PRINCIPLES of TUMOR BIOPSY
Excisional biopsy (removal of the mass) is feasible if the mass is small (<3 cm in diameter), freely moveable, and without adjacent tissue invasion. The specimen must contain a complete margin of normal tissue. Otherwise, the tumor may be spread throughout the biopsy site and may be more difficult to resect later.

Specific indications for excisional biopsy include lymph nodes, small cutaneous nodules with ample surrounding normal tissue, mammary gland tumors, tumors of the central nervous system (to provide decompression) and masses found during a laparotomy or thoracotomy since a re-excision is unlikely.

Excisional biopsies should be considered a therapeutic procedure only when complete tumor removal is histologically verified and the tumor is benign or of low grade malignancy. In other situations, excisional biopsy should be used to consider a more definitive treatment recommendation.

Incisional biopsy (removing just a piece of tissue) is recommended if a definitive diagnosis or histologic grade would influence the treatment decision. For example, histologic grade of soft tissue sarcomas and mast cell tumors are prognostic factors that can be helpful in treatment planning. Biopsy results may suggest the degree of surgical resection necessary for definitive control or indicate that additional types of therapy may be beneficial.

Ideal histologic samples contain a representative portion of the entire tumor. Superficial tumors should be sampled away from regions of ulceration, necrosis or inflammation. Deep biopsies (> 1 cm) may be necessary to avoid sampling only overlying tissue. Biopsy at the tumor margin can be undesirable in certain deep-seated tumors if it disrupts and thereby extends the tumor margin. This can necessitate wider resection or a larger radiation field for adequate treatment. Biopsy needles are preferred for deep-seated tumors. The biopsy incision or needle tract for every biopsy should be preplanned since it is a potentially contaminated region, and should be removed at the time of the definitive procedure or included in the radiation treatment field.

TUMOR STAGING
Staging is used to describe the extent of neoplastic disease, provide a framework for rational treatment planning, facilitate communication between clinicians, allow for uniform comparison and evaluation of treatment results, and aid prognostication. Accurate staging requires an understanding of the biologic behavior of different tumor types and a thorough diagnostic evaluation, including tumor measurement, lymph node assessments and imaging of the regional and whole body if indicated.

Several staging systems are available. Most are based on assessment of local, regional, and distant disease involvement. Some systems include other factors such as presence or absence of clinical signs (e.g. lymphomas), the degree of malignancy based on microscopic examination (e.g. mast cell tumors), or which site the same tumor may occur in (e.g. squamous cell carcinoma of mouth, tonsil, ear, digit).

The TNM (Tumor-Node-Metastasis) SYSTEM devised by the World Health Organization is the standard system for most tumors in veterinary medicine.

World Health Organization TNM classification of tumors.

T = Tumor size or extent.

T1-T4 represent specific size categories designated for each tumor type.

N = Lymph Node Involvement.

N1 - N3 (+ a,b) describes regional lymph node characteristics such as number of nodes enlarge, tissue adhesion, and presence or absence of neoplasia.

M = Metastasis.

M0 or M1 indicates absence or presence of distant metastasis.

Example: TNM Classification of tumors of the oral cavity in dogs or cats.

T: Primary Tumor
Tis = Preinvasive tumor (in situ)
T0 = No evidence of tumor
T1 = Tumor < 2 cm diameter
T2 = Tumor 2-4 cm diameter
T3 = Tumor 3 cm diameter subclassification of a (no bone invasion) or b (bone invasion) can be added.

N: Nodes N0 = No evidence of LN enlargement
N1 = Movable ipsilateral nodes enlarged
N2 = Movable contralateral/bilateral nodes enlarged
N3 = Fixed nodes

M: Metastasis
M0 = No metastasis
M1 = Metastasis detected