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Background. In radiotherapy, the magnitude of respiratory-induced tumor motion is often measured using a single four-dimensional computed tomography (4DCT). This magnitude is required to determine the internal target volume. The aim of this study was to compare the magnitude of respiratory-induced motion of pancreatic tumors on a single 4DCT with the motion on daily cone beam CT (CBCT) scans during a 3 -5-week fractionated radiotherapy scheme. In addition, we investigated changes in the respiratory motion during the treatment course. Material and methods. The mean peak-to-peak motion (i.e. magnitude of motion) of pancreatic tumors was measured for 18 patients using intratumoral gold fi ducials visible on CBCT scans made prior to each treatment fraction (10 -27 CBCTs per patient; 401 CBCTs in total). For each patient, these magnitudes were compared to the magnitude measured on 4DCT. Possible time trends were investigated by applying linear fi ts to the tumor motion determined from daily CBCTs as a function of treatment day.Results. We found a signifi cant (p Յ 0.01) difference between motion magnitude on 4DCT and on CBCT in superiorinferior, anterior-posterior and left-right direction, in 13, 9 and 12 of 18 patients, respectively. In the anteriorposterior and left-right direction no fractions had a difference Ն 5 mm. In the superior-inferior direction the difference was Ն 5 mm for 17% of the 401 fractions. In this direction, a signifi cant (p Յ 0.05) time trend in tumor motion was observed in 4 of 18 patients, but all trends were small ( Ϫ 0.17 -0.10 mm/day) and did not explain the large differences in motion magnitude between 4DCT and CBCT. Conclusion. A single measurement of the respiratory-induced motion magnitude of pancreatic tumors using 4DCT is often not representative for the magnitude during daily treatment over a 3 -5-week radiotherapy scheme. For this patient group it may be benefi cial to introduce breath-hold to eliminate respiratory-induced tumor motion.

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