Optimal Patient Positioning (Prone Versus Supine) for VMAT in Gynecologic Cancer: A Dosimetric Study on the Effect of Different Margins

It is unknown whether the historically found dosimetric advantages of treating gynecologic cancer with the patient in a prone position with use of a small-bowel displacement device (belly-board) remain when volumetric arc therapy (VMAT) is used and whether these advantages depend on the necessary margin between clinical target volume (CTV) and planning target volume (PTV). The aim of this study is to determine the best patient position (prone or supine) in terms of sparing organs at risk (OAR) for various CTV-to-PTV margins and VMAT dose delivery. In an institutional review board-approved study, 26 patients with gynecologic cancer scheduled for primary (9) or postoperative (17) radiation therapy were scanned in a prone position on a belly-board and in a supine position on the same day. The primary tumor CTV, nodal CTV, bladder, bowel, and rectum were delineated on both scans. The PTVs were created each with a different margin for the primary tumor and nodal CTV. The VMAT plans were generated with our in-house system for automated treatment planning. For all margin combinations, the supine and prone plans were compared with consideration of all OAR dose-volume parameters but with highest priority given to bowel cavity V45Gy (cm(3)). For both groups, the prone position reduced the bowel cavity V45Gy, in particular for nodal margins ≥10 mm (ΔV45Gy = 23.9 ± 10.6 cm(3)). However, for smaller margins, the advantage was much less pronounced (ΔV45Gy = 6.5 ± 3.0 cm(3)) and did not reach statistical significance. The rectum mean dose (Dmean) was significantly lower (ΔDmean = 2.5 ± 0.3 Gy) in the prone position for both patient groups and for all margins, and the bladder Dmean was significantly lower in the supine position (ΔDmean = 2.6 ± 0.4 Gy) only for the postoperative group. The advantage of the prone position was not present if it needed a larger margin than the supine position. For patients with gynecologic cancer, the historically found dosimetric advantages for the prone position remain for modern dose delivery techniques if large margins are needed. However, the advantage is lost for small margins and if the prone position needs a larger margin than the supine position