The Utility of Flow Cytometry for the Diagnosis of Cranial Mediastinal Malignancy in Canine Patients

Introduction:  The most common neoplasms located in the anterior mediastinum in the canine are thymoma and lymphoma. Distinguishing between the two is a diagnostic challenge. Treatment and prognosis for these diseases differs significantly. Thymomas contain a population of normally developing T cells. The majority of these T cells exhibit an immature phenotype, characterized by co‐expression of CD4 and CD8. This phenotype is rarely seen on neoplastic lymphocytes. The purpose of this study was to determine if analysis by flow cytometry could discriminate thymoma from lymphoma based on these cell surface markers.
Methods:  Fine needle aspirates were obtained from ten canine patients with mediastinal masses. Cells were analyzed by flow cytometry using a panel of T and B cell markers.
Results:  Six cases with 10% or greater CD4 + CD8+ cells were diagnosed with thymoma and were confirmed by histopathology. Four cases had fewer than 5% CD4 + CD8+ cells, having lymphocytes expressing CD4 only (3 cases) or CD21, a B cell marker(1 case). These were confirmed as lymphoma by cytology and/or a clonality assay. The sensitivity and specificity of this assay when used in the diagnostic work‐up for suspected thymoma was 100%.
Conclusion:  Flow cytometry may provide important, complementary information in the diagnostic work‐up of the canine patient with a mediastinal mass.