Chemotherapy
The use of cytotoxic drugs to treat cancer by killing or slowing the growth of rapidly dividing cells. Veterinary chemotherapy protocols are designed to extend quality life with manageable side effects rather than pursue cure at any cost.
Chemotherapy uses drugs that target rapidly dividing cells to treat cancer. In veterinary oncology, the primary goal is typically to extend quality survival time rather than achieve cure — a fundamentally different philosophy from human oncology, where more aggressive side-effect profiles may be accepted in pursuit of remission.
How Chemotherapy Works
Cytotoxic drugs interfere with cell division at various stages of the cell cycle. Because cancer cells divide more rapidly than most normal cells, they are disproportionately affected. However, normal rapidly dividing cells — in the bone marrow, GI tract, and hair follicles — are also susceptible, which accounts for most chemotherapy side effects.
Common Veterinary Protocols
Lymphoma (CHOP Protocol)
The most common cancer treated with chemotherapy in dogs. The CHOP protocol (cyclophosphamide, doxorubicin, vincristine, prednisone) achieves remission rates of 80-90% in B-cell lymphoma, with median survival times of 10-14 months. The University of Wisconsin-Madison (UW-25) protocol is a widely used variation.
Hemangiosarcoma
Doxorubicin-based protocols are used post-splenectomy for hemangiosarcoma, though median survival times remain modest (approximately 4-6 months with chemotherapy vs. 1-3 months with surgery alone).
Osteosarcoma
Carboplatin or doxorubicin following amputation or limb-sparing surgery for osteosarcoma. Chemotherapy roughly doubles median survival time compared to amputation alone.
Mast Cell Tumors
Vinblastine-based protocols for high-grade mast cell tumors or tumors not amenable to surgical cure.
Side Effects in Dogs
Veterinary chemotherapy is intentionally less aggressive than human protocols. Approximately 80-85% of dogs experience no significant side effects, and fewer than 5% require hospitalization for toxicity.
Common side effects include:
- GI effects: mild nausea, decreased appetite, soft stool (typically self-limiting, 1-3 days)
- Bone marrow suppression: low white blood cell counts (neutropenia) peaking 7-10 days after treatment. Most dogs remain asymptomatic; monitoring CBC guides dose adjustments.
- Hair changes: breeds with continuously growing hair (Poodles, Old English Sheepdogs) may lose hair. Most breeds do not experience significant hair loss.
- Fatigue: mild lethargy for 1-2 days after treatment
Monitoring During Treatment
Regular CBCs (typically weekly during induction) monitor bone marrow function. Chemistry panels assess organ function. Treatment doses are adjusted based on tolerance and response. The guiding principle is maintaining quality of life throughout treatment — if side effects become unacceptable, protocols are modified or discontinued.