Room: AAPM ePoster Library
Purpose: To assess the measured dose accuracy versus calculation when using 4D CBCT to localize a mobile target subject to motion which varies from that at the time of simulation.
Methods: A treatment plan was developed using pinnacle after acquiring a 4D CT of the CIRS dynamic thorax phantom prescribing 50 Gy over five fractions. The phantom was programmed with a previous SBRT lung patient’s respiratory motion using a 2 cm target. The plan was used to assess the accuracy of dose delivery using 4D CBCT to align the target using the planned treatment motion and an unplanned motion. The unplanned motion remained the same as planned with the addition of 1 mm and 2 mm of amplitude in the superior/inferior and lateral directions respectively. Measured dose was obtained by positioning a Standard Imaging A16 ionization chamber in the target using a phantom insert. The measurement was repeated five times and compared with the calculated dose of 1073 cGy obtained by the mean dose to a contour in the treatment plan representing the ion chamber.
Results: The mean dose percent difference between the planned motion and unplanned motion from the calculated dose was 0.8% and 2.3% respectively. Similarly, the 95% confidence interval expressed as percent difference between plans with calculated dose was [0.0%,1.5%] for planned motion and [0.8%,3.8%] for unplanned motion. Lastly, mean dose was significantly closer to calculated dose for planned motion (M=1081.28,SD=6.36) compared to unplanned motion (M=1097.63,SD=12.85), p=0.04.
Conclusion: The use of 4D CBCT for localization of mobile targets performs well for motion of that during 4D CT simulation. When additional motion was added to the target, dose accuracy appears to be comprised.