Brachytherapy is a long-established form of radiation therapy. HDR brachytherapy High-dose-rate interstitial brachytherapy (HDR-ISBT) is a variant of brachytherapy with unique features regarding dose distribution and biological effects. With brachytherapy, there is a very steep decrease in dose with increasing distance from the radiation source. This effect is very pronounced, even at small distances from the source. Therefore, the dose shows a heterogeneous dose distribution in the target volume. Nowadays, iridium-192 is the most commonly used isotope in brachytherapy. Due to its high photon density, an iridium-192 source can build small enough to pass through dedicated needles. This unique feature provides the technical basis for its interstitial use.