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Ballhausen, Hendrik; Li, Minglun; Ganswindt, Ute; Belka, Claus (2018): Shorter treatment times reduce the impact of intra-fractional motion. In: Strahlentherapie und Onkologie, Vol. 194, No. 7: pp. 664-674
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To evaluate the impact of shorter treatment times on intra-fractional motion of the prostate during external beam radiotherapy. 53h 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). The average VMAT fraction was recorded for 2min 43s and was substantially shorter than the average step-and-shoot IMRT fraction at 6min 13s. Average radial displacement of the prostate per fraction was substantially and significantly reduced from 1.31 +/- 1.28mm (n= 357 step-and-shoot IMRT fractions) to 0.96 +/- 1.04mm (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 1.12mm (-28%, p < 0.0001), from 1.16 to 0.77mm (-34%, p < 0.0001), and from 0.79 to 0.56mm (-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 4.55mm of the isocenter in case of step-and-shoot IMRT and within 2.45mm 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. 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.