Variation in prostate position quantitation and implications for three-dimensional conformal treatment planning

Purpose: This study describes and quantitates the motion, i.e., variation in position, of the prostate within the pelvis and its effect on target and normal organ dose. Methods and Materials: The motion of the planning target volume (PTV) borders and center of mass was studied in 13 patients with carcinoma of the prostate through the use of superimposed serial computerized tomography (CT) scans. Changes in bladder and rectal volumes were measured and their relationship to displacements of the PTV position were noted. The effects of this motion on target and normal organ doses were measured. Results: A variability in the position of the PTV is seen over time, which is related to changes in bladder and rectal volumes. The one standard deviation displacements of the PTV center of mass with respect to the planning scan center of mass position were 0.12, 0.40, and 0.31 cm in the lateral, anterior-posterior, and superior-inferior directions, respectively. Movement was significantly larger in the superior part of the PTV above the base of the bladder than in the inferior part. Movement of the borders of the PTV outward from the patient axis; hence, toward the edges of the treatment field, was also examined. Outward displacements of the anterior target border below the base of the bladder were less than 0.3 cm in 90% of the cases, and 1.4 cm above the bladder base. For the posterior wail these displacements were less than 0.7 cm and 1.1 cm, respectively, whereas the lateral border displacements were less than 0.3 cm throughout (90% confidence limits). These displacements would cause a median of 6% of the PTV to receive less than 95% of the planned dose for any given treatment day in these patients; the effect on rectal and bladder wall doses was greater and true doses may not be measurable through the use of only one treatment planning CT scan. Conclusions: The prostate is not a static organ, but rather has some limited motion in. the pelvis secondary to bladder and rectal volume changes. This motion has been quantified for a group of patients, and may provide a guide to further studies on the placement of field borders. (C) 1997 Elsevier Science Inc.