The Research Of The Algorithm In Planning Target Volume Of Nasopharyngeal Carcinoma By Image-guided Intensity Modulated Radiotherapy

Intensity modulated radiotherapy(IMRT)have been taken the lead in the clinical for nasopharyngeal carcinoma(NPC)in the United States in 1994,and this technology have also been carried out in most of radiotherapy centers in China in 2001.After many years of clinical practice,IMRT have been reported that it not only improved the curative effect of patients with NPC,but also improved the quality of life of patients in lots of papers.But the target of NPC is special,the exposure area is large,and the organ at risk(OAR)of the edge of the target are complicated.Therefore,the accuracy of the target contoured and the irradiated volume are the key to the IMRT.Image guided radiotherapy(IGRT)appeared was accompanied by the developed of IMRT.Image guided radiotherapy is an important tool to ensure the precise and accuracy.This study will establish a new calculation model which based on the classical Van Herk PTV model and the numbers of image guidance.The establishment of new model is based on IGRT for 30 NPC patients data which included 795 pretreatment positioning deviation data and 126 posttreatment positioning deviation data.The system error and random error of all patient were calculated based on the Van Herk calculation model.The 30 patients were divided into A、B、C groups on the basis of the 795 image guidance,respectively sampling 5-10times、10-15 times and more than 15 times the image guided placement deviation data.the system error and random error were calculated in which three groups,included the original positioning deviation of data and sampling data,and the pair t test were analyzed,the results with no significant statistical difference(P>0.05)finally.On the basis of the setup errors data,established A group PTV model:Ma=2.5Σa+0.7δa-1mm;B group PTV model:Mb=1/2(Ma+Mc);C group PTV model:Mc=2.5Σc+0.7δc+2mm。Taken a superposition plan to verify the accuracy of the three correction model.The superposition plan was used pretreatment postioning deviation data for the original plan,then got the actual positioning deviation in treatment.The data of isocenters were changed only,the other parameters of the plan remained the same,such as prescription dose,segment shape,segment weight,field irradiation,dose(MU),beam direction,organs at risk dose etc.The superposition plan(actual exposure times)and the original plan were compared.the dose deviation of each parameter was less than 5%.It is very important to select a appropriate PTV of IMRT for NPC.We study the global PTV and residual PTV as standard plans,which compared with the target and organ at risk dose of corresponding target areas.It could achieve a individual planning target volume with the strategy study of IGRT for NPC.Every time IGRT achieve the same therapeutic effect in reducing the number of IGRT.This will not only reduce the patient’s absorption of the additional dose to avoid unnecessary damage,but also to save the treatment time and the additional cost of treatment.It is great practicability to clinical application with the individual PTV calculation model.