Section 3: Tumor Diagnostics & Staging
Top 3 Takeaways
- Cancer is frequently treatable or manageable in veterinary patients. A suspicion or diagnosis of cancer should be the beginning and not the end of the diagnostic process.
- To assess a cancer patient’s prognosis and develop an optimal treatment plan, a cytologic or histopathologic diagnosis is needed, and in many cases, it is necessary to determine the tumor stage or tumor grade or both.
- Appropriate tumor staging tests vary and should be selectively performed based on their diagnostic relevance, prognostic value, and compatibility with the pet’s needs and the client’s priorities and limitations.
Making the Diagnosis
Suspicion of neoplastic disease in a patient often stems from identifying a mass on physical examination or imaging tests. The type and potential behavior of a mass cannot be determined based solely on palpation or imaging. To assess patient prognosis and develop an optimal treatment plan, a specific diagnosis and in many cases identification of the tumor stage or tumor grade or both is needed (Table 3.1).
Most tumors present as a mass or lump, although many nonneoplastic conditions can present identically. Accordingly, when physical examination or imaging reveals a mass, sampling the mass for microscopic evaluation is indicated.
Cytologic examination of a fine-needle sample (obtained with or without aspiration) of a mass often provides a definitive diagnosis of benign lesions (e.g., dermal cyst, lipoma, inflammation) and round cell tumors (e.g., lymphoma, MCT, plasma cell tumor) and can be helpful in categorizing other masses as mesenchymal or epithelial and at times benign versus malignant. In most cases, submission of cytology samples to a clinical pathologist for interpretation is recommended and may preclude the need for histopathology (Table 3.2).
Obtaining a tissue sample for biopsy and histopathology can provide a definitive diagnosis in nearly all accessible masses (Table 5.7). It also frequently provides additional information that cytology cannot (Table 3.3), such as tumor grade, mitotic index, and invasiveness that may impact prognosis and treatment recommendations.
TABLE 3.1 Tumor Grade and Stage Features
| Tumor Grade | Tumor Stage | |
|---|---|---|
Definition |
Describes the microscopic appearance of cancer cells and tissue |
Describes the size and extent of local disease and presence of regional and distant metastasis |
Features of low grade/stage |
Well-differentiated cells that closely resemble normal tissue with minimal invasion or disruption of surrounding normal tissue |
Small tumors without evidence of regional and/or distant metastasis |
Features of high grade/stage |
Poorly differentiated cells without normal tissue architecture or pattern |
Large/infiltrative tumors or tumors with regional and/or distant metastasis |
TABLE 3.2 Indications for In-Clinic Versus Diagnostic Laboratory Cytologic Examination (Selected Examples)
Cytologic Examination In-Clinic by a General Practitioner or Specialist |
Cytologic Examination by a Clinical Pathologist (Typically After Initial Review by a General Practitioner or Specialist) |
|---|---|
Clinician is confident in cytologic diagnosis (e.g., mast cell tumor) |
Clinician is uncertain of malignancy or cell types |
Client financial constraints |
No client financial constraints |
Sufficient information has been obtained to recommend and plan biopsy |
No additional diagnostics are planned before instituting therapy |
Aspirating a fatty mass, which may be poorly cellular |
Additional tumor characterization affects treatment recommendations (e.g., benign vs malignant, cytologic grading) |
TABLE 3.3 Overview of Cytologic Versus Histopathologic Examination
Cytologic Examination |
Histopathologic Examination | |
|---|---|---|
Sampling |
Minimally invasive, often requires no sedation or anesthesia |
Curative-intent surgery vs several incisional biopsy options (e.g., needle core, punch, wedge) |
Results turnaround time |
0–2 days |
3–7 days |
Information provided |
|
|
Allows grading |
No (rare exceptions for cytologic grading schemes) |
Yes |
Allows determination of tumor type |
Commonly allows determination of tumor category (e.g., round cell, epithelial, and mesenchymal) and sometimes allows a specific diagnosis |
Nearly all tumors (special stains or immunohistochemistry may be required) |
Ancillary Diagnostic Tests
In some cases, a mass may not be amenable to sampling, initial test results may be inconclusive, or no mass lesion exists, but cancer is suspected. In these cases, tests such as organ sampling (e.g., bone marrow or splenic aspirates), immunohistochemistry, proliferation markers, special tissue stains, polymerase chain reaction for antigen receptor rearrangement, and flow cytometry can provide additional diagnostic and prognostic information.1 Consult with a veterinary pathologist or oncologist to identify which ancillary tests may be indicated, how to perform them, and how they might be beneficial.
Staging
The staging process identifies the extent and distribution of cancer in a patient. For solid tumors, such as sarcomas and carcinomas, this usually involves the size of a patient’s local disease and whether regional or distant metastasis is present (Table 3.1).
Evaluating locoregional disease starts with a physical examination to determine the primary tumor’s size and appearance and its mobility or fixation to adjacent tissues. If the neoplasm is internal or a concern exists about bone or other tissue involvement, ultrasonography, radiography, computed tomography (CT), or MRI may be needed to assess the extent of local disease. It is best practice for a board-certified veterinary radiologist to review the images.
Carefully assess regional lymph nodes by palpation and, when indicated, further evaluate lymph nodes with imaging tests and cytologic or histopathologic examination. Evaluating lymph nodes by palpation and size are not always reliable indicators of metastasis. Lymph nodes may be enlarged because of metastatic disease or nonneoplastic causes such as infection or other inflammatory processes. Similarly, normal-sized lymph nodes may harbor metastatic disease. To more accurately determine whether lymph node metastasis has occurred, perform fine-needle aspiration (FNA) or biopsy. Cytologic or histopathologic examination of these samples provides a more definitive diagnosis and allows identification of metastatic cells. This approach is critical for accurate staging and treatment planning in patients with cancer.
Assess a patient’s systemic health status by obtaining a minimum database, which includes a complete blood count (CBC), chemistry panel, urinalysis, and viral testing in cats (feline leukemia virus/feline immunodeficiency virus). For potentially malignant tumors, screen for cancer spread to distant organs. Confirmed distant metastasis generally implies a worse prognosis and may dramatically affect therapeutic decisions. Staging tests to screen for distant metastasis vary, but the most common staging tests (beyond the physical examination) include thoracic radiography, abdominal ultrasonography, lymph node sampling, and/or CT. The tests selected depend on the tumor type and the impact of potential findings from each test on prognosis or treatment or both. Client education is critical at this step, and clinicians should aim to avoid unnecessary or redundant tests.
The 3 P’s of Staging Test Decision Making
Prognostic-Perform tests that impact prognosis or treatment recommendations. For example, for canine osteosarcoma, thoracic radiography is important because patients who have pulmonary metastasis have a worse prognosis and limb amputation is recommended with caution. For canine lymphoma, staging with examination of splenic or liver aspirates is unlikely to change the prognosis or treatment, but immunophenotyping tests may.
Practical-If a client has a limited budget for their pet’s care, avoid spending the majority on diagnostic testing and leaving little for treatment. Prioritize diagnostic tests that confirm a diagnosis and evaluate the patient’s health.
Pertinent-For a specific tumor, screen the sites of most frequent early metastasis. For example, for dogs with an MCT, use FNA and cytology of locoregional lymph nodes, +/- liver and spleen to screen for metastasis versus thoracic radiography, because pulmonary metastasis is less common in these patients. For dogs with osteosarcoma involving a limb, first screen for metastasis with thoracic radiography, but if the dog has an elevated serum alanine aminotransferase, abdominal ultrasound (AUS) may be recommended as a part of the work up.
The 2026 AAHA Oncology Guidelines for Dogs and Cats are generously supported by CareCredit, Hill’s Pet Nutrition, Merck Animal Health, and Zoetis.
Citations
- Thalheim L, Williams LE, Borst LB, et al. Lymphoma mmunophenotype of dogs determined by immunohistochemistry, flow cytometry, and polymerase chain reaction for antigen receptor rearrangements. J Vet Intern Med 2013;27:1509–16.