Canine lymphoma: The naive patient (Proceedings)
Incidence and signalment
Canine lymphoma (LSA) makes up approximately 18% of all malignancies in the dog, and 80% of all hematopoietic tumors in dogs are LSA. Middle-aged dogs are most commonly affected; but out of cancers affecting young dogs (as young as 6 months), LSA is common. No sex predisposition is consistently reported. Boxers, golden retrievers, basset hounds, St. Bernards, Scottish terriers, and mastiffs are breeds that are all overrepresented.
EtiologyNo definitive etiology for canine lymphoma has been identified. Risk factors investigated include viral, genetic (shown only in a limited lineage in mastiffs, otterhounds, rottweilers, Scotties, and golden retrievers), environmental (herbicides such as 2,4-dichlorophenoxyacetic acid (2,4 D) - this study was found to be erroneous), and inflammatory bowel disease. No strong associations have been found.
In dogs, the multicentric form of lymphoma (multiple peripheral lymph nodes (LNs)) is the most common. Typically, dogs are asymptomatic and present because their owners have noticed enlarged LNs ("lumps under chin"), or the LNs are noted to be enlarged on a routine physical exam. Less commonly, dogs with multicentric LSA may present clinically ill (decreased appetite, lethargy). For forms of LSA other than multicentric, the signs are consistent with the organs affected: vomiting/weight loss for GI, altered mentation/seizures for CNS, etc.
LSA is a malignancy that usually arises in lymphoid tissues (LNs, liver, spleen, bone marrow); however, it can arise in or invade any tissue. As it is almost always a systemic disease, chemotherapy is the mainstay of therapy. Different anatomic sites (or FORMS) of lymphoma respond differently to therapy. Additionally, for multicentric LSA in dogs, many prognostic factors in regards to potential response to treatment are known. Negative factors include substage b (clinically ill), hypercalcemia, T cell immunophenotype, cranial mediastinal mass, leukemia, and >50% bone marrow involvement.
The diagnosis of LSA is easy to make via fine needle aspirate (FNA). LSA is almost always a systemic disease, thus staging to determine extent of disease is important for prognosis and monitoring response to therapy. Thorough staging of animals with LSA includes:
Fine needle aspirate (FNA)
Simple and almost always diagnostic. A needle aspirate of the typical untreated canine lymphoma consists of a monomorphic population of immature (large) lymphocytes (a homogenous population of round, mononuclear cells). These cells have a large size (> neutrophil), high nuclear to cytoplasmic ratio, open and coarse ('lacy') nuclear chromatin pattern with prominent nucleoli, and deep blue cytoplasm. A normal LN contains 75 to 95% small, mature lymphocytes, a reactive LN can have up to 50% lymphoblasts (and will also have plasma cells), and > 50% lymphoblasts is diagnostic for LSA.
Complete blood count
May be normal, or may reveal cytopenias or leukemia. Cytopenias may be due to the paraneoplastic syndromes of immune-mediated hemolytic anemia or thrombocytopenia or may be due to bone marrow infiltration by the tumor cells (see below).
Serum chemistry profile
May be normal, or may reveal abnormalities related to organ infiltration (e.g. elevated liver enzymes) or production of substances by the tumor cells. Hypoglycemia is rarely identified in dogs with LSA. Hypercalcemia is a paraneoplastic syndrome that occurs secondary to malignant cell production of one or more factors, one of which can be parathyroid hormone-related peptide (PTHrP). Hyperproteinemia is a rare finding and is secondary to antibody production by tumor cells.
Important to assess prior to starting chemotherapy. Subclinical urinary tract infections can become a source for bacterial sepsis in an animal with chemotherapy-induced myelosuppression. Also, animals with LSA have decreased immune function due to their disease and can be predisposed to infections.