Room: Room 202
Purpose: To suppress the motion induced dose errors in spot-scanning proton therapy (SSPT), the beam gating technique is used in our institute. It is based on the fiducial marker positions measured at the frame rate (FR) of 30 Hz, using fluoroscopy. Higher FR will increase the dose accuracy but at the same time it will increase the imaging dose. This study investigated the influence of changing the FR on dose accuracy in SSPT.
Methods: Treatment plans were made for 7 liver cancer patients with the prescription of 76 Gy (RBE) in 20 fractions. Scanning beam delivery was simulated for different FR values (1, 3.75, 7.5, 10, 15, 30 Hz), assuming the gating window size of Â±2 mm. Dose distribution was calculated with each respiratory phase of 4DCT, which is then summed up in the end-exhale 4DCT using deformable image registration. The size of the marker displacement from the planned position was evaluated for each FR, as well as D99 and D5-D95 for clinical target volume, and V20 for normal liver.
Results: Both D99 and D5-D95 were degraded with reducing the FR. However, at FR greater than or equal to 7.5 Hz, D99 was kept higher than 95% of the prescribed dose and D5-D95 was less than 7%, except for one patient. V20 was less than 30% in the same FR range for all patients. The average of the marker displacement was 1.2 mm for both 30 and 7.5 Hz FR.
Conclusion: With few exceptions, the dose accuracy in SSPT seems to be rather insensitive to changing the FR, as long as the FR is not so small (â‰¥7.5 Hz) and the gating window size is kept small enough.