Comparison of Diagnostic Utilities in a Canine Retrobulbar Large B‐Cell Lymphoma With Multifocal Involvement

Lymphoma is the most common malignant neoplasm in dogs. However, retrobulbar lymphoma, a type of orbital lymphoma, is rarely reported in dogs. An 8-year-old neutered male Shih Tzu dog presented with bilateral exophthalmos. Computed tomography and magnetic resonance imaging revealed bilateral retrobulbar masses and enlargement of the salivary glands and cranial mediastinum. Histopathological examination and immunohistochemistry of the retrobulbar tissue confirmed large B-cell lymphoma. On the cytologic analysis, intermediate-to-large malignant lymphocytes were predominant in the retrobulbar tissue (>80%); however, they accounted for 30%-50% of the cranial mediastinum, submandibular lymph nodes and salivary glands. Flow cytometry of the retrobulbar lesion showed a B-cell phenotype (cluster of differentiation [CD]3^-/CD4^-/CD5^-/CD8^-/CD21⁺/CD34^-/CD79a⁺/major histocompatibility complex II), whereas the cranial mediastinum, submandibular lymph node and salivary gland exhibited a heterogeneous cell population. Polymerase chain reaction for antigen receptor rearrangement demonstrated clonality with immunoglobulin gene rearrangement, indicating a B-cell origin. Chemotherapy with l-asparaginase, vincristine, cyclophosphamide, doxorubicin and prednisone was initiated; however, neurological symptoms developed after the first cycle. Subsequent cerebrospinal fluid cytology and flow cytometry suggested central nervous system involvement in the lymphoma, and the dog was euthanized owing to disease progression despite the addition of lomustine. To our knowledge, this is the first report of bilateral retrobulbar large B-cell lymphoma with multifocal involvement in a dog. A comprehensive assessment of diagnostic modalities is essential to diagnose extranodal lymphoma and assess extranodal involvement.