Optimal patient positioning (prone versus supine) for VMAT in gynecological cancer: a dosimetric study on the effect of different margins

Publication date: Available online 8 June 2016
Source:International Journal of Radiation Oncology*Biology*Physics
Author(s): S.T. Heijkoop, G.H. Westerveld, N. Bijker, R. Feije, A.W. Sharfo, N. van Wieringen, J.W.M. Mens, L.J.A. Stalpers, M.S. Hoogeman
Purpose/ objectiveIt is unknown whether the historically found dosimetric advantages of treating gynecological cancer in prone position using a small-bowel displacement device (bellyboard) remain when using VMAT and if they depend on the needed CTV-to-PTV margin. The aim is to determine the best patient position (prone or supine) in terms of OAR sparing for various CTV-to-PTV margins and VMAT dose delivery.Material and MethodsIn an IRB approved study, 26 patients with gynecological cancer scheduled for primary (9) or postoperative (17) radiotherapy were scanned in prone position on a bellyboard and in supine position at the same day. The primary tumor Clinical Target Volume (CTV), nodal-CTV, bladder, bowel and rectum were delineated on both scans. Planning Target Volumes (PTVs) were created each with a different margin for the primary tumor- and nodal CTV. VMAT plans were generated using our in-house system for automated treatment planning. For all margin combinations supine and prone plans were compared considering all OAR dose-volume parameters, but giving highest priority to bowel cavity V45Gy (cm³).ResultsFor both groups, prone position reduced the bowel cavity V45Gy, in particular for nodal margins ≥10 mm (ΔV45Gy=23.9±10.6 cm³). However, for smaller margins the advantage was much less pronounced (ΔV45Gy=6.5±3.0 cm³) and did not reach statistical significance. The rectum Dmean was significantly lower (ΔDmean=2.5±0.3 Gy) in prone position for both patient groups and for all margins, while the bladder Dmean was significantly lower in supine (ΔDmean=2.6±0.4 Gy), only for the postoperative group. The advantage for prone was not present if prone needs a larger margin than supine position.ConclusionFor patients with gynecological cancer the historically found dosimetric advantages for prone position remain for modern dose delivery techniques if large margins are needed. However, the advantage is lost for small margins and if prone position needs a larger margin than supine.

Teaser

Institutes treat gynecological cancer patients in prone position using a small-bowel displacement device justified by historically found dosimetric advantages in 3DCRT and IMRT using large CTV-to-PTV margins. In this treatment planning study we compared dose to OARs in prone and supine position for various CTV-to-PTV margins and using VMAT-dose delivery. We found a significant advantage for prone setup in terms of small-bowel sparing if large margins are required, but the advantage was lost for smaller margins.


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