Internal radiation therapy uses seed implants, or brachytherapy, which places radioactive material directly into or close to the tumor. Seed implants and brachytherapy are terms used interchangeably. This procedure allows the delivery of a high radiation dose to the prostate tumor while sparing adjacent normal tissues and is often combined with external beam irradiation, which serves as a "boost dose" to the site of suspected or known malignancy.

High dose rate brachytherapy involves placing a temporary radioactive implant into the target area. The radioactive implant has an exceptionally high dose rate for rapid treatment. The procedure is generally completed on an outpatient basis under local anesthesia.

For low dose rate brachytherapy, permanent radioactive seeds (I-125, Pd-103) are placed into the prostate tumor. It may be an alternative to radical prostatectomy or external beam radiation for some patients. With this one time outpatient procedure, patients avoid the long recovery associated with surgery and retain their virility. It is generally done under regional or general anesthesia, and no hospital admission is required.