In radiation therapy for adenocarcinoma of the prostate, an arbitrary high dose to the prostate gland is limited by toxicity to the neighboring tissues namely to the urethra, the rectum, and bladder. Rectal toxicity has been directly correlated with the dose received by the rectal walls. In addition to acting principally as an immobilization device, an endo-rectal balloon may serve to reduce radiation the dose to the rectal wall. The volume of the rectal wall that receives a high radiation dose is reduced by forcing the posterior wall away from the target. In this work, point dose measurements were collected using a commercial PN-junction diode detector and electrometer using a standard radiation therapy calibration protocol. Measurements were made in an acrylic, water-filled phantom with a designed geometry that simulates a rectal cavity, prostate gland, and bladder. For comparison, predicted dosimetry was derived using CT-based treatment planning employing the Xio radiation therapy treatment planning system which uses a radiation transport model based on the collapsed cone convolution algorithm.