Does the planning dose-volume histogram represent treatment doses in image-guided prostate radiation therapy? Assessment with cone-beam computerised tomography scans

Purpose: 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 local protocol for bladder and rectal preparation. 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. Prostate dose coverage was assessed by the proportion of the CTV fully encompassed by the 95% and 98% isodose lines. Rectal and bladder volumes receiving 40 Gy, 60 Gy and 70 Gy at treatment were compared to the initial plan, with significance determined using the one-sample t-test. Results: Four patients showed marginally compromised CTV coverage by the 95% isodose at all CBCT plans. For nine patients the initial plan rectal DVH was significantly outside the range of the treatment DVH's. Conclusions: Dose coverage of the prostate was not achieved for all patients. Observed rectal and bladder doses were higher than predicted. The initial treatment plan cannot be assumed to represent accurate normal tissue doses.