• William Y Song, Bernard Chiu, Glenn S Bauman, Michael Lock, George Rodrigues, Robert Ash, Craig Lewis, Aaron Fenster, Jerry J Battista, and Jake Van Dyk.
• Radiation Treatment Program, London Regional Cancer Program, London Health Sciences Centre, London, Ontario, Canada.
• Int. J. Radiat. Oncol. Biol. Phys. 2006 Jun 1; 65 (2): 595-607.

PurposeTo evaluate the image-guidance capabilities of megavoltage computed tomography (MVCT), this article compares the interobserver and intraobserver contouring uncertainty in kilovoltage computed tomography (KVCT) used for radiotherapy planning with MVCT acquired with helical tomotherapy.Methods And MaterialsFive prostate-cancer patients were evaluated. Each patient underwent a KVCT and an MVCT study, a total of 10 CT studies. For interobserver variability analysis, four radiation oncologists, one physicist, and two radiation therapists (seven observers in total) contoured the prostate and seminal vesicles (SV) in the 10 studies. The intraobserver variability was assessed by asking all observers to repeat the contouring of 1 patient's KVCT and MVCT studies. Quantitative analysis of contour variations was performed by use of volumes and radial distances.ResultsThe interobserver and intraobserver contouring uncertainty was larger in MVCT compared with KVCT. Observers consistently segmented larger volumes on MVCT where the ratio of average prostate and SV volumes was 1.1 and 1.2, respectively. On average (interobserver and intraobserver), the local delineation variability, in terms of standard deviations [Deltasigma = radical(sigma2MVCT-sigma2KVCT)], increased by 0.32 cm from KVCT to MVCT.ConclusionsAlthough MVCT was inferior to KVCT for prostate delineation, the application of MVCT in prostate radiotherapy remains useful.

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