Interstitial brachytherapy (IRT) is a method that delivers radiation from within the tumor cavity. A device, such as a catheter or a needle, is placed into the tissue that requires treatment, and radioactive seeds are inserted into the catheter during treatment, then removed afterward. This means that radiation exposure is limited to a short distance of surrounding tissue (brachy - Greek for short distance). Healthy tissue that is near the former site of the tumor is less affected by brachytherapy than when external beam radiation is used.
Interstitial is a term that refers to the placement of the tiny radiation source within the tissue. Interstices are small narrow spaces, like catheters, which may be arranged in intervals.