Treatment of lymphoma in dogs and cats is often rewarding. However, response to conventional treatment options varies between
the two species, as well as among individuals of the same species. To better predict which patients may benefit the most from
conventional treatment options, various prognostic factors for both canine and feline lymphoma have been identified. In dogs,
clinical substage, histologic grade, immunophenotype, anatomical location, thymidine kinase activity, and histomorphologic
subtype appear to influence overall length of survival. In cats, few reliable prognostic factors are recognized, with only
initial response to therapy being consistently identified to be predictive of survival time.
Despite the advances made in veterinary oncology over the last decade, systemic chemotherapy remains the cornerstone for treating
dogs and cats with lymphoma. In some situations, radiation therapy may be useful when implemented in either an adjuvant or
palliative setting. Furthermore, radiation therapy used as a primary treatment modality for solitary extranodal lymphoma,
including nasal lymphoma in cats, can provide rewarding and durable clinical responses. Because lymphoma commonly develops
in companion animals, veterinary practitioners should be knowledgeable about the treatment options available. Therefore, the
focus of this review article is to highlight some of the more conventional therapeutic modalities useful for treating high-grade
lymphoma in dogs and cats.
TREATING HIGH-GRADE, MULTICENTRIC CANINE LYMPHOMA
Because of the rapid proliferative capacity of malignant lymphocytes, dogs not receiving appropriate treatment for lymphoma
often die of terminal tumor burden within several weeks. Thus, it is important that dogs with lymphoma receive prompt and
appropriate medical therapy. Systemic chemotherapy remains the treatment of choice for canine high-grade, multicentric lymphoma.
Older, yet effective, chemotherapeutic protocols were composed of two distinct treatment phases referred to as induction and maintenance. During the induction phase, dose-intense cytotoxic chemotherapy is administered at frequent intervals over a relatively
short time frame, killing most viable cancer cells. After induction chemotherapy, lower dose intensity, long-term chemotherapy
called maintenance therapy is instituted to prevent or delay microscopic tumor regrowth. Although maintenance therapy is well-tolerated, it necessitates
long-term follow-up treatments, which may be inconvenient and time-consuming for pet owners. Furthermore, it is possible that
administering chemotherapy over a prolonged period may ultimately result in cumulative and irreversible bone marrow toxicity.
In the past few years, new chemotherapeutic protocols for treating canine high-grade, multicentric lymphoma have excluded
the use of maintenance chemotherapy, yet these shorter treatment protocols retain high therapeutic efficacy. Eliminating the
requirement for long-term chemotherapy may help pet owners be more receptive to treating their ailing pets with systemic chemotherapy.
Although shorter chemotherapeutic protocols provide durable first remission times, you should inform pet owners that most
dogs with lymphoma are not cured of their disease. So pet owners should routinely monitor their pets' behavior and activity
levels, and you should encourage them to have their dogs evaluated by a veterinarian at least three or four times a year.
Through routine health screening, lymphoma relapses may be detected earlier, allowing for the institution of additional chemotherapy
before an advanced tumor burden develops.
Most dogs (about 75% to 90%) with high-grade, multicentric lymphoma will achieve complete remission with doxorubicin-based
multidrug protocols (Table 1). With numerous chemotherapeutic protocols available, you may find it difficult to decide which therapeutic regimen is best.
As a general guideline, you should adopt a protocol that is reliably efficacious but is tolerable even in a compromised patient.
Although it is often easier to treat dogs with single-agent oral drug formulations, such as prednisone or lomustine, this
practice should be limited to special circumstances in which multiagent protocols cannot be instituted. Multiagent protocols
provide superior response rates, disease-free intervals, and survival times when compared with simpler single-agent protocols.
Make sure to educate pet owners about the advantages and disadvantages of different treatment regimens.
TABLE 1. Summary of Selected Canine Lymphoma Protocols
Even with the use of highly effective, multiagent chemotherapeutic protocols, most dogs ultimately relapse. Dogs experiencing
lymphoma relapse may achieve long-lasting and clinically relevant second and third remissions with additional treatment. The
best combination of chemotherapeutic agents necessary to achieve meaningful second and third remissions depends on several
factors including prior drug usage and the amount of time since the patient last received chemotherapy. In many situations,
dogs may present with or develop drug-resistant lymphomas that are refractory to traditional chemotherapeutic agents. For
these patients, using less conventional antineoplastic drugs or a combination of agents, called rescue therapy, may provide some clinical benefit (Table 2).
TABLE 2. Summary of Selected Rescue Protocols for Canine Lymphoma