Canine lymphoma has long been regarded and addressed as a single disease entity, while human lymphomas are classified in numerous
classes and subclasses, each with a distinct presentation, prognosis, and accepted therapy.Indolent, or low-grade, lymphomas
constitute a unique subgroup of lymphomas that often may behave differently than the archetypical high-grade canine multicentric
lymphomas. The veterinary literature is scarce regarding the many variants of canine lymphomas, their clinical presentation,
prognosis, and therapy. Recent findings suggest that canine indolent lymphomas are not as uncommon as they once were thought
to be, and that their optimal therapeutic approach may need to differ from classical multicentric high-grade lymphomas.
Epitheliotropic T-cell lymphoma (CTCL), including mycosis fungoides, Sézary syndrome, and pagetoid reticulosis, is the most
common form of canine cutaneous lymphoma, and also appears to necessitate therapy that is different than standard therapy
for multicentric high-grade lymphomas.
VARIANTS OF INDOLENT LYMPHOMA
Similar to high-grade lymphomas, indolent lymphomas can be classified based on the cell type, via immunophenotyping, and then
further categorized in specific subtypes by an experienced anatomic pathologist. Using the human WHO classification, the most
common variants of canine indolent lymphomas include marginal zone lymphoma (MZL), follicular lymphoma (FL), and mantle cell
lymphoma (MCL), all from B-cell lineage, and T-zone lymphoma (TZL), a lymphoma of T-cell type.
Also often considered a low-grade malignancy, canine CTCL is most commonly a disease of CD8+ cytotoxic T cells.
CLINICAL PRESENTATION
In general, dogs with indolent lymphomas will be middle-age to older (median 9 years), and have no clinical signs of illness
associated with their lymphoma (substage a) at initial diagnosis. Currently, there is no known breed or sex predilection,
although retriever and terrier breeds may be overrepresented. It may be more common with certain types of indolent lymphomas
(FL, MZL) to have only one or two lymph nodes enlarged at diagnosis, to involve only the spleen (MZL, MCL), or to have a history
of long-term (months to years) slowly developing multicentric lymph node enlargement (TZL). This latter presentation is in
marked contrast to the traditional high-grade lymphomas that present with acute lymphadenopathy, typically in a multicentric
fashion, and rarely live longer than 1-2 months when left untreated. While hypercalcemia is a common paraneoplastic syndrome
observed with approximately 40% of classic intermediate or high-grade T-cell lymphomas, we have not observed hypercalcemia
in more than 40 cases of TZL.
Dogs with CTCL have a mean age of 9 to 12 years, and can present with lesions at an early stage such as erythroderma, patch,
or plaque stage, or with the more advanced tumor (nodular) or disseminated stage, or even the leukemic stage also known as
Sézary syndrome. Cutaneous lesions often have been present for months prior to the definitive diagnosis of CTCL, and oral
or mucocutaneous lesions are other fairly common sites of involvement.
