Introduction
Cancer is never something we want to diagnose a patient with, but it is unfortunately a leading cause of morbidity and mortality in dogs and cats, affecting ~50% of dogs and ~30% of cats over 10 years of age.
There are various treatment options available in veterinary oncology, and early detection is key to helping attain a more favorable treatment outcome. It is important to individualize the treatment plan for each patient, remembering to take into account the owner’s abilities and wishes.
Treatment options consist of (i) specialty referral to a veterinary oncologist, (ii) treatment with the GP or primary care veterinarian, and/or (iii) palliative and end-of-life care.
Diagnostic options include cytology and/or histopathology, along with imaging and/or lymph node aspirations. Thoracic radiographs are commonly taken to help assess for possible metastasis.
Therapy and prognosis are based on the tumor type, grade, and stage.
Therapy is multimodal, and may include surgery, chemotherapy, radiation therapy, immunotherapy, nutritional support, and/or pain management.
It is important to protect the patient, team, and the owner as related to the handling of certain chemotherapeutic agents.
Common Cancers of the Dog
- anal sac carcinoma
- lymphoma
- mammary gland cancer
- mast cell tumor
- oral malignant melanoma
- osteosarcoma
- soft tissue sarcoma
- splenic hemangiosarcoma
- transitional cell carcinoma/urothelial carcinoma
- nasal tumors
Common Cancers of the Cat
- lymphoma
- mammary gland cancer
- squamous cell carcinoma
- soft tissue sarcoma (including injection site sarcoma)
- nasal tumors
- mast cell tumor
Tumor Grading and Staging
Tumor grade is the microscopic appearance, whereas tumor stage is the size and extent of the local disease as well as whether there is presence of metastasis.
Low grade consists of well-differentiated cells that closely resemble normal tissue with minimal surrounding tissue invasion.
High grade consists of poorly differentiated cells with abnormal tissue architecture or pattern.
Low stage consists of small tumors without evidence of metastasis.
High stage consists of large, infiltrative tumors with metastasis.
Prednisone/Prednisolone
Prednisone/prednisolone can be used during cancer treatment, and as GP veterinarians, this is a drug that we readily have available within the hospital or can easily write a prescription for. It is also very cost-effective for the owner.
Prednisone can be used in dogs, but remember that cats need to be prescribed prednisolone.
The principal indications for prednisone/prednisolone include lymphoma, mast cell tumor, myeloma, and chronic lymphocytic leukemia.
The noncytotoxic indications for prednisone/prednisolone include central nervous system tumors, insulinoma, and hypercalcemia management.
Prednisone/prednisolone adverse effects include, but are not limited to, polyuria, polydipsia, polyphagia, muscle wasting, and behavioural changes. Since prednisone/prednisolone can induce insulin resistance, prednisone/prednisolone-induced diabetes mellitus (PIDM) is also a potential adverse effect, particularly in cats.
Special considerations and monitoring include steroid hepatopathy with chronic use, as well as blood glucose monitoring for potential PIDM.
Do not use steroids (e.g. prednisone or prednisolone) concurrently with non-steroidal anti-inflammatory drugs (NSAIDs) (e.g. carprofen); a washout period between NSAID and steroid use is usually indicated.
“Depending on the patient’s clinical presentation and the owner’s preferred treatment option, if the owner opts to start their pet on prednisone/prednisolone with me in a GP setting, I generally start patients at a dosage of 2 mg/kg PO every 24 hours for 7 days, then taper the dosage based on the cyclophosphamide, doxorubicin, vincristine, and prednisone/prednisolone (CHOP) protocol, as well as based on the patient’s clinical response.” Dr. Danelia de Kock
Chemotherapy Adverse Effects
Approximately 15 to 30% of dogs and approximately 10 to 15% of cats receiving chemotherapy experience adverse effects.
Mild adverse effects (e.g. decreased appetite, vomiting, diarrhea, and/or lethargy) may be manageable with medical intervention.
Severe, life-threatening adverse effects (e.g. febrile neutropenia) occur in ~5 to 7% of dogs and < ~5% of cats, and may require hospitalization.
Neutropenia commonly occurs due to bone marrow suppression, as a result of the myelosuppressant effects of the chemotherapeutic drugs. It can also occur due to bone marrow infiltration by neoplastic cells, as well as neutrophil consumption due to infection.
Neutropenia is the primarily dose-limiting toxicity.
The neutropenic nadir is the lowest neutrophil count, and typically occurs ~7 days (note: this timeframe can vary) post treatment with chemotherapeutic drugs that have a high potency for bone marrow suppression (e.g. doxorubicin, carboplatin, and lomustine).
In order to determine if antibiotics are indicated and/or whether a chemotherapeutic dose reduction is necessary, a complete blood count (CBC) is recommended in order to assess the neutropenic nadir post administration of chemotherapeutic drugs with high potency for bone marrow suppression (e.g. doxorubicin, carboplatin, and lomustine).
At the time of the neutropenic nadir appointment: obtain a CBC, a thorough patient history, vital signs (particularly temperature to assess for the presence of a fever), and perform a complete physical examination.
The primary indicator is the neutrophil count, not the total white blood cell count.
There are slightly differing opinions and study outcome recommendations regarding when to prescribe prophylactic antibiotics.
Some consider prescribing prophylactic antibiotics at the expected neutropenic nadir, however this recommendation is considered controversial by others.
Some consider prophylactic antibiotics when administering chemotherapeutic drugs with high potency for bone marrow suppression (e.g. doxorubicin, carboplatin, and lomustine), particularly in high-risk patients.
Some opt for a more individualized approach and practicing antimicrobial stewardship.
Antibiotics are recommended when the neutrophil count falls below 1,000/µL, although a recent publication suggested when the neutrophil count falls below 750/µL*.
Below is a table (adapted from Table 5.4 in the AAHA Guidelines) summarizing the various neutropenic nadir appointment recommended action plans:
| Neutrophil Count * | Fever and/or Signs of Illness | Recommended Action Plan |
| 1,000 to 2,000/µL | No | – monitor – antibiotics are not necessary – delay chemotherapy if due for treatment |
| <1,000/µL | No | – prescribe oral antibiotics – delay chemotherapy if due for treatment – consider chemotherapeutic dose reduction |
| <1,500/µL | Yes | – hospitalization – provide IV fluid therapy – provide IV antibiotics – delay chemotherapy if due for treatment – chemotherapeutic dose reduction |
If a patient has a fever and is neutropenic (febrile neutropenia), it is considered an oncological emergency and hospitalization is required.
Common antibiotic choices include trimethoprim sulfamethoxazole and amoxicillin- clavulanic acid.
Gastrointestinal (GI) adverse effects are less common in cats compared to dogs, and are usually self-limiting, lasting ~3 days.
Gl adverse effects include decreased appetite, vomiting, diarrhea, and nausea, and can typically be managed with medical intervention.
Use of antiemetic drugs (e.g. maropitant or ondansetron) can be used to help prevent or treat nausea.
Use of appetite stimulants (e.g. mirtazapine or capromorelin) can be used to help stimulate appetite and prevent anorexia.
Note that antiemetic drugs and appetite stimulants can be used concurrently.
Transdermal mirtazapine can be very beneficial in cats.
Use of Crofelemer-CA1, dietary modifications and/or probiotics can be used to help treat diarrhea.
Antibiotics (e.g. tylosin or metronidazole) can also be used to help treat diarrhea, but remember to practice antimicrobial stewardship.
Encourage small, frequent, palatable meals.
If GI adverse effects are more severe, then hospitalization with intravenous (IV) fluids and/or injectable medications as indicated.
Alopecia is a possible adverse effect that is more common in dogs compared to cats; it is a rare side effect in cats. Hair and whiskers generally will regrow following chemotherapy completion, but note that they may regrow with different colors and/or textures.
Other drug specific toxicities include:
- doxorubicin: contraindicated in dogs with cardiac dysfunction, can exacerbate renal disease in cats
- cyclophosphamide: can cause sterile hemorrhagic cystitis in dogs (rarely in cats)
- rabacfosadine: can cause cumulative dermatopathy, rarely can cause life-threatening pulmonary fibrosis
Immunotherapy, Radiation Therapy, and Surgery
Please visit AAHA’s site for the complete guidelines.
https://www.aaha.org/resources/2026-aaha-oncology-guidelines-for-dogs-and-cats/
Supportive and Symptomatic Care
Since these patients are immunocompromised, and are therefore more susceptible to food borne illnesses (e.g. Salmonella and Escherichia coli), do not feed raw diets to these patients.
Nausea and/or vomiting: maropitant, ondansetron, metoclopramide.
Hyporexia: capromorelin oral solution, mirtazapine, prednisone/prednisolone.
Diarrhea: Crofelemer-CA1, probiotics, smectite +/- metronidazole, fiber, tylosin.
Pain: NSAIDs (do not use concurrently with steroids e.g. prednisone/prednisolone), buprenorphine, opioids, gabapentin/pregabalin.
Other GI support: dietary support, therapeutic diet, antacids, motility drugs, vitamin B12, probiotics, feeding tube.
Bone marrow suppression: see neutropenic nadir recommended action plans above.
Monitoring and Follow-up Care
Please visit AAHA’s site for the complete guidelines.
https://www.aaha.org/resources/2026-aaha-oncology-guidelines-for-dogs-and-cats/
References
- Christensen J, Johnson K, Ettinger S, Garrett L, Gordon I, Ireifej S, Love A, Wisecup M. AAHA Oncology Guidelines for Dogs and Cats. Journal of the American Animal Hospital Association. 2026 Jan 1;62(1):1-37. doi: 10.5326/JAAHA-MS-7549
- University of Wisconsin Veterinary Care, Chemotherapy Protocols. Accessed January 4, 2026.
https://uwveterinarycare.wisc.edu/small-animal/cats-and-dogs/oncology/chemotherapy-protocols
