What Is It and How Far Has It Spread? Grading and Staging Explained
Grading and staging are two of the most critical concepts in veterinary oncology, yet they are often confused by both pet parents and veterinary team members alike. While they may sound similar, grading and staging describe different aspects of a cancer diagnosis. Understanding the distinction between the two terms and their meaning is essential for determining prognosis, guiding treatment decisions, and communicating clearly with clients.
Below, we'll clarify the difference between grading and staging, explaining why both are essential in the treatment of pets with cancer and the management of pet owner-veterinarian relationships.
Evaluating Pets With Cancer
When evaluating a dog with a lump or bump that may be cancer, two questions form the foundation of the diagnostic approach:
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What type of tumor are we dealing with? And how aggressive is it?
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Has the tumor spread to any other parts of the body?
When communicating with a client about cancer, I often simplify the approach with this framework: What is it, and how far has it spread?
Grading: How Aggressive Is It?
After diagnosis, grading answers: How aggressive is this cancer?
Often, the first step when a dog has a lump or bump is performing a fine needle aspirate (FNA) to collect cells for microscopic evaluation. Results may quickly rule out cancer (e.g., neutrophils filled with bacteria suggesting an abscess or lipid-filled cells consistent with a benign lipoma) or identify cells that raise suspicion for cancer (e.g., individual basophilic granule-filled cells suggesting a mast cell tumor).
FNA provides direction or an indication of cancer, triggering the next steps in a workup. For example, veterinarians will likely order a biopsy to obtain a tissue sample for histopathology, which is the gold standard for diagnosis for most tumors. Evaluation of a tissue sample enables a pathologist to assess a tumor's microscopic features and assign a grade, if applicable.
Grade predicts how aggressive a tumor is likely to be using a numerical scale, with higher numbers usually (but not always) indicating more aggressive disease.
High‑grade tumors typically carry a greater risk of recurrence or metastasis than low‑grade tumors. Features associated with a higher grade can include:
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Increased mitotic count and other markers of rapid growth or proliferation.
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Increased areas of tumor necrosis.
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Evidence of invasion into blood vessels and/or lymphatics.
Surgical specimens can also be evaluated for completeness of surgical margins, or how close residual microscopic cancer cells are to the cut edge of tissue.
Staging: How Far Has It Spread?
Staging answers the second question: How far has it spread?
Staging defines the extent of cancer that is present at diagnosis and is periodically performed after treatment to monitor for disease progression. Staging typically involves:
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Measuring the primary tumor with imaging or physical measurement.
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Imaging studies, such as radiographs, ultrasound, or CT scans, of body cavities to look for evidence of dissemination.
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Sampling lymph nodes or at‑risk organs based on tumor location and type.
For lymphoma, staging systems are on a scale of one to five, with higher numbers indicating greater spread or metastasis.
There are a variety of commonly used staging systems, including the modified World Health Organization (WHO) systems; Tumor, Node, Metastasis (TNM); and other tumor-specific staging systems. As shown in the example below regarding nasal tumors in canines, these systems present information about the extent of cancer in different ways:
| Stage | Description |
|---|---|
| Stage 1 | Tumor only involves one nasal passage or sinus, and doesn't involve bone |
| Stage 2 | Involves bone, but doesn't invade the orbit, subcutaneous, or submucosal tissues |
| Stage 3 | Tumor invades the orbit and/or nasopharynx or is subcutaneous or submucosal |
| Stage 4 | Tumor extends into the cribriform plate |
| Stage | TNM Classification | Meaning |
|---|---|---|
| Stage I | T1 N0 M0 | Small/localized primary tumor; no lymph node involvement; no distant metastasis |
| Stage II | T2 N0 M0 | Larger primary tumor; no lymph node involvement; no distant metastasis |
| Stage III | T3 N0 M0 | Largest/more advanced primary tumor; no lymph node involvement; no distant metastasis |
| Stage IV | Any T N1 M0 | Any tumor size with regional lymph node involvement; no distant metastasis |
| Stage V | Any T Any N M1 | Distant metastasis present, regardless of tumor size or lymph node status |
Similar to grade, a higher stage is generally associated with more advanced cancer.
How Grade and Stage Guide Treatment
Together, grade and stage form the roadmap for oncology management:
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Grade: Predicts the likelihood of recurrence or spread after surgery. High‑grade tumors often require systemic therapy, such as chemotherapy, in addition to local treatment.
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Stage: Influences treatment recommendations. Local therapies like surgery or radiation may be less effective if cancer has already spread and the chance of a cure is lower. The chance of a cure is highest for tumors that are low grade and amenable to complete surgical removal.
Veterinarians should perform staging tests before treatment begins, repeat them during treatment (restaging), and continue them after treatment ends to monitor for progression.
In cases where metastasis is already present or the tumor can’t be removed at the time of diagnosis, restaging helps determine whether the recommended treatment is working. Evidence of spread on restaging signals the need to adjust the plan and consider new treatments.
Grading and Staging Guides Treatment and Client Compliance
A good approach to any dog with a tumor is to determine the grade and perform staging tests. Grade predicts a tumor's biological behavior; stage defines how far the tumor has spread. By understanding and clearly explaining these concepts, veterinarians can set realistic expectations, support informed decision‑making, and build trust with clients navigating difficult choices.
