Canine Lymphoma Isn't One Disease: What the WHO Classification Actually Changes

Vetopathy · Pathology Deep Dive · Canine

"Lymphoma" tends to function as a single-word diagnosis in the exam room — and to an owner, that word sounds like one disease with one prognosis. It isn't. Under the WHO classification system for canine lymphoma, more than twenty distinct subtypes are recognized, and they range from disease that responds predictably to standard chemotherapy, to disease that progresses slowly enough to be watched rather than treated aggressively, to disease that looks deceptively low-grade under the microscope but behaves aggressively regardless. The subtype — not the word "lymphoma" — is what should be driving the prognosis conversation.

What "lymphoma" actually covers

The WHO classification used in veterinary pathology is adapted from the human WHO/REAL system and incorporates tissue architecture (nodal versus extranodal, diffuse versus follicular), cell size and morphology, and immunophenotype — B-cell versus T-cell lineage, and often a specific marker profile beyond that. More than twenty subtypes are formally recognized in dogs, though in clinical practice the large majority of cases cluster into a handful of well-characterized categories.

Morphology alone, especially on a limited sample such as a fine-needle aspirate or a small core biopsy, is frequently not sufficient to assign a subtype. Full architectural evaluation on an excisional node, combined with immunophenotyping, is often what's actually required to move from "lymphoma" to a specific, prognostically meaningful diagnosis.

The first split: B-cell versus T-cell

The majority of canine lymphomas are of B-cell origin, with a smaller but clinically significant proportion of T-cell origin. Immunophenotyping is what establishes lineage — IHC markers such as CD79a, CD20, and PAX5 identify B-cell lymphomas, while CD3 identifies T-cell lymphomas. Flow cytometry is a common alternative when fresh aspirate material is available. PARR (PCR for antigen receptor rearrangement) can serve as a molecular adjunct when morphology and immunophenotyping remain equivocal, or when the available sample is limited to cytology — though, as with any clonality assay, it supports the diagnosis rather than making it outright.

As a broad generalization, T-cell lymphomas as a group tend to carry a worse overall prognosis than B-cell lymphomas as a group. That generalization breaks down badly once you get to the subtype level, which is exactly why stopping the workup at "B-cell" or "T-cell" still leaves the most useful clinical information on the table.

Diffuse large B-cell lymphoma: the most common subtype

Diffuse large B-cell lymphoma (DLBCL) is the most common subtype diagnosed in dogs. It is CD20-positive, CD79a-positive, and PAX5-positive, and while it is an aggressive disease, it is also the subtype most reliably responsive to multi-agent CHOP-based chemotherapy protocols, with treated dogs commonly achieving remission measured in months to roughly a year. Genomic subtyping of canine DLBCL — identifying molecular subtypes analogous to those described in human DLBCL — is an active area of research, but it is not yet part of routine clinical classification.

Peripheral T-cell lymphoma, not otherwise specified

Peripheral T-cell lymphoma, NOS, is the diagnostic category for T-cell lymphomas that don't fit a more specifically defined T-cell subtype. It is CD3-positive, generally aggressive, and has historically been associated with a poorer response and shorter remission duration than DLBCL — a meaningful distinction to make explicit in a report rather than leaving the reader to assume "T-cell lymphoma" behaves like "B-cell lymphoma," just with a different marker.

T-zone lymphoma: when small cell size actually means indolent disease

T-zone lymphoma is a distinct, well-characterized entity defined by expansion of the paracortical T-zone of the lymph node, typically with compressed but still-identifiable residual follicles. The neoplastic cells are small to intermediate in size with clear to pale cytoplasm. The immunophenotypic signature that separates it from other T-cell lymphomas is loss of CD45 expression — these CD45-negative T cells are distinctive enough to be effectively diagnostic for the entity.

Clinically, T-zone lymphoma behaves indolently. Many affected dogs survive for a long time without aggressive chemotherapy — sometimes managed with nothing more than oral chlorambucil and prednisone, sometimes simply monitored. This matters because, on morphology alone, a paracortical lymphocytic infiltrate can look concerning to a non-specialist reader, and an unqualified "lymphoma" report on a case that is actually T-zone lymphoma can trigger a treatment conversation — and an emotional one — far more urgent than the disease actually warrants.

The exception that proves the rule: diffuse small B-cell lymphoma

It would be convenient if small cell size reliably predicted indolent behavior the way it does with T-zone lymphoma. It doesn't. Diffuse small B-cell lymphoma in dogs is recognized as a high-grade, aggressive malignancy despite its small-cell morphology — a notable divergence from several small B-cell lymphomas in human medicine, which tend to behave indolently. This is the clearest illustration of why cell size alone is an unreliable prognostic shorthand in canine lymphoma: the same "small, mature-looking lymphocyte" morphology can represent an indolent T-cell disease or an aggressive B-cell one, and only immunophenotyping resolves which.

Marginal zone and follicular lymphoma: the other indolent forms

Marginal zone lymphoma is a relatively indolent B-cell subtype, seen nodally or in the spleen, and considerably less common than DLBCL. Follicular lymphoma, which retains at least partial follicular architecture rather than effacing it diffusely, is also generally indolent and is uncommon in dogs relative to its frequency in human lymphoma. Both are worth naming specifically in a report rather than folding into a generic "B-cell lymphoma" diagnosis, since both carry a materially better prognosis than DLBCL.

Lymphoblastic lymphoma

Lymphoblastic lymphoma is an aggressive subtype, more often seen in younger dogs, and can present with mediastinal involvement or overlap clinically with acute lymphoblastic leukemia when bone marrow is involved. TdT (terminal deoxynucleotidyl transferase) immunohistochemistry is the key marker that distinguishes it from other small-to-intermediate cell lymphomas, including T-zone lymphoma — a distinction that matters given how differently the two are managed.

Why the subtype belongs in the prognosis conversation

"Lymphoma, high-grade" and "diffuse large B-cell lymphoma, CD20-positive, CD79a-positive" describe very different amounts of clinical information, even when they're both technically accurate for the same case. The classification work — architecture, cell morphology, and immunophenotyping, with PARR as an adjunct when needed — is what turns "lymphoma" from a label into an actionable prognosis and a treatment plan matched to how the disease actually behaves.

Quick reference: selected canine lymphoma subtypes

Subtype	Typical behavior	Key immunophenotype	Notes
Diffuse large B-cell lymphoma	Aggressive, chemo-responsive	CD20+ / CD79a+ / PAX5+	Most common subtype in dogs
Peripheral T-cell lymphoma, NOS	Aggressive	CD3+	Generally poorer prognosis than DLBCL
T-zone lymphoma	Indolent	CD3+, CD45-negative	Distinct entity; often does not require aggressive chemotherapy
Diffuse small B-cell lymphoma	Aggressive despite small cell size	CD20+ / CD79a+	Small cell size does not equal indolent behavior in this subtype
Marginal zone lymphoma	Indolent	CD20+ / CD79a+	Nodal or splenic; less common than DLBCL
Follicular lymphoma	Often indolent	CD20+ / CD79a+, retains follicular architecture	Uncommon in dogs
Lymphoblastic lymphoma	Aggressive	TdT+	Often younger dogs; may show marrow involvement

Eric Snook, DVM, PhD, DACVP — Vetopathy. Questions about immunophenotyping options, or which subtype a specific case might represent? Happy to discuss before or after submission.