Prostatic displacement during extreme hypofractionated radiotherapy using volumetric modulated arc therapy (VMAT)

Background: To determine prostate displacement during extreme hypofractionated volume modulated arc radiotherapy (VMAT) using pre-and post-treatment orthogonal images with three implanted gold seed fiducial markers. Methods: A total of 150 image pairs were obtained from 30 patients who underwent extreme hypofractionated radiotherapy to a dose of 40 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 prostate displacements were 0.03 +/- 1.23 mm (1SD), 0.18 +/- 1.55 mm, and 0.37 +/- 1.95 mm in the left-right, superior-inferior, and anterior-posterior directions, respectively. The mean 3D displacement was 2.32 +/- 1.55 mm. Only 6 (4%) fractions had a 3D displacement of > 5 mm. The average time of treatment delivery for a given fraction was 195 +/- 59 seconds. Conclusions: The mean intra-fraction prostate displacement during a course of extreme hypofractionated radiotherapy delivered via VMAT, continues to be small. Clinical margins typically used in a similar fixed-angle IMRT treatment are adequate. The use of VMAT in further extreme hypofractionation may limit prostatic motion uncertainties that would be otherwise be associated with longer treatment times.