This book considers how Nuclear Medicine treatments have been further developed in the past several years and are now widely applied in clinical practice, especially in oncological patients, in addition to radioiodine therapy for various thyroid diseases. These treatments are very effective when properly indicated, but side effects and complications specifically related to the radioactive drugs and other potential clinical problems should also be considered, since other treatments, e.g. chemotherapy or radiotherapy, are frequently administered before, simultaneously with and/or after radionuclide therapies.   Consequently, cumulative toxicities and treatment interactions must be considered, and physicians prescribing treatments should be capable of preventing (when possible) and properly diagnosing and treating such side effects and complications. Accordingly, expertise in radionuclide therapy and extensive clinical expertise are required to appropriately manage critical cases.