Although nuclear medicine is fundamentally a diagnostic specialty, radioisotopes can be used as treatment in specific applications in the form of nuclear molecular radiotherapy.

Radiotherapy involves the administration of ionizing radiation with curative intent through the destruction of malignant tissues or tumours. About a century ago, two French physicians, Bergonié and Tribondeau, demonstrated that cell radiosensitivity is directly related to cell differentiation and reproduction capacity – with less differentiated cells with a higher growth rate being comparatively more sensitive. Since malignant tissue cells generally meet these criteria, their exposure to radiation will result in tumour tissue death, while the healthy tissues, which are more resistant, since they are composed of more differentiated and slower-growing cells, will therefore survive.

The different radiotherapeutic modalities are named according to the characteristics of the radiation involved and the equipment used to generate it.

Teletherapy is a form of radiotherapy that uses radiation from a generating system located at a certain distance from the zone to be irradiated. This form of radiation can be generated using low- or mixed-energy X-ray generators or high-energy systems such as cobalt therapy and linear accelerators.

Brachytherapy is radiotherapy that uses closed or sealed radioactive material sources that are placed in contact with the tumour or inside it. Its main advantage is that it ensures that a maximum radiation dose will reach the tumour tissue, with scant irradiation of the surrounding healthy tissues.