Staging of the Canine Cancer Patient

An important clinical application of Dr. Baum’s work is staging of dogs diagnosed with cancer. Staging a cancer patient involves determining the anatomical extent of disease, which is classified according to the extent of primary tumour (T), involvement of regional lymph nodes (N) and presence of distant metastases (M) (the TNM staging system). For example, for thyroid tumours, lymph nodes are classified as N0 (no evidence of regional lymph node involvement), N1 (ipsilateral regional lymph node involved), and N2 (bilateral regional lymph node involvement). NX is used when the regional lymph nodes are not assessed^[1]. The stage of cancer will affect a patient’s prognosis, as well as the treatment plan recommended by an oncologist. For example, a lymph node group known to contain metastases may be included in radiation treatment.

Knowledge of the biological behaviour of a tumour will determine the importance of staging regional lymph nodes. Some malignant tumour types are more likely to involve regional lymph nodes; examples of these tumour types in the dog include mast cell tumours, thyroid tumours, oral melanoma and anal sac tumours.

Staging of regional lymph nodes may be accomplished through physical examination, imaging (ultrasonography, computed tomography [CT], lymphography, lymphoscintigraphy, positron emission tomography [PET], magnetic resonance imaging [MRI]) and cytological or histological examination. In order to decide which regional lymph nodes draining a tumour should be assessed, a clinician must understand the lymphatic drainage patterns. For example, lymph vessels leaving the non-diseased thyroid gland drain to the medial retropharyngeal lymph node, the deep cervical lymph nodes, and the cranial mediastinal lymph nodes, and these lymph nodes should be considered when staging a dog with a malignant thyroid tumour.

Cancer may change the normal pattern of lymphatic drainage. Oncologists may use lymphography (injection of a contrast agent to image lymphatic drainage), lymphoscintigraphy (injection of a radioactive tracer to image lymphatic drainage) or injection of dye to identify the first lymph node(s) draining a patient’s tumour. This lymph node(s) is called the sentinel lymph node (SLN). The presence or absence of tumour cells in the SLN predicts the lymphatic spread of a tumour and may predict outcome, as well as guide treatment^[2].

Lymph node size alone is not specific or sensitive for regional metastasis but can be used to evaluate response to treatment. Computed tomography is the preferred imaging method for measurement of lymph node size, based on reproducibility of measurements. Size measurements should be made on the plane of acquisition (usually the transverse plane). Lymph nodes less than 1 cm in diameter are considered non-pathological in response evaluation of dogs with solid tumours^[3].