A prostate immobilizer has a shaft with a fluid passageway extending therethrough, a balloon affixed over an end of the shaft such that the fluid passageway communicates with an interior of the balloon, a lumen extending within the shaft so as to have an end opening adjacent an end of the balloon, and a device for locking a position of the shaft. The device has a positioning member having a stem portion extending from a head portion, and a locking member having a locked position and an unlocked position. The locking member cooperates with the stem portion of the positioning member so as to fix a position on the shaft. A channel is formed in the head portion so as to receive the shaft therein.