While some diagnostic and therapeutic applications of radioisotopes have become germane to routine specialty practice, such as technetium-based scintigraphy and radioiodine therapy, a rapidly expanding armamentarium of isotopes will change veterinary cancer care. Radioisotopes such as 18F-fluorodeoxy glucose (18F-FDG) can exploit the metabolism of the cancer cell, allowing improved and minimally invasive staging, as well as the ability to locate an elusive pathologic process. Additionally, by using inherent and manufactured targeting characteristics, and by selecting the desired energy for a biologic effect, radioisotopes can be used to target the cancer cell while sparing normal tissue. In veterinary medicine, facilities that work with radioactive diagnostic and therapeutic isotopes are sparse, and the goal of this talk is to inform the listener of the appropriate applications for nuclear medicine for veterinary patients. This discussion will help identify cases for which referral to a facility with nuclear medicine will be beneficial.
Radiation therapy causes single and double strand breaks in DNA. Double strand breaks are often lethal to a cell, and an accumulation of single strand breaks can have the same effect. Repeated treatments allow DNA damage to accumulate within a cell. The effects of radiation on both tumor and normal tissue can be seen at various intervals depending on when the cell attempts to complete the cell cycle. A cancer cell can lie dormant for an extended period of time. When the cell eventually attempts mitosis, the damaged cell undergoes a mitotic catastrophe and death. Therefore, tumors can shrink slowly over weeks to months, or sometimes simply stop growing. Lymphocytes are unusual in that they can undergo an intermitotic death.
External beam radiation therapy has been the mainstay of radiation therapy for many years. This entails fractionated treatments over the course of approximately 3–6 weeks. The use of cross sectional imaging (CT scans) for computer- guided planning allows relative sparing of normal tissue and targeting of tumor tissue. Brachytherapy involves implanting a radioactive source into the tumor, and the energy that is emitted is highly concentrated around the point sources, resulting in sometimes complete sparing of the surrounding normal tissue. However, proper orientation of sources can be challenging. Brachytherapy can also be accomplished using targeted pharmaceuticals such as radioiodine and samarium. The University of Missouri houses the largest research reactor on a university campus in North America, and the active radiopharmaceutical program offers unique opportunities for research and clinical treatment of animals with cancer.