Section 9: Post-Treatment Monitoring and Follow-Up Care

Assessing Response

What constitutes a patient’s response to therapy can be difficult to generalize because each cancer process is unique. Response to therapy must be viewed in the context of the original therapy, whether it be cure or palliation. Assessing response in oncology patients is categorized as: ¹

• Complete response (CR): regression of all evidence of disease with normal-sized lymph nodes.
• Partial response (PR): greater than a 30% reduction in the sum of longest diameters of target lesions (up to five specific lesions measured at the start of treatment and tracked throughout therapy), with no new lesions appearing.
• Progressive disease (PD): increase of at least 20% in the sum of the longest diameters of target lesions or the appearance of new lesions.
• Stable disease (SD): neither sufficient shrinkage to qualify as PR nor sufficient increase to qualify as PD.

For patients with PD, restaging is appropriate to look for systemic changes/metastasis before initiating another treatment modality (see Section 5, Therapeutic Interventions).

New Clinical Signs in Cancer Patients

Because many patients receiving treatment are geriatric, it is common for them to develop new conditions or symptoms. Whenever a patient presents after cancer treatment with a new clinical sign, consider the possibility that the new sign can be caused by one of the following three things.

• Tumor progression
• Treatment toxicity
• An unrelated new problem

If the new clinical sign is not one that would be expected as a result of tumor progression or treatment toxicity, it is more likely that the pet has a new and potentially undiagnosed problem not related to the tumor or treatment.

Post-RT Monitoring

Many patients, especially those treated with definitive intent, have a fair-to-excellent prognosis following initial RT. Acute effects of RT often resolve with minimal intervention. Late side effects can develop more than a year after therapy and are more common in patients receiving higher doses but typically occur in less than 5% of patients. Although rare, they can be permanent. Supportive care is often sufficient and includes pain management (opioids or NSAIDs), pentoxifylline, and vitamin E. Side effects are rarely life threatening, so euthanasia is seldom the best option (see Section 5, Therapeutic Modalities: Radiation Therapy for a description of acute and late side effects).

Patients must have periodic post-therapy examinations to assess for recurrence, metastasis, new tumor development, or complications of initial therapy.

Upon completion of initial therapy, patients are often restaged to determine the extent of disease. Patients should be monitored for resolution, improvement, progression, or recurrence of their initial presenting signs and for new signs potentially associated with recurrence, metastasis, new tumor development, or complications of initial therapy (see Box). The frequency of such evaluations should be based on the patient’s condition, diagnosis, time since therapy, and client preferences and limitations. It is common for pets to be evaluated every 2–4 weeks for 2–3 months after they complete radiation therapy (usually by the treating oncologist), then every 2–3 months for the first year after treatment, followed by every 3 6 months during the second year. In many cases, this long-term follow-up may include evaluations by the primary care veterinarian.

Continuous Therapy

Continuous therapy is used either after an intense induction period or as the primary intervention. Additional chemotherapy, metronomic chemotherapy, or TKIs and cyclooxygenase-2 inhibitors have been used as ongoing therapy in such cases. During this time, continued diagnostic monitoring is tailored to the patient’s therapeutic plan.

Monitoring for Recurrent or Metastatic Disease

PD can occur at any time during the therapeutic or remission stages. All patients must have a follow-up after receiving any cancer diagnosis regardless of their status. This allows the practitioner to monitor for recurrent or progressive disease. Follow-up is tailored to the patient and their tumor type. Keep in mind that abnormalities found during this period are not always related to the primary condition. Side effects secondary to treatment may arise or new problems often occur. It is important to be vigilant and distinguish the cause of clinical signs because it is common for clients to attribute changes in their pet to their cancer diagnosis, even when other etiologies are more likely.

As a general guideline, reassessments are recommended every 4–6 months for patients with low-grade tumors and every 1–3 months for those with high-grade tumors. Although not required at all visits, reassessment may include a physical examination, minimum database (CBC, serum chemistry, urinalysis, blood pressure), and radiographs. If disease recurs, it may indicate a guarded prognosis and return to normalcy may not be possible, so assessing these patients’ quality of life is critical. Goals of therapy in such cases are often dynamic and affected by the extent of disease, expectations for the patient’s quality of life, and client wishes.

Continuing Care

Veterinarians and clients often question whether cancer patients should continue to receive vaccines. The published veterinary literature does not support a consensus on this topic and opinions vary with the cancer type, so consult with an oncologist for their recommendations regarding specific patients.

Supportive care for cancer patients does not stop after the intense induction period of therapy. The post diagnosis and post therapy time frames are crucial for communication between the client and the veterinary team. It is important to reinforce the client-clinic relationship, so patients receive ideal care. Follow-up care includes open communication, long-term pain management, recognizing and treating secondary conditions, and improving quality of life for as long as possible (See also Section 7, Supportive and Symptomatic Care).

The 2026 AAHA Oncology Guidelines for Dogs and Cats are generously supported by CareCredit, Hill’s Pet Nutrition, Merck Animal Health, and Zoetis.