SU-F-T-542: Using Calypso Electromagnetic Beacons to Determine When to Modulate and When Not to Modulate SBRT Treatment Plans When Breathing Motion Is Present




To use the Calypso system to conduct a comprehensive study to determine what is the maximum breathing amplitude and length of breathing cycle allowed to accurately deliver IMRT or VMAT plans for liver tumors treated with SBRT when breathing motion is present.


A phantom that simulates breathing motion was constructed using solid water. The phantom contains a 5 cm diameter insert of low density material to simulate a tumor, 3 calypso beacons to track breathing motion and a micro ion chamber for point dose measurements. Radiochromic films were used to measure spatial dose distribution. Gamma 3%, 3 mm metrics were used for plan evaluation. With phantom placed on a respiratory gating platform, 4DCT data sets were acquired for breathing amplitudes of 5, 10, 15, 20, 25, 30, and 40 mm respectively. Breathing cycles of 2, 4, and 6-seconds-long were used for each case. Minimum intensity projection (MINIP) datasets were generated for each case. Eclipse treatment planning system was used to generate IMRT and VMAT plans. Calypso system was used to gate all treatment deliveries.


Point dose measurement did not show significant difference for different breathing amplitude. This is because the ion chamber is placed at isocenter, and film analysis revealed most dose variations occur on the edges of the tumor. Gamma analysis shows significant reduction in passing rate for plans with motion due to MLC interplay effect and dose blurring. With 15 mm breathing amplitude, gamma passing rate dropped by 10% and 25% for IMRT and VMAT respectively.


With a breathing motion management, it is safe to modulate SBRT plans when breathing amplitude is less than 10 mm and 15 mm for IMRT and VMAT respectively. Longer breathing cycles lead to less accurate dose delivery. More analysis is being performed for breathing amplitudes greater than 15 mm.