The accuracy of surface imaging in detecting secondary patient motion caused by pitch and roll corrections in pelvic radiotherapy.

Background And Objective: Correction of rotational setup errors by tilting the treatment couch improves target dose accuracy and prevents healthy tissue overdosage in external beam radiotherapy. However, couch tilts may cause secondary patient motion. This study aimed to quantify the secondary motion caused by pitch and roll corrections and to evaluate the feasibility of surface imaging for detecting the secondary motion in pelvic radiotherapy.

Methods: Setup cone-beam computed tomography images and surface imaging data of 337 fractions of 22 patients were retrospectively collected. The secondary motion was quantified by the residual setup deviations observed in verification cone-beam computed tomography images acquired in 91 fractions with >1° pitch or roll corrections. The secondary motion observed in the surface imaging data was compared to that observed in the cone-beam computed tomography images in 73 fractions.

Results: Average residual errors of 0.7 mm/1° (R=0.95) and -1.3 mm/1° (R=-0.87) were observed in longitudinal and lateral directions after pitch and roll corrections, respectively. The 95% limits of agreement between surface imaging and cone-beam tomography imaging residual errors were (-2.2, 1.5) mm in longitudinal direction and (-1.3, 1.1) mm in lateral direction.

Conclusion: Pelvic radiotherapy patients seemed to have shifted downwards on the treatment couch during couch tilts. The secondary motion could be accurately detected by surface imaging, although respiratory motion reduced the accuracy in longitudinal direction.