What is Tumor Staging (TNM)? — How Are Dog and Cat Cancers Staged?

ImmunityQ&AMeongsiljang Veterinary Advisory Board

Tumor staging (TNM) is a diagnostic procedure that evaluates the extent of cancer progression in pets based on three key factors: the primary tumor, lymph nodes, and distant metastasis. It serves as a critical criterion for determining treatment direction and prognosis.

What Is Tumor Staging (TNM)?

A veterinarian palpating a dog’s lymph nodes while conducting a tumor staging examination.

Tumor staging (TNM) is a diagnostic process that evaluates how advanced your pet’s cancer is by assessing three key factors: the size of the primary tumor (T), lymph node involvement (N), and distant metastasis (M). The most important thing to understand is that the treatment plan and prognosis are determined by these staging results. Rather than immediately starting chemotherapy upon finding a lump, we first confirm the TNM stage to determine whether surgery, radiation, or chemotherapy is the most appropriate course of action.

What Each T, N, and M Component Means

The TNM system originated from a tumor classification framework for companion animals proposed by the World Health Organization (WHO). - T (Tumor): The size of the primary tumor and the extent of its invasion into surrounding tissues - N (Node): Whether and to what extent the cancer has spread to local lymph nodes - M (Metastasis): The presence or absence of distant metastasis to organs such as the lungs, liver, or bones By assigning a number to each category (T1–T4, N0–N2, M0–M1) and combining them, we determine the overall stage (Stage I–IV). Because the numerical criteria vary depending on the type of tumor, specific cancers such as mammary tumors, lymphoma, and mast cell tumors each have their own unique TNM staging tables.

Basic Classification by T, N, and M Stage

Item	T (primary tumor)	N (lymph nodes)	M (distant metastasis)
Stage 0	No tumor/carcinoma in situ	No lymph node metastasis	No distant metastasis
Stage 1	Small and localized	No lymph node metastasis	No distant metastasis
Stage 2	Medium size	No lymph node metastasis	No distant metastasis
Stage 3	Large or invading surrounding tissue	Regional lymph node metastasis present (N1)	No distant metastasis
Stage 4	Regardless of size (including organ invasion/rupture)	Regardless of lymph node metastasis	Distant metastasis present (M1)

In the modified WHO staging of feline mammary tumors, confirmed distant metastasis (M1) classifies the case as Stage IV regardless of T and N, and regional lymph node metastasis (N1) is reflected from Stage III onward. Because the tumor size (cm) criteria for each stage differ by tumor type (mammary, oral, mast cell tumor, etc.), the actual determination is made by the veterinarian applying the standard chart for that tumor.

Tests Required for Staging — How It’s Done

To establish a TNM stage, we perform a series of tests in a stepwise manner. - Physical examination and palpation: Measure tumor size and check for swelling in nearby lymph nodes. - Fine-needle aspiration (FNA) and biopsy: Determine tumor type and malignancy (essential). - Blood work and biochemistry: Assess overall health and anesthesia risk. - Three-view thoracic radiographs: Check for lung metastasis. - Abdominal ultrasound: Examine the liver, spleen, and abdominal lymph nodes. - CT or MRI: Precisely measure tumor size and extent of invasion (if needed). The order of tests may vary depending on the tumor’s location.

A cat undergoing a chest X-ray to check for distant metastasis.

Why Treatment Should Not Begin Without Staging

Simply removing the lump isn’t the end of the process. If surgery is performed before staging, there’s a risk of missing already metastatic tumors or determining an incorrect surgical margin, which can increase the likelihood of recurrence. This is especially true for mast cell tumors, soft tissue sarcomas, and mammary gland tumors, where the required surgical margins vary significantly depending on the stage. The standard approach is to first confirm the tumor type through fine-needle aspiration (FNA) and biopsy, complete metastasis screening, and then develop a comprehensive treatment plan.

How Treatment Direction Changes by Stage

Once the cancer stage is determined, the range of treatment options naturally narrows. - Stage I (localized, no metastasis): Primary treatment is complete surgical excision, with adjuvant radiation therapy considered in certain cases. - Stage II (intermediate size, suspected regional lymph node involvement): Surgery combined with lymph node removal and adjuvant chemotherapy. - Stage III (multiple lymph nodes, extensive local invasion): Chemotherapy and/or radiation therapy are typically administered first, followed by consideration of surgery. - Stage IV (distant metastasis): Palliative care focused on maintaining quality of life, with chemotherapy used to slow disease progression. Textbook guidelines indicate that prognosis generally worsens once distant metastasis is confirmed. In such cases, chemotherapy is recommended, which also influences the owner’s treatment decisions. Therefore, even for the same type of tumor, treatment goals (curative intent versus maintaining quality of life) and expected prognosis vary depending on the stage. This makes accurate staging the essential starting point for developing a treatment plan.

A consultation scene where the veterinarian explains a treatment plan tailored to each stage of the disease to the pet owner.

Common TNM Example — Mammary Tumors

Mammary tumors in dogs are a prime example of a cancer type where TNM staging is particularly crucial. - T1: Less than 3 cm in diameter, no skin adhesion - T2: 3–5 cm - T3: Greater than 5 cm or adhesion to muscle/skin - N0/N1: No local lymph node metastasis / Local lymph node metastasis present - M0/M1: No lung metastasis / Lung metastasis present Feline mammary tumors have a higher rate of malignancy than those in dogs, so more aggressive treatment is recommended even at the same T stage. Simply removing the mass without confirming the stage makes it easy to miss recurrence or metastasis.

What Owners Should Prepare in Advance

Staging tests may not be completed in a single day. Histopathology results typically take 3–7 days, while imaging interpretation takes 1–2 days. In advance, ask whether fasting is required the day before the test, whether sedation or anesthesia is needed, and the estimated cost range, then coordinate the schedule accordingly. Bring photos taken when the tumor was first detected, records of any changes in its size, and a list of current medications; these will help your veterinarian make an accurate assessment.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

Is a biopsy always necessary for TNM staging?

Yes, fine-needle aspiration (FNA) can help estimate the tumor type, but a biopsy is the standard for determining the exact malignancy grade and T classification. This is essential for surgical planning and prognosis.

How much does staging testing typically cost?

The cost varies depending on the tumor’s location and the veterinary clinic. Since many clinics bundle basic blood tests, chest X-rays, abdominal ultrasounds, and biopsies together, you’ll need to consult with your attending veterinarian to get an accurate estimate.

Does the absence of metastasis mean the disease is cured?

This simply means that no metastasis has been detected at this time; whether the condition is fully cured will be determined by evaluating the surgical margins and the results of regular follow-up examinations. Typically, follow-up check-ups are scheduled at 3, 6, and 12 months after surgery.

Do cats use the same TNM staging system as dogs?

The basic concept is the same, but the numerical criteria vary by tumor type. For example, the T2 threshold for feline mammary tumors is lower than that for dogs, and lymphoma staging may combine WHO and Lugano classification systems. Your veterinarian will apply the appropriate standard staging chart for the specific tumor.

Should we give up on treatment if the tumor stage is high?

Not at all. Even with Stage IV cancer, we can extend your pet’s life while maintaining their quality of life through chemotherapy and palliative care. It’s simply a shift in treatment goals from “cure” to “maintaining quality of life,” not a reason to give up.

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References

[1] Withrow, Vail, Page, Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition, 2019

[2] Owen LN, TNM Classification of Tumours in Domestic Animals, World Health Organization, 1980

[3] Ettinger, Feldman, Cote, Textbook of Veterinary Internal Medicine, 8th Edition, Chapter 343 Tumor Biology and Metastasis