Influence Of Tumor,Anatomical Structure Changes During IMRT For Patients With NPC On Dose Distribution

Objective:To observe the changes of the tumor,anatomical and explore the impact of dose distribution on the changes by weekly Cone Beam Computer Tomography(CBCT)scan,during intensity modulated radiation therapy(IMRT)for patients with nasopharyngeal carcinoma(NPC).Methods:The linear accelerator with CBCT was used to gather CBCT images weekly during IMRT for patients with NPC.The dynamic changes of volume and position for GTVnx,GTVnd,body contour,parotid and submandibular gland were analyzed.CT was performed again at the 21 st fraction and the anatomical structure was contoured again.Beams of plan based on the initial CT were imported into the new scanned CT,and TPS was used to calculate the changes of dose at PGTVnx,PGTVnd,parotid gland,submandibular gland,spinal cord and brainstem because of anatomical changes.Rusults:The volume of GTVnx,GTVnd,body contour,parotid and submandibular gland were reduced and the location were changed significantly.The rate of changes were obvious at the 21 st time radiotherapy.All changes of volume were positively related to its initial size besides GTVnx.At the 21 st time,due to the changes of anatomical structure,dose of PGTVnd(D95)was reduced of 169.55 cGy on average,parotid(Dmean)was increased of 175.70 cGy,147.45 cGy,submandibular gland(Dmean)was increased of 391.15 c Gy,479.25 cGy,and spinal cord(Dmax)was increased of 68.55 cGy.The differences were statistically significant(P<0.05).There were no obvious changes for dose of PGTVnx(D95)and brainstem(P>0.05).Change of body contour volume was positively correlated to the change of parotid gland and GTVnd volume(P<0.05),and also have significant correlation to the changes of PGTVnd,parotid gland,submandibular gland and spinalcord dose(P<0.05).Conclusions:During radiotherapy,the volume of anatomic structure is reduced and the position is also changed.This changes lead to the reduction in dose of PGTVnd and increase of parotid gland,submandibular gland and spinal cord.Change of the body contour can reflect the volume and dose changes of target area and organs at risk to a certain extent,which is a sensitive observation index during IMRT for NPC.