Histopathology
The microscopic examination of tissue samples to diagnose disease. Histopathology provides definitive diagnosis of tumors, inflammatory conditions, and organ diseases by revealing cellular architecture and pathological changes invisible to the naked eye.
Histopathology is the microscopic examination of tissue samples — obtained by biopsy or surgical excision — to diagnose disease at the cellular and architectural level. A veterinary pathologist processes the tissue, stains thin sections (typically with hematoxylin and eosin), and examines them under a microscope to identify abnormalities in cell morphology, tissue architecture, and disease processes.
Why Histopathology Is the Gold Standard
Many conditions look similar on imaging, bloodwork, or gross examination but have fundamentally different diagnoses, treatments, and prognoses. Histopathology resolves this ambiguity:
- A skin mass could be a benign lipoma, a mast cell tumor (varying grades with vastly different prognoses), or a soft tissue sarcoma. Only histopathology provides a definitive diagnosis and grade.
- Chronic vomiting and diarrhea could reflect inflammatory bowel disease or intestinal lymphoma. Endoscopic biopsies with histopathology differentiate these conditions.
- A splenic mass could be a hematoma, nodular hyperplasia, or hemangiosarcoma. Post-splenectomy histopathology determines the actual diagnosis.
How Tissue Samples Are Obtained
Incisional biopsy: A small piece of tissue is removed from a larger lesion for diagnosis before definitive surgery. Used when the surgical approach depends on the diagnosis.
Excisional biopsy: The entire mass is removed and submitted. Provides both diagnosis and treatment in one step. The pathologist evaluates surgical margins to determine whether excision was complete.
Endoscopic biopsy: Small tissue samples obtained via endoscope from the gastrointestinal tract, nasal cavity, or airways. Critical for diagnosing chronic GI disease.
Core needle biopsy: Ultrasound-guided needle retrieval of tissue cores from internal organs (liver, kidney, spleen). Less invasive than surgical biopsy but provides less tissue architecture.
What the Pathology Report Includes
A standard histopathology report provides:
- Diagnosis: the specific disease identified (e.g., grade II mast cell tumor, lymphoplasmacytic enteritis)
- Tumor grade (for neoplasms): a standardized assessment of cellular differentiation, mitotic rate, and invasiveness that predicts biological behavior
- Margin assessment: whether surgical margins are clean (no tumor cells at cut edges), narrow, or incomplete — directly guiding the need for revision surgery or adjunctive therapy
- Special stains and immunohistochemistry: additional tests (Ki-67 proliferation index, c-KIT staining) when standard H&E staining is insufficient for diagnosis or prognosis
Histopathology vs. Cytology
Cytology (fine-needle aspirate) examines individual cells and is faster, cheaper, and less invasive. However, cytology cannot assess tissue architecture, tumor grade, or surgical margins. For cancer diagnosis and staging, histopathology is usually necessary after cytology provides a preliminary direction.
Relevance to Longevity
Accurate diagnosis drives appropriate treatment. A misdiagnosed tumor type leads to wrong treatment and lost time. For cancer-prone breeds, understanding when histopathology is indicated — and acting on results promptly — is essential for maximizing treatment outcomes and survival time. Routine submission of all excised masses for histopathology, even those presumed benign, is standard of care.