Impact of anatomical changes on dose distribution of intensity-modulated radiotherapy for nasopharyngeal carcinoma

Objective To observe the physique and anatomy changes in patients with nasopharyngeal carcinoma (NPC) during intensity-modulated radiotherapy (IMRT), using repeated CT images and deformable registration technique, and analyze their impact on delivery dose distribution.Methods Ten NPC patients were randomly selected from those who had received IMRT treatment.Gross tumor volume of nasopharyn (GTVnx), GTV of metastastatic lymph node (GTVnd), clinical target volume (CTV) and normal tissue or organ (OAR) were re-contoured on the in-course repeated CT images using a kind of deformable registration and auto-segmentation software according to the original planning contouring.Changes in volume of treatment targets and organs at risk were evaluated and the trends were then analyzed.Dose distributions were recalculated with repeated CT images and compared to the original plans.Results The volume of GTVnx were decreased by 6.44%,10.23% and9.72%(F=1.34,P=0.278) in the 2-,4-and 6-week after IM RT comparing with before IM RT, with 6.59%, 30.98 % and 35.13 % (F = 9.22, P =0.000) in GTVnd, 0.73%, 1.86% and 1.41% (F=0.33,P=0.722) in CTV1, -1.78%, -6.47%and -9.34% (F =16.89 ,P =0.000) in CTV2, 13.96%, 32.97% and 37.77%(F=17.17,P=0.000)in the left parotid , and 3.56% , 29.57% and 35.63% (F = 13.49 , P = 0.000) in the right parotid.The mean dose change rate of GTVnx were -0.39%, 0.08% and 0.32% (F =0.15 ,P =0.860) in the 2-,4- and 6-week after IMRT comparing with planning faction dose, with 0.53%, 1.19% and 0.69% (F=0.81,P=0.455) in GTVnd, 1.95%, 2.70% and 3.78% (F=0.61,P=0.552) in the spinal cord,0.32%, 0.81% and 0.62% (F=0.03,P=0.975) in the brain stem, 4.50%, 4.66% and 7.20% (F=0.33,P=0.725) in the left parotid, 2.20%, 7.17% and 7.12% (F= 1.24,P=0.306) in the right parotid.Conclusions The GTVnd, CTV2 and parotids shrinks obviously along with the treatment times for NPC patients during IMRT.Although changes in fraction dose of GTV, CTV, spinal cord, stem and parotids are not significant, further study with larger samples is needed. Key words: Nasopharyngeal neoplasms/intensity-modulated raliotherapy;  Anatomical change; Deformable registration;  Dosimetry