Nuclear medicine or scintigraphy is a less common form of diagnostic testing used to screen cancer in dogs and cats. The primary reason is the cost and general availability of these tests. It involves the use of expensive equipment, along with special requirements regarding handling of radiopharmaceutical (radionuclide) materials, and is done mostly at veterinary teaching hospitals, but is also available in the private sector. This type of testing is very sensitive at detecting lesions, but does not give information as to the specific nature of the lesion. Combination nuclear imaging and CT scanners exist, which allows for both imaging procedures to be performed at the same time. The two images created can be fused together and allow for the anatomical information obtained from CT to be placed together with the physiological information obtained from the nuclear imaging procedure.
Type of information obtained from nuclear medicine in the staging of cancer
Information obtained from nuclear medicine studies does not provide detailed information regarding the size and shape of structures compared to other imaging procedures used in staging of cancer. However, it does provide information on the physiological or functional aspects of the patient based on the type of radiopharmaceutical used. This type of testing is thought to be very sensitive at detecting lesions, but will not provide information on the true nature of the lesion, as benign and malignant lesions can look the same. Once a lesion is detected with nuclear imaging, additional testing may be necessary to characterize the true nature of the lesion. Therefore, nuclear imaging is often considered a two step process, where sites of activity detected by this form of imaging is further imaged with other diagnostic procedures such as plain radiographs, computerized tomography (CT), magnetic reasonance imaging (MRI) and cytology or histopathology. Since this type of imaging is so sensitive, it is thought to be beneficial at detecting early recurrence of tumors. However, it is not known yet whether earlier detection of tumor recurrence in pets has a significant impact on long-term survival.
Examples of radionuclides used in the staging of cancer in dogs and cats
Radionuclides are administered to the patients that are directed to sites of interest by metabolic pathways. Technetium-99m (99mTC) is the most commonly used radionuclide in veterinary medicine. It is an excellent radionuclide because it has great imaging properties, has a short half life (6 hours) and is easily bound to other agents. Two of the most common types of studies performed with 99mTC in veterinary medicine involves use of 99mTC bound to pertchnetate to image the thyroid gland (particularly in cats with hyperthyroidism) and use of 99mTC bound to methylene diphosphonate (MDP) to evaluate the skeletal system in dogs with bone cancer.
Another radionuclide commonly used is a glucose-like molecule, 18R-fluorodeoxyglucose (FDG). It acts like a glucose molecule and is transported into cells, but is not metabolized the way glucose normally is and remains trapped inside the cell. Cells that are more active metabolically take up more FDG. It is felt that metastatic lesions are more active metabolically and that those cells will take up more FDG allowing for detection of those lesions.