An 11-year-old mixed breed dog
was presented with signs of anorexia. Radiographic and ultrasound examinations revealed a
large mass in the heart, between the right atrium and the right ventricle. Upon gross
inspection, a multilobulated tumor arising from the right atrial wall and occupying the
right atrium was identified. Microscopical analysis demonstrated that this tumor consisted
of neoplastic cells with granular cytoplasm, which were separated into nests by fine
fibrovascular stroma and were negative for Grimelius’s method. Immunohistochemical
examinations revealed that the neoplastic cells expressed chromogranin A, synaptophysin
and neuron specific enolase. Electron microscopy revealed that the cytoplasm of the
neoplastic cells held secretory granules. Based on these pathological findings, the tumor
was diagnosed as a paraganglioma. This report is a rare case of primary paraganglioma
deriving from the right atrium and provides a detailed characterization of its
morphological features.