Dosimetric evaluation of image-guided radiation therapy for prostate cancer

The aim of this study was to investigate the dosimetric accuracy of imaged-guided radiation therapy for prostate patients using the in-room computed tomography (CT) target localization technique. A Siemens CT-on-rails system was used for patient setup and target localization for intensity-modulated radiation therapy (IMRT) of prostate cancer. Fifteen previously treated prostate patients were included in this retrospective study. CT-on-Rails scans were performed before and after the IMRT treatment under local IRB approval. A total of 15 original simulation CT scans and 98 post-treatment CT scans were contoured by the same oncologist to delineate the prostate target, bladder, and rectum. IMRT plans were generated on the original simulation CTs and the same MUs and leaf sequences were used to compute the dose distributions using post-treatment CTs. Varian Velocity deformable registration was used for the summation of the fractional doses and the cumulative doses were compared with the planned doses. For the 15 patients investigated, the mean isocenter shift was up to 4.0 mm in the left-right direction, 5.9 mm in the anterior-posterior direction and 5.6 mm in the superior-inferior direction due to interfractional organ motion. The mean rectal volume varied from 0.6 to 1.73 times and the mean bladder volume varied from 0.59 to 3.65 times between simulation and the end of treatment. The prescription dose to 95% of the PTV, D_p, was set to 76 Gy for all treatment plans. The dose to 95% of the clinical treatment volume (CTV), D₉₅, was 74.0 to 77.6 Gy and the minimum CTV dose, D_min, was 61.0 to 71.6 Gy, respectively, in the cumulative dose distributions. Detailed analyses showed that 7.1% of the treatment fractions had cold spots (< 85% of D_p) in the peripheral CTV, leading to D_min < 64 Gy in the cumulative dose distributions for 4 patients. The rectal dose-volume constraints were violated in 35.7% of the treatment fractions while the bladder dose was much improved in 82.7% of the treatment fractions. The current IGRT procedure for patient setup and target localization using rigid-body registration based on contour/anatomy matching is effective for population-based PTV margins. For a small group of patients, specific PTV margins and/or real-time target monitoring/tracking will be necessary due to significant prostate deformation/rotation caused by inter- and intrafractional bladder and rectal volume variation.