Analysis of Dose Shifts Induced by Organ Movements during Treatment with TomoTherapy Using a Motion Phantom and GafChromic EBT Films

Physiological organ movements during radiotherapy are a limiting factor for the dose applied to the clinical target volume. In particular the use of highly conformal techniques like TomoTherapy (TomoTherapy Inc.) is considered controversial if significant motion is supposed to occur during treatment. A motor-driven motion phantom with a movable wagon was used in this work to simulate organ motion in anterior- posterior and cranio-caudal direction. Therefore we analyzed different tumor entities in our study. Treatment plans were created from 2.0 to 4.0 Gy dose per fraction for different tumor sites. Especially, Planning target volumes (PTV) close to organs at risk (OARs) were planned to measure the motion-induced shift of the dose gradient. This is important to assess the quality of verification in the static state of the phantom. Motion patterns with the same characteristics as those of 4D-CT patient data were simulated. Moreover, some methods of dose statistics were used to analyze the clinical suitability of a treatment plan. A major effect of motion on the dose distribution is dose blurring. Due to dose blurring the dose gradient decreases. Moreover, dose differences in the region of large parallel to motion direction situated dose gradients (mainly underdosages >10 %) were observed. The number of underdosages increased with the motion speed. Especially in the high-dose areas with characteristic ripples, dose differences with an underdosage of about > 10% (increasing with motion speed) were observed.