Physics. Chapter 2

The absorbed doses delivered by internally distributed radiopharmaceuticals are the result of emissions emanating from radioactive decay of the radionuclide. The emitted photons and charged particles interact with the tissue, and deposit energy, resulting in excitations and ionizations of atoms. These initiate chemical reactions that may affect sensitive biomolecules such as deoxyribonucleic acid (DNA). The DNA damage may occur either via direct interaction or be mediated by reactive oxygen species generated by radiolysis of water in tissue. Thus, information about the number of radioactive atoms that are present in organs and tissues combined with information about their emissions are both necessary to determine the absorbed doses delivered and the radiobiological effects obtained in radiopharmaceutical therapy (RPT). There are two types of radioactive emissions — particles (alphas, electrons, positrons, neutrinos and antineutrinos) and electromagnetic radiation (gamma and X ray emissions) — and they can originate from the atomic shells and/or from the nucleus itself. A short discussion of the emissions that are most relevant to RPT follows.