Abstract
There are still uncertainties in delivering prostate IMRT plans accurately and safely due to inter- and intra-fractional prostate motion. The purpose of this study was to evaluate prostate motion based on fiducial markers using two modern online IGRT systems and to determine the optimum PTV margin for prostate IMRT. The IGRT system used in this study was a stereoscopic kV X-ray system or a kV cone beam CT (CBCT) system, both of which have automatic repositioning functions. To evaluate inter- and intra-fractional prostate motion, the pre- and post-treatment X-ray images automatically matched to bony structures and the implanted gold markers on digitally reconstructed radiographs constructed from the planning CT images were obtained using the X-ray IGRT system, and the pre- and post-treatment CBCT images were obtained using the CBCT IGRT system. At the beginning of treatment, the pre-treatment CBCT was manually matched to the planning CT images based on implanted 125I substituted for fiducial markers in this study. Daily prostate set-up error based on fiducial markers and the displacement of each patient's body and prostate during treatment were calculated using the images and movement data recorded on the IGRT systems through these procedures. Inter- and intra-fractional systematic (Σ) and random (σ) errors according to the ICRU-62 report and the optimum PTV margins by the equation of Stroom for prostate IMRT were evaluated using the calculated positioning data. The PTV margins for prostate IMRT were evaluated in 9 patients set-up with the X-ray IGRT system between July 2005 and September 2006, and in 15 patients set-up with the CBCT IGRT system in 2011. IMRT doses of 76 Gy/38 fractions and 45 Gy/25 fractions after 125I seed implantation therapy were delivered to the patients using the X-ray and CBCT IGRT systems, respectively. The daily pre- and post-treatment positioning data of each patient and prostate were clearly obtained through the two IGRT systems based on fiducial markers. On the X-ray IGRT system, the calculated PTV margins in this study were: vertical, 2.8 mm; longitudinal, 2.6 mm; and lateral, 2.5 mm. On the CBCT IGRT system, the corresponding values were 3.6 mm, 3.7 mm, and 2.7 mm. The two results were almost the same, although the data obtained from the CBCT IGRT system were slightly larger due to the uncertainty of manual CT image matching. The two modern IGRT systems provided clear data on inter- and intra-fractional prostate motion and the PTV margin for prostate IMRT. The results showed that a minimum of at least 4 mm is needed as the optimum PTV margin for prostate IMRT, even using fiducial settings.
Published Version
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