the effects of low-dose-rate irradiation (brachytherapy) were investigated in vivo using a murine mammary adenocarcinoma (mtg-b) growing in the flank of c3h mice. for local tumor irradiations, a noninvasive cap was devised to cover the tumor and house three /sup 125/i seeds (average apparent activity 5.2 mci each) located at 120 degrees intervals around the circumference of the hemispherical cap (13 mm i.d.). mice were secured during treatment in a tube allowing limited mobility while restricting access to the seeds. tumors were exposed to a series of dose rates ranging from 14-40 cgy/h, and the total dose over the treatment interval (48 to 72 h) ranged from 830 to 2378 cgy. a total of nine experiments were conducted using the caps over a 10-week interval. the brachytherapy results are compared to the effects of external beam irradiation in the same tumor system. a linear relationship was observed between the total radiation dose and doubling volume growth delay or treatment volume growth delay for the brachytherapy and external beam irradiation