Fifty-seventh annual meeting of the American association of physicists in medicine
SU-E-J-40: Improving Uncertainty of Prostate Positioning by Using Implanted Fiducial Marker-Based Hybrid Evaluation Combined with KV Portal Imaging and CBCT for Prostate IMRT
The aim of this work was to present our single-institution experience with fiducial marker-based hybrid evaluation of prostate positioning combined with orthogonal kV portal image which inform the central position of prostate and CBCT which provides the information on localization of the peripheral structures.
Twenty-five patients received prostate IMRT with daily localization by use of implanted fiducials. Orthogonal kV portal imaging and CBCT preceded all 977 treatments. After shifts in the AP, SI, and LR dimensions were made from kV imaging, following another shifts were made when more than 2-mm prostate deviation as total sum of all dimensions were detected by CBCT. Post-treatment kV imaging and CBCT were also obtained in 203 treatments (21%). Shifts between two techniques were analyzed, and compared by Bland-Altman limits of agreement. Intrafraction motion and PTV margin were determined from shifts between pre- and post-treatment in each technique.
Pre-treatment isocenter shift were made based on CBCT in addition to kV imaging in 315 (32%) treatments. Mean differences between kV imaging and CBCT in the AP/SI/LR were −0.10±0.93/− 0.06±0.91/0.15±0.51 mm. The correlation between these techniques was follows: R2 = 0.90, 0.93, 0.94. Bland-Altman 95% confidence intervals were −1.7 to 1.9/−1.7 to 1.8/−1.2 to 0.8 mm, without significant trends. Intrafraction motion for prostate obtained from pre- and post-CBCT were 0.00±0.15/0.02±1.49/0.15±0.64 mm, which were smaller than the results from kV imaging (0.42±1.90/0.12±1.98/0.26±0.80 mm). PTV margins calculated from the results of CBCT were also smaller than kV imaging (3.95 mm vs 5.15 mm, 4.02 mm vs 5.34 mm, 1.75 mm vs 2.18 mm).
Hybrid evaluation combined with identification of cranial and caudal edge of prostate by using CBCT compensates the uncertainty of prostate positioning originated from independent utilization of kV portal imaging, which enables to set PTV margins that reflect more realistic intrafraction organ motion.