Accuracy of the planning dose volume histogram to predict delivered dose in image-guided prostate radiation therapy : assessment with cone beam computerised tomography scans

Full text: To assess the accuracy of the initial CT plan dose-volume histograms (DVH's) for prostate, rectum and bladder by comparison to delivered doses determined from cone beam CT (CBCT) scans acquired during image-guided treatment. Materials and Methods Twelve prostate patients were treated using daily implanted fiducial guidance and following a bladder and rectal preparation protocol. CBCT scans were acquired twice weekly and contoured for prostate, rectum and bladder. The planned beams were applied to all CBCT scans to determine the delivered doses. A uni form CTV to PTV margin of 7 mm was used. Prostate dose coverage was assessed by the proportion of the CTV fully encompassed by the 95 and 98% isodose lines. Rectal and bladder doses were assessed by the volume receiving at least 70 Gy (V70). The mean value of V70 at treatment was compared to the initial plan, with signi ficance dete mined using the one-sample t-test. Results CTV coverage was largely obtained with 99% of the prostate covered by the 95% isodose at all CBCT plans.