In Medical Applications that use Radioactive sources, How are long-term effects avoided?

Considering: 1) The nature of the decay (alpha, beta, gamma) 2) The half-life (long enough for the isotope to produce useful measurements, but short enough for the radioactive sources to decay to safe levels soon after use) 3) Toxicity If the half-life chosen is too long, the damaging effects of the radiation would last for too long and the dose received would continue to rise