Quantification and predictors of prostate position variability in 50 patients evaluated with multiple CT scans during conformal radiotherapy

Purpose: To determine the extent and predictors for prostatic motion in a l arge number of patients evaluated with multiple CT scans during radiotherap y, and evaluate the implications of these data on the design of appropriate treatment margins for patients receiving high-dose three-dimensional confo rmal radiotherapy. Materials and methods: Fifty patients underwent four serial computerized to mography (CT) scans, consisting of an initial planning scan and subsequent scans at the beginning, middle, and end of the treatment course. Each scan was performed with the patient in the prone treatment position within an im mobilization device used during therapy. Contours of the prostate and semin al vesicles were drawn on the axial CT slices of each scan, and the scans w ere matched by alignment of the pelvic bones with a chamfer matching algori thm. Using the contour information, distributions of the displacement of th e organ center of mass and organ border from the planning position were det ermined separately for the prostate and seminal vesicles in each of the thr ee principle directions: anterior-posterior (AP), superior-inferior (SI) an d left-right (LR). Each distribution was fitted to a normal (Gaussian) dist ribution to determine confidence limits in the center of mass and border di splacements and thereby evaluate for the optimal margins needed to contain target motion. Results: The most common directions of displacement of the prostate center of mass (COM) were in the AP and SI directions and were significantly large r than any LR movement. The mean prostate COM displacement ( +/- 1 standard deviation, SD) for the entire population was -1.2 +/- 2.9 mm, -0.5 +/- 3.3 mm and -0.6 +/- 0.8 mm in the, AP and SI and LR directions respectively (n egative values indicate posterior, inferior or left displacement). The mean ( +/- 1 SD) seminal vesicle COM displacement for the entire population was -1.4 +/- 4.9 mm, 1.3 +/- 5.5 mm and -0.8 +/- 3.1 mm in the AP and SI and L R directions, respectively. The data indicate a tendency for the population towards posterior displacements of the prostate from the planning position and both posterior and superior displacements of the seminal vesicles. AP movement of both the prostate and seminal vesicles were correlated with cha nges in rectal volume (P = 0.0014 and < 0.0001, respectively) more than wit h changes in bladder volume (P = 0.030 for seminal vesicles and 0.19 for pr ostate). A logistic regression analysis identified the combination of recta l volume > 60 cm(3) and bladder volumes > 40 cm(3) as the only predictor of large ( > 3 mm) systematic deviations for the prostate and seminal vesicle s (P = 0.05) defined for each patient as the difference between organ posit ion in the planning scan and mean position as calculated from the three sub sequent scans. Conclusions: Prostatic displacement during a course of radiotherapy is more pronounced among patients with initial planning scans with large rectal an d bladder volumes. Such patients may require more generous margins around t he CTV to assure its enclosure within the prescription dose region. Identif ication and correction of patients with large systematic errors will minimi ze the extent of the margin required and decrease the volume of normal tiss ue exposed to higher radiation doses. (C) 1999 Elsevier Science Ireland Ltd . All rights reserved.