(Thursday, 8/2/2018) 7:30 AM - 9:30 AM
Room: Davidson Ballroom A
Purpose: To investigate the use of lateral head flexion to increase beam entrance angles and extend the available solid angle space for ring gantry stereotactic radiosurgery.
Methods: Seven healthy volunteers were scanned in a Philips 1.5T MRI scanner at three different head flexion positions: neutral, left flexion and right flexion. The three scans for each volunteer were co-registered using rigid registration and the rotational transformation extracted for each registration. The head pitch, roll, and yaw were computed to evaluate the natural range of motion for all volunteers. Two sets of single fraction SRS plans were computed for five datasets to 21Gy at the 70-80% isodose line. One coplanar set for head neutral scans, and a three arc set using the head neutral and head flexion scans. The conformity index (CI), gradient measure (GM) and R10 (V10%/V100%) were used to evaluate both sets of plans.
Results: The average pitch, roll, and yaw was 4.1°±4.7°, 16.9°±3.7°, and 2.54°±4.9° for right flexion and 4.9°±4.3°, 14.0°±3.7° and 2.8°±5.4° for left flexion. The CI decreased an average of 7.2% from 1.52 on the head neutral plans to 1.41 on the head flexion plans. The GM decreased 2.8% from 5.5 to 5.3 mm, and the R10 (V10%/V100%) decreased 19.9% from 61.1 to 48.9.
Conclusion: Lateral head flexion was shown to increase beam entrance angles improving conformity, dose gradient, and normal tissue sparing. Rigid registrations demonstrated each lateral flexion to be analogous to a 15° couch kick. Ring gantry radiotherapy devices like the Viewray MRIdian, Accuray Tomotherapy, Varian Halcyon, and many proton units are limited to deliver beams in a single axial plane, severely limiting beam entrance angles and rendering non-coplanar beam delivery impossible. The head flexion technique outlined here was shown to be a feasible solution for SRS treatments for such ring gantry devices.
Funding Support, Disclosures, and Conflict of Interest: Disclosures for Geoffrey Hugo: Research support from Varian, NIH
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