Purpose: Studies have shown that tumor motion during radiotherapy can cause reduced target dose coverage and increased healthy tissue exposure. However, research mainly focuses on translational motion and dosimetric analysis has been limited to simulation. This investigation aims to experimentally evaluate the dosimetric impact of intrafraction prostate rotations during Stereotactic Body Radiotherapy (SBRT) treatment using film measurements.
Methods: Four clinically recorded 6 degree-of-freedom (DoF) intrafraction prostate motion traces and four 7.25Gy/Fx SBRT treatment plans were selected for the experiment. The traces covered typical patterns including small-angle rotations (â‰¤3Â°), transient movement, persistent excursion (5Â°) and erratic rotations (â‰¥6Â°). Gafchromic film placed inside a custom-designed phantom was held by a 6 DoF robotic arm (high precision, up to 0.1mm and 0.2Â°) at the linacâ€™s isocenter for measurement during treatment plan delivery. For each combination of the trace and plan, two film measurements were made while the robot reproduced 6 DoF motion and only 3D translations of the trace respectively. A static measurement of the plan was obtained as the reference film. The pass rate in a 2% relative dose/2 mm distance gamma test was evaluated for each measured film, using the reference filmâ€™s PTV average as reference dose and a 90% test threshold.
Results: For each of the sixteen plan and trace pairs, a reduction of the gamma pass rate was observed for 6DoF motion compared with only 3D translations. The mean reduction was 5.8%, with a standard deviation of 7.1% and a range from 0.2% to 25.8%. The average gamma pass rate was 80.7% for 6DoF motion and 86.5% for translation only.
Conclusion: For the first time, the dosimetric impact of intrafraction prostate rotations during SBRT treatment was measured experimentally. Results proved rotational tumor motion would cause degradation to dose delivery accuracy beyond the tolerance level of 2%/2 mm.
Funding Support, Disclosures, and Conflict of Interest: We acknowledge funding from NHMRC and Cancer Institute NSW fellowships. This work is supported by an Cancer Australia grant.