Teletherapy: is radiation deleivered externally to the patient using a linear accelaerator

Brachytherapy: is radiation delivered internally, directly at the site of the tumour using radiation sources such as pellets or seeds.

The prefixes, 'Tele' and 'Brachy' are applied here, to the distance of the source of the radiation from the target tumour.
Tele - long distance - external radiation, applied from afar
Brachy - short distance - internal radiation applied directly on site