Brachytherapy delivers radiation from a sealed source placed directly inside or immediately next to the tumour, rather than from outside the body. Because the source sits so close to the target, dose falls off sharply with distance — allowing a very high dose to be delivered to the tumour itself while sparing nearby organs and tissue far more than external radiation alone can achieve. It is commonly used for cervical, prostate, and breast cancers, either as the primary treatment or as a boost alongside external beam radiation.

Treatment begins with imaging and careful planning to determine the best approach — intracavitary, where an applicator is placed within a body cavity such as the cervix, or interstitial, where thin catheters or needles are placed directly into the tumour or tissue, as is often done for the breast or prostate. Depending on the case, a high-dose-rate (HDR) source may be used for short, outpatient-style sessions delivered over one or more visits, or a low-dose-rate (LDR) source may be used for continuous treatment over a longer period. Image guidance during applicator or catheter placement ensures the radioactive source is positioned with precision before each dose is delivered.