Room to Move: Enhancing Head and Neck Radiotherapy With a Six-Degree-of-Freedom Couch

Purpose Image-guided radiation therapy (IGRT) allows for a reduction in target volume margins, but relies on accurate daily set-up. Six-degree-of-freedom couches offer rotational adjustments during alignment. Given the often large and irregular target volumes treated in head and neck (H&N) cancers, rotational errors may contribute significantly to inter-fraction variability in both target coverage and critical-structure dose. We seek to evaluate the impact of rotational adjustments on target coverage and critical-structure dose in H&N radiotherapy. Methods We reviewed 214 radiotherapy fractions for H&N cancer. Patients were treated using a six-degree-of-freedom couch that allows a maximum of ±3° of roll, pitch, and yaw, in addition to linear shifts. Daily couch adjustments were recorded, and the CT simulation images and treatment plan were imported into the oncology management system to allow fusion. The delivered dose was recalculated with and without rotational components, with deviation thresholds for planning target volume (PTV) V70Gy coverage taken to be 3% and 5%. Results A rotational shift in at least one plane was used on 212 of 214 treatment fractions. Without rotational shifts, PTV V100% was lower by a median of 1.97% (1.77%-2.18%; 95% CI) compared to the planned coverage. Specifically, 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 a ≥3% drop in PTV coverage required at least 1.5° rotational correction in at least one direction. Organs at risk (OAR) dose was less dependent on rotational correction, without significant correlations between the magnitude of rotational correction and increased OAR dose. Conclusion Rotational corrections are applied frequently when treating H&N cancers on a six-degree-of-freedom couch. Rotational adjustments had the greatest benefit on PTV coverage, while OAR doses were less dependent. These data highlight the utility of a six-degree-of-freedom couch for H&N radiotherapy and suggest a target set-up goal of less than 1.5° rotational error in all planes.