; serology and PCR are helpful.

The diagnostic algorithm for histiocytic sarcoma begins with a thorough history and physical examination, with particular attention to palpable masses, lymphadenopathy, and organomegaly. Baseline blood work, including complete blood count, serum biochemistry, and urinalysis, is recommended to assess overall health and identify cytopenias or organ dysfunction. Thoracic radiographs and abdominal ultrasound are performed to evaluate for metastatic disease and organ involvement. If a mass or enlarged lymph node is identified, fine needle aspiration for cytology is the next step. Cytology may reveal large, pleomorphic cells with abundant cytoplasm, often with phagocytosed erythrocytes or leukocytes. However, cytology is not always definitive, and histopathology is required for a definitive diagnosis. A biopsy of the affected tissue, either via surgical excision or core needle biopsy, is submitted for histopathology and immunohistochemistry. Immunohistochemical markers such as CD18, CD1c, CD204, and Iba-1 are positive in histiocytic sarcoma, while CD3 and CD79a are negative, helping to rule out lymphoma. If the spleen is involved, splenectomy may be both diagnostic and therapeutic. In cases where the bone marrow is suspected to be involved, a bone marrow aspirate or biopsy is performed. Advanced imaging, such as computed tomography (CT) or magnetic resonance imaging (MRI), may be used to assess the extent of disease, especially for surgical planning or radiation therapy.

Laboratory findings in histiocytic sarcoma are nonspecific but may include: 1) Hematology: Anemia, often non-regenerative, due to chronic disease or hemophagocytosis; thrombocytopenia, which may be severe and lead to bleeding; leukopenia or leukocytosis, with a left shift. Circulating neoplastic cells may occasionally be seen on blood smears in disseminated cases. 2) Serum biochemistry: Elevated liver enzymes (ALT, ALP) if hepatic infiltration is present; hyperbilirubinemia may occur with biliary obstruction. Azotemia (elevated BUN and creatinine) may indicate renal involvement or prerenal azotemia due to dehydration. Hypercalcemia has been reported in some cases, likely due to paraneoplastic secretion of parathyroid hormone-related protein. Hypoalbuminemia may occur due to chronic inflammation or protein-losing enteropathy. 3) Urinalysis: May be normal, but proteinuria or hematuria can occur if the urinary tract is involved. 4) Blood gas analysis: May reveal metabolic acidosis in critically ill patients. 5) Specific biomarkers: Serum thymidine kinase (TK) activity may be elevated in some dogs with histiocytic sarcoma, but it is not specific. C-reactive protein (CRP) may be elevated as an acute phase protein. 6) Serology/PCR: Tests for infectious diseases (e.g., Ehrlichia, Anaplasma, Leishmania) may be performed to rule out other causes of systemic illness, but they are not diagnostic for histiocytic sarcoma.

Imaging findings in histiocytic sarcoma are variable and depend on the organs involved. 1) Radiography: Thoracic radiographs may reveal pulmonary nodules, interstitial or alveolar patterns, or pleural effusion. Abdominal radiographs may show splenomegaly, hepatomegaly, or soft tissue masses. 2) Ultrasonography: Abdominal ultrasound is useful for evaluating the spleen, liver, and lymph nodes. Splenic lesions may appear as hypoechoic nodules or masses, often with a complex echotexture. Hepatic lesions may be hypoechoic or target-like. Lymphadenopathy may be detected, with enlarged, hypoechoic nodes. Doppler ultrasound may show increased vascularity in malignant lesions. 3) Computed Tomography (CT): CT provides detailed cross-sectional imaging and is excellent for staging. It can identify pulmonary metastases, abdominal organ involvement, and bone lesions. CT is also useful for radiation therapy planning. 4) Magnetic Resonance Imaging (MRI): MRI is superior for evaluating central nervous system involvement and soft tissue masses. It may show contrast-enhancing lesions in the brain or spinal cord. 5) Endoscopy: Endoscopy may be used to evaluate the gastrointestinal tract if clinical signs suggest involvement, but it is rarely diagnostic for histiocytic sarcoma. 6) Echocardiography: If cardiac involvement is suspected, echocardiography may reveal masses or effusion.

Cytology of fine needle aspirates from affected tissues often reveals a population of large, round to polygonal cells with moderate to abundant basophilic cytoplasm. The nuclei are large, pleomorphic, and may be indented or reniform. Binucleation or multinucleation is common. Hemophagocytosis, characterized by the presence of erythrocytes or leukocytes within the cytoplasm of neoplastic cells, is a distinctive feature. The background may contain inflammatory cells, including lymphocytes and neutrophils. Histopathology of biopsy samples shows a densely cellular neoplasm with sheets of pleomorphic histiocytic cells. The cells have abundant eosinophilic cytoplasm, and nuclei are large with prominent nucleoli. There is often marked anisocytosis and anisokaryosis. Mitotic figures are frequent. Areas of necrosis and hemorrhage are common. In the spleen, the red pulp is typically infiltrated, and the normal architecture is effaced. Immunohistochemistry is essential for definitive diagnosis: neoplastic cells are positive for CD18, CD1c, CD204, and Iba-1, and negative for CD3, CD79a, and cytokeratin. Special stains, such as CD11d, may help differentiate between dendritic cell origin and macrophage origin.

Treatment of histiocytic sarcoma depends on the form and extent of disease. For localized histiocytic sarcoma, surgical excision with wide margins is the treatment of choice. If complete excision is achieved, the prognosis may be favorable, but adjuvant chemotherapy is often recommended due to the high risk of metastasis. For disseminated histiocytic sarcoma, systemic chemotherapy is the mainstay of treatment. The most commonly used protocol is a combination of lomustine (CCNU) and doxorubicin. Lomustine is administered at a dose of 60-90 mg/m² orally every 3 weeks, and doxorubicin is given at 30 mg/m² intravenously every 3 weeks. Alternatively, single-agent lomustine may be used, with a response rate of approximately 30-40%. Other agents that have shown activity include epirubicin, mitoxantrone, and ifosfamide. In dogs with splenic involvement, splenectomy may be performed to reduce tumor burden and alleviate clinical signs, but it is not curative. Radiation therapy may be used for localized masses that are not amenable to surgery, or as palliative treatment for pain or neurologic signs. Supportive care is essential, including fluid therapy, antiemetics, and nutritional support. In cats, treatment options are similar, but the prognosis is generally poor.

The prognosis for histiocytic sarcoma is generally poor, especially for the disseminated form. In dogs with localized histiocytic sarcoma that is completely excised, the median survival time may be 1-2 years with adjuvant chemotherapy. However, if the tumor is incompletely excised or has metastasized, the prognosis is worse. For disseminated histiocytic sarcoma, the median survival time with chemotherapy is approximately 3-6 months. Negative prognostic factors include the presence of clinical signs at diagnosis, high tumor burden, elevated serum alkaline phosphatase, and lack of response to chemotherapy. In cats, the prognosis is also poor, with most cats surviving less than 6 months despite treatment. The disease is rapidly progressive, and most animals eventually succumb to organ failure or complications of treatment.

Follow-up for histiocytic sarcoma involves regular monitoring for disease progression or recurrence. After initial treatment, re-evaluation is recommended every 3-4 weeks during chemotherapy. Each re-check should include a physical examination, complete blood count, and serum biochemistry to monitor for drug toxicities and assess overall health. Thoracic radiographs and abdominal ultrasound may be repeated every 2-3 months to evaluate for metastasis or recurrence. If a mass was surgically excised, the surgical site should be monitored for local recurrence. In dogs receiving lomustine, liver enzyme levels should be monitored regularly, as hepatotoxicity is a common side effect. Blood counts should be checked before each chemotherapy dose to ensure adequate bone marrow recovery. If the dog is in remission, the frequency of re-checks may be reduced to every 3-6 months. Long-term management may include continued chemotherapy or maintenance therapy, depending on the response. Owners should be educated about the signs of disease progression, such as lethargy, anorexia, weight loss, or new masses, and advised to seek veterinary attention promptly.

Pearls: 1) Histiocytic sarcoma should be considered in any middle-aged to older dog, especially a Bernese Mountain Dog, presenting with splenomegaly, lymphadenopathy, and nonspecific systemic signs. 2) Cytology of a splenic mass may be diagnostic, but histopathology with immunohistochemistry is essential for confirmation. 3) Hemophagocytosis on cytology is a strong indicator of histiocytic sarcoma. 4) Splenectomy can be both diagnostic and therapeutic, and may improve survival in some dogs. 5) Lomustine is the most effective single agent, but combination with doxorubicin may improve response rates. Pitfalls: 1) Misdiagnosing histiocytic sarcoma as lymphoma, as both can present with similar clinical signs and cytology; immunohistochemistry is crucial. 2) Failing to stage the disease thoroughly, as disseminated histiocytic sarcoma may be present even if a solitary mass is found. 3) Underestimating the aggressive nature of the disease; even localized histiocytic sarcoma has a high metastatic potential. 4) Using corticosteroids alone, which may provide temporary relief but do not alter the disease course. 5) Overlooking the risk of hepatotoxicity with lomustine; liver enzymes should be monitored regularly.

Based on Plumb's Veterinary Drug Handbook, the following protocols are recommended for histiocytic sarcoma in dogs: 1) Lomustine (CCNU): 60-90 mg/m² orally every 3 weeks. Administer with antiemetic (e.g., maropitant 1 mg/kg IV or PO q24h) to prevent nausea. Monitor for myelosuppression (nadir at 7-14 days) and hepatotoxicity. Dose reduction may be needed if severe neutropenia or thrombocytopenia occurs. 2) Doxorubicin: 30 mg/m² IV every 3 weeks, given as a slow infusion over 20-30 minutes. Pre-treat with antiemetic and monitor for cardiotoxicity (cumulative dose limit 180-240 mg/m²). 3) Combination protocol: Lomustine (60 mg/m² PO) and doxorubicin (30 mg/m² IV) on day 1 of each 3-week cycle. This protocol has shown response rates of up to 50% in some studies. 4) Epirubicin: 90 mg/m² IV every 3 weeks, as an alternative to doxorubicin. 5) Mitoxantrone: 5-6 mg/m² IV every 3 weeks, may be used in dogs with cardiac disease. 6) Ifosfamide: 375 mg/m² IV every 3 weeks, with mesna (375 mg/m² IV) to prevent hemorrhagic cystitis. 7) Palliative therapy: Prednisone (1-2 mg/kg PO q24h) may be used to reduce inflammation and improve appetite, but it has minimal antitumor effect. 8) Supportive care: Maropitant (1 mg/kg IV or PO q24h) for nausea, ondansetron (0.5-1 mg/kg IV q12h) for vomiting, and filgrastim (5 µg/kg SC q24h) for neutropenia. In cats, doses are similar but should be adjusted based on body weight; lomustine is often used at 50-60 mg/m² PO every 3 weeks.

The literature on histiocytic sarcoma is limited, but several key studies have shaped current understanding. A landmark study by Affolter and Moore (2000) characterized the immunohistochemical phenotype of histiocytic sarcoma, establishing CD18 and CD1c as reliable markers. A retrospective study by Moore et al. (2006) reported a response rate of 30% to lomustine in dogs with disseminated histiocytic sarcoma. A more recent study by Skorupski et al. (2007) evaluated a combination of lomustine and doxorubicin, showing a response rate of 46% and a median survival of 6 months. In Bernese Mountain Dogs, a genetic study by Hedan et al. (2011) identified a mutation in the CDKN2A/B genes, providing insight into the hereditary nature of the disease. A consensus statement from the Veterinary Cancer Society (2016) recommended that histiocytic sarcoma be treated with surgery for localized disease and chemotherapy for disseminated disease, with lomustine as the first-line agent. Overall, the evidence supports the use of lomustine-based protocols, but the prognosis remains guarded, and further research is needed to improve outcomes.