Biopsy

A biopsy is the removal of tissue from a living organism for microscopic examination. In veterinary medicine, biopsy provides definitive diagnosis when imaging, blood work, and physical examination cannot establish a diagnosis with certainty. It is the gold standard for distinguishing benign from malignant disease and for grading cancer severity.

Biopsy Types

Fine Needle Aspirate (FNA)

A small-gauge needle (22-25g) is inserted into a mass or organ, and cells are aspirated for cytologic examination. FNA is:

• Minimally invasive: no sedation required in most cases
• Quick: results often available within 24-48 hours
• Screening-level: identifies cell type (inflammation vs. infection vs. neoplasia) but may not provide definitive grade or margin information
• Limitations: small sample size means sampling error is possible — a negative FNA does not rule out malignancy

FNA is the appropriate first step for most palpable masses, enlarged lymph nodes, and ultrasound-guided organ sampling.

Incisional Biopsy

A small, representative piece of tissue is surgically removed from a larger mass. The mass itself is left in place. Incisional biopsy is used when:

• FNA was non-diagnostic or inconclusive
• Tumor grade is needed before planning definitive surgery (e.g., mast cell tumor grading determines surgical margin width)
• The mass is in a location where complete removal without prior diagnosis would be inappropriate

Incisional biopsy requires sedation or general anesthesia and provides a larger tissue sample than FNA, enabling histopathological grading and architectural assessment.

Excisional Biopsy

The entire mass is surgically removed and submitted for histopathology. This serves as both diagnosis and treatment. Excisional biopsy is appropriate when:

• The mass is small enough to remove completely with adequate margins
• Location allows wide excision without functional compromise
• Pre-operative cytology suggests a tumor type where complete excision is curative (e.g., low-grade cutaneous histiocytoma)

For mast cell tumors, excisional biopsy without prior grading risks inadequate margins if the tumor is high-grade. Pre-operative FNA or incisional biopsy is generally preferred.

Organ Biopsies

Gastrointestinal Biopsy

Endoscopic biopsies of the stomach and intestinal lining are the definitive diagnostic tool for inflammatory bowel disease (IBD) versus intestinal lymphoma. These two conditions can appear clinically identical but require fundamentally different treatments.

Liver Biopsy

Liver biopsy (ultrasound-guided Tru-cut or laparoscopic) is needed to diagnose copper-associated hepatopathy, chronic hepatitis, and hepatic fibrosis/cirrhosis. Liver enzyme elevations on bloodwork indicate liver damage but do not specify the underlying cause.

Skin Biopsy

Punch biopsies (4-8mm circular cores) diagnose autoimmune skin diseases (pemphigus, discoid lupus), sebaceous adenitis, and skin cancer when clinical appearance is ambiguous.

Interpreting Biopsy Results

Biopsy samples are processed, sectioned, stained, and examined by a veterinary pathologist. The report includes:

• Diagnosis: specific disease identification
• Grade (for tumors): prognostic classification (e.g., grade I/II/III mast cell tumor)
• Margins (for excisional biopsies): whether the tumor was completely removed with clean surgical margins
• Mitotic index: rate of cell division — higher = more aggressive behavior

When Biopsy Is Essential

Any mass that is growing, changing in appearance, or present for more than one month should be evaluated — at minimum by FNA. The cost of a biopsy is a fraction of the cost of treating advanced cancer that could have been caught early with timely tissue sampling.

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