Internal organ motion in prostate cancer patients treated in prone and supine treatment position

Background and Purpose: To compare supine and prone treatment positions for prostate cancer patients with respect to internal prostate motion and thr required treatment planning margins. Materials and methods: Fifteen patients were treated in supine and fifteen in prone position. For each patient, a planning computed tomography (CT) sc an was used for treatment planning. Three repeat CT scans were made in week s 2, 4, and 6 of the radiotherapy treatment. Only for the planning CT scan, laxation was used to minimise the rectal content. For all patients, the cl inical target volume (CTV) consisted of prostate and seminal vesicles. Vari ations in the position of the CTV relative to the bony anatomy in the four CT scans of each patient were assessed using 3D chamfer matching. The overa ll variations were separated into variations in the mean CTV position per p atient (i.e. the systematic component) and the average 'day-to-day' variati on (i.e. the random component). Required planning margins to account for th e systematic and random variations in internal organ position and patient s et-up were estimated retrospectively using coverage probability matrices. Results: The observed overall variation in the internal CTV position was la rger for the patients treated in supine position. For the supine and prone treatment positions, the random components of the variation along the anter ior-posterior axis (i.e, towards the rectum) were 2.4 and 1.5 mm (1 standar d deviation (1 SD)), respectively; the random rotations around the left-rig ht axis were 3.0 and 2.9 degrees (1 SD). The systematic components of these motions (1 SD) were larger: 2.6 and 3.3 mw, and 3.7 and 5.6 degrees, respe ctively. The set-up variations were similar for both treatment positions. D espite the smaller overall variations in CTV position for the patients in p rone position, the required planning margin is equal for both groups (about 1 cm except for 0.5 cm in lateral direction) due to the larger impact of t he systematic variations. However, significant time trends cause a systemat ic ventral-superior shift of the CTV in supine position only. Conclusions: For internal prostate movement, it is important to distinguish systematic from random variations. Compared to patients in supine position , patients in prone position had smaller random but somewhat larger systema tic variations in the most important coordinates of the internal CTV positi on. The estimated planning margins to account for the geometrical uncertain ties were therefore similar for the two treatment positions. (C) 1999 Elsev ier Science Ireland Ltd. All rights reserved.