摘要：Background This study aimed to evaluate the dosimetric effect of anatomical and volumetric changes in tumour size on target volumes (TVs) and organ at risks (OARs) during intensity-modulated radiotherapy (IMRT), and to compare the dose distribution with or without replanning in locally advanced head and neck cancer (LAHNC). Methods 20 patients with LAHNC treated by IMRT were enrolled in a prospective study. Planning computed tomography (CT) scans were acquired both pre-treatment and after 20 fractions (midcourse) of radiotherapy. An original treatment plan based on initial planning CT scan was generated and implemented. A hybrid plan was generated for each patient by applying beam configuration of the original plan to the anatomy of the midcourse CT scan. The dose-volume histograms of the original plan and hybrid plan were compared for second phase of treatment. A midcourse adaptive replan based on the interim CT scan was done, and dose distribution for TVs and OARs was compared with or without replanning. Findings Planned doses (D98%, D95%, and D2%) to TVs for second phase of radiotherapy (hybrid plan) were not significantly different from the actually delivered doses. However, an increase in volume by 110% was observed in the HPLAN in comparison with the original plan. With replanning, dose distribution to TV was made more homogenous as compared with the hybrid plan (p = 0.00). Re-planning significantly reduced V > 110% and V < 93% as compared with the hybrid plan (p < 0.05). There was a significant increase in delivered doses (Dmax and D2%) to spinal cord as compared with the planned dose (p < 0.05). Adaptive replanning resulted in decrease in Dmax and D2% of spinal cord (p < 0.05). The mean dose to ipsilateral parotid was significantly higher in the hybrid plan in comparison with the original plan (p < 0.05). On comparing the hybrid plan and adaptive replan, dose to ipsilateral parotid was significantly reduced with replanning. Interpretation Replanning with midcourse CT simulation significantly improves the TV coverage and reduces the dose to OARs.