ABSTRACT
When treating patients with systemic malignant diseases, external radiation treatment is sometimes not an option because of the wide spread of the tumors. A possible alternative for local treatment with external radiation is therefore the use of radionuclide therapy. In this modality, a radionuclide with a proper decay scheme is carried to the location of the disease by an appropriate radiopharmaceutical or biomolecule and thereby delivers energy by the emission of charged particles to kill the tumor cells. A classic example of radionuclide therapy is the treatment of cancer in the thyroid where orally administered 131I is taken up naturally by the thyroid as part of its metabolism. If the radionuclide is not active in the metabolism of the tumor/ organ, then it can be chemically labeled to a pharmaceutical, antibody, or a peptide. In radionuclide therapy, the major component to the absorbed dose is delivered by radionuclides that emit β-particles or α-particles. However, absorbed dose planning for radionuclide therapy is not a trivial task since the source cannot be turned on-off as is the case in external beam therapy. Instead, the radionuclide decays exponentially with characteristics depending both on the biokinetic properties and on the physical half-life of the radionuclide. On a small-scale level, the radiopharmaceutical is usually heterogeneously distributed, which indicates that the energy deposition may be nonuniform. The biokinetic may also change over time by internal redistribution, which means that activity distribution needs to be
measured at several time points to estimate the total number of decays and the related emitted kinetic energy. The dose rate for radionuclide therapy is also much lower than in external beam therapy, which has radiobiological implications. Since radionuclide treatment is systemic, some circulating activity is unavoidable and may result in uptakes in normal organs and tissues. This poses a problem that may restrict the amount of activity to be possibly administered.
