Patient positioning in prostate radiotherapy: Is prone better than supine?

Purpose: To assess potential dose reductions to the rectum and to the bladd er with three-dimensional conformal radiotherapy (3D-CRT) to the prostate i n the prone as compared with the supine position; and to retrospectively ev aluate treatment position reproducibility without immobilization devices. Methods and Materials: Eighteen patients with localized prostate cancer und erwent pelvic CT scans and 3D treatment planning in prone and supine positi ons. Dose-volume histograms (DVHs) were constructed for the clinical target volume, the rectum and the bladder for every patient in both treatment pos itions. "Comparative DVHs" (cDVHs) were defined for the rectum and for the bladder: cDVH was obtained by subtracting the organ volume receiving a give n dose increment in the prone position from the corresponding value in the supine position. These values were then integrated over the entire dose ran ge. The prescribed dose to the planning target volume (PTV) was 74 Gy using a 6-field technique; To evaluate reproducibility, portal films were subseq uently reviewed in 12 patients treated prone and 10 contemporary patients t reated supine (controls). No immobilization devices were used. Deviations i n the anterio-posterior (X) and cranio-caudal (Y) axes were measured. Mean treatment position variation, total setup variation, systematic setup varia tion, and random setup variation were obtained. Results: Prone position was associated with a higher dose to the rectum or to the bladder in 6 (33%) and 7 (39%) patients, respectively. A simultaneou sly higher dose to rectum and bladder was noted in 2 (11%) patients in pron e and in 7 (39%) patients in supine. Rectal and bladder volumes were freque ntly larger in prone than in supine: mean prone/supine volume ratios were 1 .21 (SD, 0.68) and 1.03 (SD, 1.32), respectively. In these cases cDVH analy sis more often favored the prone position. Mean treatment position variatio n and total setup variation were similar for both prone and supine plans. A higher systematic setup variation was observed in prone positioning: 2.7 m m vs. 1.9 mm (X axis) and 4.1 mm vs. 2.2 mm (Y axis). The random variation was similar for both prone and supine: 4.0 mm vs. 3.6 mm (X axis) and 3.7 m m vs. 3.6 mm (Y axis). Conclusions: Prone position 3D-CRT is frequently, but not always, associate d with an apparent dose reduction to the rectum and/or to the bladder for p rostate cancer patients. As suggested by the increased mean prone/supine re ctal volume ratio, the advantage of prone positioning for the rectum may be artifactual, at least partly reflecting a position-dependent rectal air vo lume, which may significantly vary from treatment to treatment. In the abse nce of immobilization devices, daily setup reproducibility appears less acc urate for the prone position, primarily due to systematic setup variations. (C) 2000 Elsevier Science Inc.