Daily online bony correction is required for prostate patients without fiducial markers or soft-tissue imaging.

AIM: To compare online position verification strategies with offline correction protocols for patients undergoing definitive prostate radiotherapy.

MATERIALS AND METHODS: We analysed 50 patients with implanted fiducial markers undergoing curative prostate radiation treatment, all of whom underwent daily kilovoltage imaging using an on-board imager. For each treatment, patients were set-up initially with skin tattoos and in-room lasers. Orthogonal on-board imager images were acquired and the couch shift to match both bony anatomy and the fiducial markers recorded. The set-up error using skin tattoos and offline bone correction was compared with online bone correction. The fiducial markers were used as the reference.

RESULTS: Data from 1923 fractions were analysed. The systematic error was ≤1 mm for all protocols. The average random error was 2-3mm for online bony correction and 3-5mm for skin tattoos or offline-bone. Online-bone showed a significant improvement compared with offline-bone in the number of patients with >5mm set-up errors for >10% (P<0.001) and >20% (P<0.003) of their fractions.

CONCLUSIONS: Online correction to bony anatomy reduces both systematic and random set-up error in patients undergoing prostate radiotherapy, and is superior to offline correction methods for those patients not suitable for fiducial markers or daily soft-tissue imaging.