Sealed source radiotherapy or brachytherapy is the application of radiation from close range and is used for techniques where the radioactive source is placed inside the area requiring treatment.

The Greek word "brachy" means close or nearby, and is the opposite of "tele" which means far or at a distance. Teletherapy has been used to refer to the use of gamma-emitting sources (usually Cobalt-60) positioned perhaps 0.8m from the target volume.

Brachytherapy can be split into three main types:

• Surface Applicator or "Mould" brachytherapy. Superficial tumours can be treated using sealed sources placed close to the skin. Dosimetry is often performed with reference to the Manchester system ; a rule-based approach designed to ensure that the dose to all parts of the target volume is within 10% of the prescription dose.

• Interstitial brachytherapy. Here the sources are inserted into tissue. The first treatments of this kind used needles containing Radium-226, arranged according to the Manchester system, but modern methods tend to use Iridium-192 wire. Iridium wire can be arranged either using the Manchester or the Paris system ; the latter was designed specifically to take advantage of the new nuclide. Prostate cancer treatment with Iodine-125 seeds is also classified as interstitial brachytherapy.

• Intracavitary brachytherapy places the sources inside a pre-existing body cavity. The most common applications of this method are gynaecological in nature, although it can also be performed on the nasopharynx.