During the 7 to 8 weeks of conformal radiotherapy for prostate carcinoma, the prostate has an intra-pelvic motion which is not detectable by the conventional portal imaging. This prostate motion may lead to a mistargeting, which possibly decreases the local control. On the other hand, the new 3D on board imaging (3D OBI) allows to localize the target but is expensive. The cost/effectiveness ratio is not well established, as well as the frequency of using such imaging during the course of radiation.