Room to Move: Enhancing Head and Neck Radiotherapy with a 6 Degree of Freedom Couch

Summary: Image guided radiation therapy (IGRT) has allowed for a reduction in target volume margins but relies on accurate daily set-up. Recent introduction of 6 degree of freedom (DOF) couches have allowed for rotational adjustments during alignment. We seek to evaluate the impact of rotational adjustments on both target coverage and critical structure dose in head and neck radiotherapy. We reviewed 214 fractions from 6 courses of definitive radiotherapy for head and neck cancer. All patients were planned for VMAT with thermoplastic mask immobilization. Patients were treated on a LINAC with a 6 DOF couch that allows a maximum of +/- 3 degrees of roll, pitch, and yaw, in 0.1 degree increments, in addition to linear shifts. Daily CBCT images and couch adjustments were recorded, and brought into the treatment planning software, where delivered dose was recalculated based on couch adjustments with and without rotational components. Deviation thresholds for PTV V70Gy coverage were 3% and 5%, in line with daily LINAC QA recommendations per AAPM TG-142. Rotational adjustment of at least one plane was used on 212 of 214 treatment fractions. 101 (47.2%) fractions required rotation of 2.1-3° on any plane, 77 (36%) fractions between 1.1-2°, and 34 (15.9%) fractions between 0.1 – 1°. In the absence of rotational shifts, PTV V100% was lower by a median of 1.97% (1.77% - 2.18%; 95% CI) compared to planned. 43 (20%) fractions had a decrease in PTV V100% of ≥3% when rotational shifts were not applied, and 10 (4.7%) fractions had a decrease in PTV V100% of ≥5%. All 43 fractions with ≥3% drop in PTV coverage had required at least 1.5° rotational correction in any direction. When degree of rotation remained 1.5° or less in all planes (68 fractions), PTV coverage never decreased by ≥3% (2.69% maximum). OAR dose was less dependent on rotational correction. Contralateral parotid Dmean had 96 fractions receiving >105% of planned, and 25 fractions >110%, with an overall average Dmean 103% of planned. Cord Dmax had 14 fractions >105%, and 1 fraction > 110%, with an average Dmax 100.6% of planned. Mandible Dmax had 1 fraction >105%, and none >110%, with an average 99.5% of planned. Rotational corrections are applied frequently when treating head and neck cancers with daily CBCT guided radiotherapy on a 6 DOF couch. Greatest rotational adjustments were made along the roll axis. Rotational adjustments had greatest benefit on PTV coverage, while OAR doses were less dependent. These data highlight the utility of a 6 DOF couch for head and neck radiotherapy, and suggest a target set-up goal of less than 1.5° rotational error in all planes, with emphasis on roll, when rotational shifts are not available. Peter Lee, MD, PhD (Presenting Author) - Fox Chase Cancer Center; Joseph Panetta, PhD (Co-Author) - Fox Chase Cancer Center; Thomas Galloway, MD (Co-Author) - Fox Chase Cancer Center