Research On Image Guided Adaptive Radiation Therapy For Nasopharyngeal Carcinama Based On The CT-ON-RAILS System

Intensity modulated radiation therapy or the multidisciplinary treatment with IMRT as the main treatment is the most effective method for the treatment of nasopharyngeal carcinoma.It can not only improve the survival rate and long-term efficacy of nasopharyngeal carcinoma patients,but also significantly reduce the dose of the surrounding normal tissues,and reduce the side effects of radiotherapy.However,the anatomic structure and dose distribution of the tumor and surrounding normal tissues during radiotherapy in patients with nasopharyngeal carcinoma can be significantly changed.If the treatment planning based on the localization CT images is lasted for 6-7weeks,it can not only cause the leakage of target,but also may cause additional complications.Therefore,it is necessary to adjust the radiotherapy plan in time according to the feedback information obtained in the course of radiotherapy.The research selected 16 NPC patients who get IMRT on the CT-ON-RAILS system.Make ensure that all patients throughout the course of treatment get three times daily verification CT a week at the same position of treatment on the CT-ON-RAILS system.On the verification CT images manually outline the nasopharyngeal primary tumor,lymph nodes and parotid glands and other important organs at risk,observed changes in their size and location.The original treatment plan will be ported to the verification CT images,re-calculation of the dose volume histogramsand to get the radiation dose changes of targets and organs at risk.To analysis how the receiving dose lead to the changes of volumes.The results showed that GTVs decreased throughout the course of IMRT,at a median rate of 0.22cm~3 per treatment day(range,0.02-0.86cm~3/d).In terms of the percentage of the initial volume,the GTVs decreased at a median rate of 2.06%per treatment day(range,1.35-2.58%/d).On the last day of treatment,this corresponded to a median total relative loss of 68.12% of the initial GTV(range,44.41-85.31%).In addition,the centre of mass of shrinking tumors changed position with time,indicating that GTV loss was frequently asymmetric.At treatment completion,the median centre of the mass displacement was 2.7mm(range,0-15.7mm).Parotid glands also decreased in volume(median,0.10cm~3/d;range,0.02-0.22 cm~3/d),and generally shifted medially(median,2.9mm;range,0-10.4mm)with time.At the end of treatment,the actual cumulative dose of parotid glands of the plans on the verification CT images wasincreased.Brainstem,spinal radiation receiving dose by 15% more than originally planned dose.The dose changes of the parotid glands correlated highly with the weight loss that occurred during treatment.The dose changes of GTV was not highly correlated.So it is concluded that measurable anatomic changes occurred thoughout IMRT process for NPC.These changes in the enternal contour,shape,and location of the target and critical structures resulting in changes in dose distribution,affect the accuracy of treatment.Especially for patients with significant anatomical volume changes,it is necessary to choose the right time to get CT scans and promptly re-design a treatment plan to reduce the radiation dose and radiation reaction by normal organs after 3-4 weeks of treatment.IMRT based on the CT-ON-RAILS system could access instant CT images of diagnostic level before treatment fused to CT images of the original plan,dynamic observation the changes of target and organs at risk in the anatomy and dose.To provides some basis for individual adaptive radiation therapy for NPC.