Key Points

• Lymphoma is a common canine cancer and is a systemic disease that requires chemotherapy in almost all cases.
• The majority of dogs achieve a complete remission with chemotherapy (approximately 80%). Higher remission rates are typical with CHOP multi-agent chemotherapy protocols.
• Early accurate diagnostics and careful staging are keys to proper clinical decision-making.
• To determine the best protocol for a patient and owners, it is important to understand the efficacy of the various protocols, the potential toxicities, and prognostic factors.
• Dogs treated with chemotherapy live significantly longer than untreated dogs, and chemotherapy is generally well-tolerated in most dogs. Only a minority develops significant toxicity.
• The diagnostic and treatment choices can be confusing and overwhelming. In this talk, we will take “My 3 Ps” approach - prognostic, practical and pertinent.

Biology of Lymphoma

Lymphoma is a collection of cancers arising from the malignant transformation of lymphocytes. Even though lymphoma is clinically a diverse group of neoplasms, the common origin is the lymphoreticular cells. Lymphoma is one of the most common canine cancers, accounting for 7–24% of all canine tumors and 85% of hematopoietic tumors. Dogs of any age, gender, and breed can be affected with lymphoma. Affected dogs are typically middle aged to older dog.

Anatomic Classification

Multicentric (PLN) is the most common form, accounting for 80% of lymphomas. Most dogs are typically asymptomatic, and 20–40% are clinical (substage b) with anorexia, lethargy, fever, V/D , weight loss, melena.

Clinical Appearance

Historic findings: The most common complaint is generalized lymphadenomegaly. Owners commonly report that lymph node size is rapidly increasing - over days to 1 to 3 weeks. In the early stages, dogs appear healthy and are not showing clinical signs. When present, clinical signs tend to be nonspecific and include vomiting, diarrhea, melena, anorexia, fever, and weight loss (substage b).

Common examination findings: Lymphoma can be indolent or aggressive, solitary or multicentric, or node-based or associated with any organ. Non-painful generalized lymphadenomegaly is most common physical exam finding. Multicentric lymphoma involving the peripheral lymph nodes is most common, accounting for 80% of patients.

Most dogs are “healthy” substage a. T-cell dogs tend to be sick (b). In dogs, multicentric LSA is generally the NHL (non-Hodgkin’s LSA) form. Hepatosplenomegaly is common. Diffuse pulmonary infiltration has been reported in 27–34% based on CXR but on BAL, lung involvement may be higher. The lack of generalized lymphadenomegaly does not eliminate the possibility of lymphoma, as some dogs will have internal involvement only (i.e., hepatosplenic form, GI). Another scenario that can lead to confusion is hypercalcemia, often without peripheral lymphadenomegaly so lymphoma is not suspected.

Preliminary Diagnosis

Cytology - Confirmation of lymphoma starts with fine needle aspirate of an affected lymph node. Cytology is minimally invasive, less expensive than biopsy, and typically provides rapid results, in 1 to 2 days. Cytology reveals monomorphic abnormal lymphocyte populations. Cytology does not provide complete classification, grading, or phenotype. Avoid reactive LN, such as the mandibular LN.

Diagnostic Work Up

The minimum tests required for treatment are cytological confirmation (lymph node or affected organ), CBC, chemistry panel and urinalysis. The next diagnostic I encourage owners to submit is phenotyping to determine B vs. T-cell subtype. Phenotyping is typically determined with immunocytochemistry from aspirates, immunohistochemistry from biopsy, or flow cytometry or PARR from aspirates. If there is a peripheral lymphocytosis on CBC (stage V), flow cytometry can be submitted on a whole blood sample to determine phenotype. Phenotype is the best independent prognostic factor; prognosis is worse with T-cell than B-cell.

Lymph node biopsy is ideally performed for histologic grading but is often only collected when cytology was inconclusive. Baseline chest radiographs and abdominal ultrasound are recommended for staging purposes to determine extent of disease. While stage is prognostic, I also find it valuable to have these baseline imaging tests to be able to compare treatment response or progression. Bone marrow cytology is also considered part of the basic staging but it is often not done in the majority of cases, factoring in the additional cost and sedation for most cases. Bone marrow cytology is of less clinical utility in most cases. However, if there are cytopenias and/or a lymphocytosis, a bone marrow should be considered to identify bone marrow involvement.

To stage or not to stage? Complete lymphoma staging includes lymph node cytological confirmation, CBC, chemistry panel, urinalysis, lymph node histology, urinalysis, thoracic radiographs, abdominal ultrasound, bone marrow cytology and phenotyping. These tests are useful and informative, as they provide prognostic factors and a baseline for a patient‘s response. These tests can also help determine if there is large tumor burden and risk for acute tumor lysis syndrome with induction chemotherapy. Still, we must consider the owner’s financial issues. While it is ideal to perform all the tests, we can also consider each test on a case by case basis and help the owner make an educated decision. We can treat without but review pros and cons with the owner and let owner make educated decision and maybe choose more important tests for that dog.

Histology

NIH WF & Kiel System most useful, and both describe architecture and cell morphology, including mitotic index, cell size, and cell shape. Why do I care about histology? It’s prognostic. Positive: Low-grade LSA, including mantle-zone, follicular, T-cell. But low-grade LSA may only partially respond to chemotherapy and is often incurable. Negative: Intermediate and high-grade LSA BUT have a high mitotic rate & are more likely to completely respond to chemotherapy.

Phenotype: 60–80% of LSA are B-cell, and this is an important positive predictor, associated with higher rate of CR, longer remission, increased ST, and most high grade are B-cell. Breed prevalence with B-cell includes Cockers and Dobies. Goldens have equal B- and T-cell. 10–38% of LSA are T-cell, and this is an important negative predictor, associated with lower rate of CR, shorter remission, shorter ST, and tends to be associated with hypercalcemia. Boxers are over-represented.

Flow cytometry (FCM) involves staining live cells with labeled antibodies that bind to cell surface proteins. These live cells are suspended in liquid (saline, tissue culture media). Different types of lymphocytes express different proteins. Flow cytometer tells us how many cells of each type are present and can determine the lineage of the cells present. Flow could not identify LSA in 30% of newly diagnosed cases.

PARR: PCR antigen receptor rearrangement is a polymerase chain reaction (PCR) assay that amplifies DNA with PCR primers in the dog or cat. It tells us if the majority of cells in the sample are clonal: same original clone - most consistent with neoplasia, or from multiple clones/polyclonal - lymphoid proliferation - most consistent with a reactive process. It is useful to determine: whether lymphoid neoplasia, phenotype (B vs. T), and to monitor for MRD in treated patients, it must be interpreted with history, clinical signs, cytology, flow cytometry, IHC.

For sensitivity & specificity, both are ∼90% in dogs, and it is more sensitive for circulating cells>blood, bone marrow. In cats, it is better for T-cell (89%, 80%) vs. B-cells (60%, 70%). FCM and PARR are not useful for neutrophilia to r/o chronic myelogenous leukemia, when hypercalcemia is only sign, not helpful on LN, fluid, etc., or as a screening test for healthy dogs and cats without clinical signs.

Prognostic factors: There are many prognostic factors, but the more significant predictors include:

• Phenotype: B-cell is better than T-cell. 60–80% are B-cell and this is associated with higher rate of CR, longer remission rates, and increased ST. Most high-grade LSA are B-cell.
• Histologic grade: High grade has better CR rate than low grade, but low grade often has comparable survival times with less intensive chemotherapy protocols.
• Administration of prednisone prior to chemotherapy is a negative predictor
• Substage: Clinically healthy dogs tend to do better than sick dogs
• Higher stage (stage IV and V) tend to do worse than lower stage (I to III)
• Hypercalcemia: Negative predict or due to association with T-cell phenotype
• Mediastinal mass: Negative predictor due to association with T-cell phenotype

Remember, prognostic factors cannot predict an individual’s response, and lymphoma is typically treatable and rewarding to treat for the patient, owner and the veterinarian.

Treatment Modalities

Treatment pearls - Chemotherapy is the mainstay of therapy to promote a rapid, durable and complete remission (CR), while maintaining a good to excellent quality of life even during chemotherapy. Complete remission is complete disappearance of all detectable lymphoma and resolution of clinical signs. Lymphoma is typically rewarding to treat with high response rates, and most dogs tolerate chemotherapy quite well.

Treatment: Chemotherapy - The goal of therapy is to achieve a complete remission and a good to excellent quality of life. Dogs that respond and achieve CR are usually free of clinical signs of lymphoma and live longer and live well. Only a minority develops significant toxicity or do not respond to therapy. Most patients are treated on an outpatient basis. Newly diagnosed lymphoma patients that are sick (substage b), dehydrated, and have a large tumor burden (advanced stages) are at increased risk for acute tumor lysis syndrome with induction chemotherapy. In such cases, the dogs should be admitted for IV fluid therapy, supportive care, and intensive monitoring prior to chemo and for 24 to 72 hours after.

Combination chemotherapy provides improved remission rates and duration in comparison to single-agent protocols. Multi-agent CHOP protocols are the most successful, with complete remission rates of >80% and remission durations of typically 6–11 months. Median survival times (MST) are 1 year when followed by rescue protocol, and 25% of dogs are long-term survivors >2 years. There are numerous CHOP protocols that vary in drug dosages, scheduling, and dose intensity. The UW-Madison protocol is often recommended for owners choosing a combination protocol for its high complete remission rates, higher remission duration, and lower morbidity and mortality rates. Commonly used UW protocols are the 25- and 19-week protocols

Multi-agent CHOP protocols typically combine vincristine, cyclophosphamide, doxorubicin and prednisone. Recent studies suggest the inclusion of l-asparaginase at induction does not significantly impact remission duration or survival times and can be omitted and saved for the rescue protocol.

Additionally, recent studies suggest there is no survival benefit of maintenance phase. Most current protocols are discontinuous without a chronic maintenance phase and provide comparable remission durations. It is thought the period without chemotherapy may lead to greater responsiveness at relapse by lack of selection of resistant cells.

For some clients, alternative protocols are elected over the multi-agent CHOP protocol due to budget, toxicity profile on par with clients’ willingness to assume risks of chemo, and schedule and time commitment. In some cases, it is to avoid drugs that target a patient’s weakness or concurrent illness. For example, lomustine is avoided with liver dysfunction and doxorubicin can cause cardiotoxicity so should be used cautiously in dogs with some pre-existing cardiac disease.

Alternative chemotherapy protocols include COP (vincristine, cyclophosphamide, and prednisone), single agent doxorubicin for B-cell lymphoma, and single agent lomustine for T-cell lymphoma. These protocols generally have lower response rates ranging from 50–80% and shorter remission durations of 6 to 7 months.

New therapies for lymphoma include monoclonal antibodies, a lymphoma vaccine, and Tanovea. It is hopeful these new therapies will increase survival times. Canine remission times on CHOP have plateaued at about 9 months.

In humans, monoclonal antibodies are standard of care. Before rituximab, results of CHOP-based chemotherapy plateaued in human medicine. Since its launch in 1997 it is the standard of care for non-Hodgkin’s lymphoma in humans and the addition of rituximab to standard CHOP has increased overall survival by 55%. More recently, monoclonal antibodies have been introduced as targeted therapy for both T- and B-cell canine lymphoma, but efficacy and administration schedule are still being worked out.

If chemotherapy is declined, another option is single agent steroids. Typical response rates are 50% with duration of 2 to 3 months. Prednisone should not be started prior to chemotherapy since it may decrease response rate to chemotherapy started after the steroids. Pre-chemotherapy steroid use is associated with shorter remission and survival times due to induction of multidrug resistance. If staging tests are done after prednisone is started, higher stage patients may appear to be lower stage (down-stage). Without chemotherapy the prognosis for lymphoma is poor, with MST of 1 month.

Relapse: The majority of lymphoma patients relapse as there is the emergence of tumor clones that are more resistant to chemotherapy, or survival-of-the-fittest lymphoma cells. These MDR (multi-drug-resistance) clones are more likely to express MDR-1 gene that encodes for protein transmembrane pump associated with multidrug resistance. Other reasons for relapse include inadequate chemotherapy dosing, inadequate chemotherapy frequency, or failure to achieve high chemotherapy concentrations at certain sites, such as the CNS.

When a patient relapses, I recommend reintroducing the initial protocol if it was successful, meaning the expected remission duration was achieved. For example, if a dog relapses one month after completing a CHOP protocol I will not recommend restarting front-line chemotherapy. However, if the dog was off chemotherapy for 4–5 months with a 1st remission of 9–10 months, I will recommend restarting the induction protocol as re-induction rates of 90% can be expected. Remember there is a cumulative dose of doxorubicin, so doxorubicin is typically replaced after a total of 6 doses. When a dog no longer responds front-line chemotherapy, rescue protocols are recommended. There is decreased likelihood of response (30–50%) and shorter remission durations, typically half the length of the initial remission. Still some patients experience long-term re-inductions. Some commonly used protocols include MOPP, doxorubicin or mitoxantrone with DTIC, lomustine/l-asparaginase/prednisone, and single agent lomustine.

Overall: Lymphoma is one of the most successfully treated cancers in dogs, and many patients with lymphoma outlive animals with other noncancerous diseases such as kidney, heart, and liver disease. Dogs treated with chemotherapy live significantly longer than untreated dogs, and chemotherapy is generally well tolerated in most dogs.

References

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2.  Williams LE. Lymphoma, Dog (Multicentric) in Clinical Veterinary Advisor Dogs and Cats. 2nd ed. St. Louis, MO: Elsevier Mosby; 2011:675–678.

3.  Bryan JN. Lymphoma. In: Cancer Management in Small Animal Practice. Saunders; 2010:343–350.

4.  Alexandrakis I, et al. Vet Comp Oncol. 2014.

5.  Vaughan MA, et al. J Vet Intern Med. 2007;1(6):1332–1339.