Purpose: To evaluate the impact of shorter treatment times on intra-fractional motion of the prostate during external beam radiotherapy.
Methods: 53 h of intra-fractional motion of the prostate were recorded in real-time by 4D ultrasound (4DUS) during 720 fractions in 28 patients, 14 of which whom treated with step-and-shoot intensity-modulated radiotherapy (IMRT) and 14 of whom were treated with volumetric arc therapy (VMAT).
Results: The average VMAT fraction was recorded for 2 min 43 s and was substantially shorter than the average step-and-shoot IMRT fraction at 6 min 13 s. Average radial displacement of the prostate per fraction was substantially and significantly reduced from 1.31 ± 1.28 mm (n = 357 step-and-shoot IMRT fractions) to 0.96 ± 1.04 mm (n = 363 VMAT fractions), p = 0.00004. Radial, vertical, and longitudinal root-mean-square (r. m. s.) error per fraction was reduced from 1.55 to 1.12 mm (-28%, p < 0.0001), from 1.16 to 0.77 mm (-34%, p < 0.0001), and from 0.79 to 0.56 mm (-29%, p = 0.0002), respectively. Lateral intra-fractional motion was generally small and did not differ significantly. The prostate remained during 95% of fraction time within 4.55 mm of the isocenter in case of step-and-shoot IMRT and within 2.45 mm in case of VMAT. The variance of displacements increased linearly with time, and the rate was the same for both step-and-shoot IMRT and VMAT patients.
Conclusions: The position of the prostate changed less during shorter fractions, limiting fraction-average and end-of-fraction variance. This substantially and significantly reduced the impact of intra-fractional motion during shorter VMAT fractions as compared to longer step-and-shoot IMRT fractions.
Keywords: Organ motion; Prostate cancer; Radiotherapy, intensity-modulated; Ultrasound imaging; Volumetric-modulated arc therapy.