Variation in prostate position relative to adjacent bony anatomy

In prostatic cancer, the prostate cannot be discerned from a portal image. Setup information is, therefore, obtained from bony anatomy. To perform high-precision conformal therapy, knowledge about the variation in distance between the prostate and adjacent bony anatomy during an external radiotherapy treatment is mandatory. This report is concerned with that variation.

Nine patients previously treated interstitially for prostatic cancer with implantation of 125I seeds, agreed to cooperate in a study. They underwent a number of simulations of external radiotherapy treatment. After the first patient setup, this setup was repeated five times for each patient as if the patient was treated by external radiotherapy. Simulator radiographs were made from each setup in the anterior-posterior and left-right lateral directions. The seeds were clearly visible in the simulator film images and reflect the position of the prostate. No bladder instructions were given and the filling of the rectum was not quantified.

Variation in distance between the prostate and the bony anatomy was measured and is presented in one standard deviation of the normal distribution function: 0.8 mm lateral, 1.5 mm ventrodorsal, and 1.7 mm craniocaudal.

The bone structure of the pelvis reflects fairly well the position of the prostate. The prostate is least mobile in a lateral direction and most mobile in the craniocaudal and ventrodorsal directions.