Intra-fraction Motion during Extreme Hypofractionated Radiotherapy of the Prostate using Pre- and Post-treatment Imaging

Aims: To determine intra-fraction displacement of the prostate during extreme hypofractionated radiotherapy using pre- and post-treatment orthogonal images with three implanted gold seed fiducial markers. Materials and methods: In total, 265 image pairs were obtained from 53 patients who underwent extreme hypofractionated radiotherapy to a dose of 35 Gy in five fractions on standard linear accelerators. Position verification was obtained with orthogonal X-rays before and after treatment and were used to determine intra-fraction prostate displacement. Results: The mean intra-fraction prostate displacements were -0.03 ± 0.61 mm (one standard deviation), 0.21 ± 1.50 mm and -0.86 ± 1.73 mm in the left-right, superior-inferior and anterior-posterior directions, respectively. The mean intra-fraction displacement during the first two fractions was moderately correlated with the displacement in the remaining three fractions, with correlation coefficients of 0.63 (95% confidence interval 0.43-0.77) and 0.47 (95% confidence interval 0.22-0.65) in the superior-inferior and anterior-posterior directions, respectively. There was no significant correlation in the left-right direction with a coefficient of -0.04 (95% confidence interval -0.31-0.23). Conclusions: The mean intra-fraction prostate displacement during a course of extreme hypofractionated radiotherapy is small. A strategy using the first two fractions to predict future displacements >5 mm warrants further validation.