Flow cytometry confirmation of canine histiocytic sarcoma

Rebecca Gans, Kevin Finora, Kimberly Ho, Dorothee Bienzle, Felipe Reggeti

Central Toronto Veterinary Referral Clinic, Toronto, ON (Gans, Finora, Ho), Department of Pathobiology, University of Guelph, Guelph, ON (Bienzle), Animal Health Laboratory, University of Guelph, Guelph, ON (Reggeti)

AHL Newsletter, 2021;25(2):30.

A 10-year-old female spayed Miniature Schnauzer presented to the family veterinarian for evaluation of acute onset of partial anorexia and lethargy.  The patient was found to be mildly pyrexic and was tender on cranial abdominal palpation.  Blood work demonstrated a normal complete blood count, and moderately elevated liver values [AST: 351 U/L (RI 16-55 IU/L), ALP: 407 U/L (RI 5-131 IU/L) and bilirubin: 10.9 umol/L (RI 0.0-5.2 umol/L.)].  Thoracic and abdominal radiographs showed a cranial thoracic mass, possible pneumonia in the cranial lung lobes, trace pleural effusion, and a small liver.

The patient was referred to the Central Toronto Veterinary Referral Clinic, Oncology Service, for further assessment.  Thoracic auscultation noted diminished breath sounds in the ventral lung fields.  A CT scan of the thorax demonstrated the presence of two thoracic masses.  One large (7.2 x 6.5 x 9.3 cm), cavitary soft tissue mass was located in the right hemithorax, suspected to be arising from the cranial mediastinum. Another smaller soft tissue mass (2.9 x 2.3 x 1.2 cm) was within the caudodorsal aspect of the left caudal lung lobe.  The CT scan also showed multiple mildly to moderately enlarged lymph nodes within the thorax (cranial, mediastinal, tracheobronchial, and right sternal), mild bilateral pleural effusion, and mild microhepatica.

The primary differential diagnoses were metastatic carcinoma, thymoma, lymphoma, and less likely, sarcoma.  Ultrasound-guided fine needle aspirates of the mediastinal mass and samples of pleural fluid were sent to the Animal Health Laboratory, University of Guelph, for further testing.  Cytology of the mediastinal mass revealed a pleomorphic population of atypical round cells with moderate to abundant pale blue cytoplasm, discrete cytoplasmic vacuolation, round to oval nuclei with coarsely reticular chromatin, and single to multiple indistinct nucleoli.  Binucleated and multinucleated cells were numerous and mitotic figures were also identified (Fig. 1).  The pleural effusion cytologic preparation was mildly cellular, but a few large hyperchromatic round cells were also noted.  The cytologic findings from the mediastinal mass were most consistent with histiocytic sarcoma (HS), however, a poorly differentiated anaplastic carcinoma could not be ruled out.  To reach a definitive diagnosis, immunophenotyping by flow cytometry was performed on aspirates from the same mediastinal mass.   This method identified a population of large mononuclear cells highly positive for CD14, CD18, MHC II and partially positive for CD4.  These results indicated a monocyte/histiocyte population consistent with an exfoliating histiocytic sarcoma.

Histiocytic sarcoma is an aggressive neoplasm of the innate immune system, most commonly of dendritic cell origin.  It can be localized or disseminated (1).  In this case, the patient was diagnosed with the disseminated form of disease which is more aggressive.  Left untreated, disseminated HS progresses quickly and is rapidly fatal (2).  Chemotherapy with Lomustine (1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea - CCNU) is most effective, with a reported median survival time of 106 days (3).  The owner accepted the recommended chemotherapeutic treatment, and prednisone was also started to help decrease inflammation and support the patient's appetite.  However, during the first course of treatment, the patient experienced increased lethargy, difficulty breathing and subsequently died, 61 days after thoracic radiographs at rDVM identified the thoracic mass.   AHL

                             Figure 1. Aspirate of mediastinal mass showing a population of large neoplastic histiocytes with prominent criteria of malignancy, including numerous multinucleated cells. Wright’s stain (600x).

Figure 1. Aspirate of mediastinal mass showing a population of large neoplastic histiocytes with prominent criteria of malignancy, including numerous multinucleated cells. Wright’s stain (600x).


1. Fulmer AK, Mauldin GE. Canine histiocytic neoplasia: An overview. Can Vet J 2007;8(10):1041–1050.

2. Clifford CA, et al. Chapter 34 Miscellaneous tumors ; Section F: Histiocytic Diseases. In: Withrow and MacEwen’s Small Animal Clinical Oncology, 6th ed. Liptak JM, Thamm DH and Vail DM, eds. Elsevier, 2020:791-797.

3. Skorupski K, et al. CCNU for the treatment of dogs with histiocytic sarcoma. J Vet Int Med 2007;21:121-126.