CANINE

Canine lymphoma, a prevalent neoplasm in dogs, is characterized by the monoclonal expansion of lymphoid cells, analogous to non-Hodgkin lymphoma in humans. The etiology remains multifactorial and includes environmental influences such as exposure to 2,4-D herbicides and genetic predisposition in certain breeds like Boxers and Golden Retrievers. The disease manifests in various clinical forms, with the most common presentation involving asymptomatic, markedly enlarged peripheral lymph nodes. Diagnostic approaches include fine needle aspiration, cytologic evaluation, and advanced techniques like polymerase chain reaction for antigen receptor rearrangements and flow cytometry. Therapeutic strategies range from multiagent chemotherapy protocols, such as CHOP (Cyclophosphamide, Doxorubicin, Vincristine, Prednisone), to single-agent treatments and rescue therapies for refractory cases. Prognosis and treatment response vary based on the lymphoma subtype and clinical stage at diagnosis.

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Etiology/Pathophysiology:
Common neoplasm, analogous to non-Hodgkin lymphoma in humans.
Characterized by monoclonal expansion of lymphoid cells.
Etiology: Unknown, likely multifactorial.
Environmental influences: Exposure to 2,4-D herbicides, magnetic fields, environmental waste.
No viral causative agents identified in dogs, unlike Epstein-Barr virus, HIV in humans.
Immune-mediated disorders (IMHA, thrombocytopenia), organ transplantation, atopic dermatitis implicated.

Epidemiology:
Incidence: 84 per 10,000 dogs/year.
Higher risk breeds: Boxers, Golden Retrievers, Basset Hounds, Bulldogs.
Lower risk breeds: Dachshunds, Pomeranians.
Average age: 5-10 years, no sex predilection.

Clinical Presentation and Staging:
Multicentric lymphoma: Variable, dependent on disease extent.
WHO staging system: Five categories based on dissemination and site.
WHO substage: Presence (b) or absence (a) of constitutional signs (anorexia, vomiting, diarrhea, weight loss, cachexia, fever).
Common presentation: Asymptomatic with enlarged peripheral lymph nodes (Stage III, substage a).
Stage V: Clinical signs related to anatomic location (bone marrow infiltration, pulmonary involvement).

Diagnosis/Clinical Staging:
Fine needle aspiration (FNA) and cytologic evaluation.
Cytology: Large or intermediate-sized lymphocytes (>50%).
Polymerase chain reaction (PCR) for antigen receptor rearrangements (PARR) for clonality.
Flow cytometry: Monoclonal antibodies, fluorescent markers to confirm diagnosis.
Histology: Preferred for classification, excisional biopsy ideal.
Immunophenotyping: CD3 (T cell) vs. CD79a (B cell).
Minimum database: CBC, serum biochemistry, urinalysis.
Radiographs/ultrasound: Evaluate visceral organ involvement.

Therapy:
Chemotherapy: Multiagent protocols (CHOP-based) superior to single-agent.
CHOP: Cyclophosphamide, Doxorubicin, Vincristine, Prednisone.
Single-agent options: Doxorubicin (caution for cardiotoxicity).
Prednisone: Short-lived remission, risk of multidrug resistance.
Rescue chemotherapy: Rabacfosadine (Tanovea), Lomustine, Vinblastine.
Complications: Nausea, vomiting, diarrhea, myelosuppression.
CBC before each treatment to monitor WBC and platelets.

Extranodal Forms:
Alimentary: Poorly responsive to chemotherapy, CCNU recommended.
Cutaneous: Epiteliotropic (T cell) vs. Nonepiteliotropic (B/T cell), treatment varies.
Other sites: CNS, nasal cavity, etc.

Prognosis:
Varies with clinical picture, histologic classification, immunophenotype.
B cell: Better response and prognosis.
T cell: Modified protocols with more alkylating agents recommended.

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Reference:

Burgess, K.E. (2020). Lymphomas. In D.S. Bruyette (Ed.), Clinical Small Animal Internal Medicine (Vol. II, pp. 1231-1239). John Wiley & Sons, Inc.