Abstract: Objective The study was to determine the effect of set-up errors on dosimetric evaluation of synchronous boost intensity-modulated radiotherapy（SIB-IMRT） in pancreatic cancer according to simulated systematic set-up errors by moving the isocentre. Methods SIB-IMRT plan were generated in 9 patients with pancreatic cancer. The effects of set-up errors of 3 and 5 mm were simulated in the treatment planning system by moving the isocentre and then recalculated the dose distribution without changing the field fluence distributions. Dose distribution of target volume and organs at risk were compared before and after the shift of isocentre. ResultsCompared with non-mobile group， a significant effect of set-up errors in all 6 directions were obtained for Dmean， Dminand V95of planning target volume of clinical target volume（PTV-C）， particularly more than 5mm（P＜0.05）. No significant change was obtained for Dmaxand V105（P＞0.05）. A significantly effect of set-up errors in the direction of caudal， cranial， superior were obtained for Dmeanof planning target volume of gross target volume（PTV-G）. Others has little effect（P＞0.05）. For clinical target volume（CTV） and gross target volume（GTV）， others’ set-up errors have little effect except for the directions of superior and caudal（P＞0.05）. For spinal cord Dmax， the greatest influence was from the posterior set-up errors（P＜005）.Except for the cranial set-up errors， bilateral renal dose distributions changed significantly（P＜0.05）. A significant effect of set-up errors in the direction of cranial，caudal and right were obtained for liver（P＜0.05）.
Conclusion For SIB-IMRT in pancreatic cancer， there was a little effect of set-up errors on dose distribution of target volume， but greater effect on organs at risk such as spinal cord，bilateral kidney and liver，This mentioned that the margin of organs at risk should be paid more attention when SIBIMRT in pancreatic cancer was worked out.