Room: AAPM ePoster Library
Purpose: For stereotactic brain radiosurgery(SRS), there are multiple options available for imaging and patient immobilization. It is important to use clinical data to quantify the setup and motion error to ensure the correct PTV margin specific to imaging and immobilization technique is used. The purpose of the study is to validate the current PTV margin and to determine the effectiveness of using a bite block.
Methods: A total of 54 patients (180 total fractions, 25 patients with bite block and 29 patients without bite block) were retrospectively analyzed. The prescription dose was 24-30 Gy in 3-5 fractions using 2 coplanar VMAT arcs. A GTV to PTV margin of 2mm was used for treatment planning. The typical brain SRS workflow was as follows: daily ExacTrac X-ray image pairs were co-registered to DRRs generated from the planning CT data set using automated software. The shifts were applied to a 6D robotic couch and images were re-acquired to verify correct positioning(post-correction). ExacTrac image pairs are re-acquired after each arc and the tolerance for patient re-positioning was 1° and 1mm. The setup error was evaluated using post-correction data and the difference between post-correction data and data from the last monitoring image was used to determine the intrafractional motion. The van Herk formula (2.79?+0.7s) was used to obtain the PTV margin.
Results: The setup and motion error for the 54 patients undergoing brain SRS was 2.5mm translation: 1.5mm(X), 1.5mm(Y) and 1.4mm(Z). For patients with a bite block it was 1.9mm: 1.2mm(X), 1.1mm(Y) and 1.0mm(Z), and for patients with a bite block it was 3.2mm: 1.9mm(X), 1.9mm(Y) and 1.7mm(Z).
Conclusion: The current PTV margin scheme is valid and using a bite block can help reduce setup error and intrafractional motion. However, before reducing margins, a clinical follow-up study is also needed to perform.