Influence Factors And Analysis Of The Bladder And Rectum Dose During Intensity Modulated Radiation Therapy For Cervical Cancer

ObjectiveTo analyse impacts of tumor variation and organs at risk(bladder and rectum)filling on target volume and bladder and rectum for definitive treatment of cervix cancer with helical tomotherapy and guide clinical practice.MethodsThis study selected twenty cervical patients for definitive treatment with helical tomotherapy from September 2013 to August 2017 in general hosipital of Beijing miltary region(called PLA Army General Hosipital since May 2016).Patients underwent CT simulation in the supine position with custom immobilization.The planning kilo voltage computed tomography(KVCT)images were sent to a Pinnacle 9.2 station,and contours for gross target volume(GTV),clinical target volume(CTV),planning target volume(PTV)and bladder and rectum were drawn.The CT images and contours were sent to a helical tomotherapy planning 4.1.2.2 system and the treattnent plans were optimized then.After the plans passed the validation of Delta 4 system,the patients began receiving treatment.Then an megavoltage computed tomography(MVCT)scan was performed,which were automatically registered with the planning KVCT images before the manual registration was taken by the radiation therapists in order to correct the patients’ set up errors.The KVCT images and the MVCT images of the first and third day of each week since the start of treatment were chosen,and dose distributions were recalculated on the MVCT images,which were the actual dose of each fraction.Each single dose distribution and the corresponding CT image were transferred to software(MIM 6.5),and deformable image registration was performed to the CT images,and then the sum of actual dose was acquired by accumulating the single dose together.The bladder and rectum were contoured on the MVCT images according to the initial treatment plans with the same principles.The dose distribution of targets and bladder and rectum of Planl and Plan2 were compared on DVH.Data analysis was performed using an SPSS 20.0Results1.Tumor regressionWith the increase of radiation dose,the volume of tumor gradually narrowed and decreased significantly at 11～16 times,narrowing more than 10%,and slowing down after 16 times.At the end of the external irradiation,the average tumor decreased by about 70%(Table 1).The bladder and rectum displacement in Planl and Plan2 was associated with tumor contraction,with the head,abdomen and back in the direction(P<0.05).During the radiotherapy,there was a positive correlation between Plan-1 and Plan-2 bladder and rectal dose parameters and tumor regression(P<0.05).When the tumor withdrew around 40～60%,the V45 level of bladder was significantly higher than that of the prescription bladder and rectum limit(P<0.05)(Table 5).2.bladder and rectum volume variationIn the course of radiotherapy,the consistency of bladder filling was generally smaller than that in location(Table 6).The displacement of the foot,abdomen,dorsal side and the dose of the bladder were positively correlated with the filling changes(P<0.05)(table 7,8).The changes of more than 400ml(60%)made the dose of the bladder significantly higher than the prescription limit(P<0.05)(table 7).In the course of radiotherapy,the consistency of rectum filling was also poor,and it was generally higher than that of the rectum.The displacement of the foot,the dorsal side and the dose of the rectum were positively correlated with the filling changes(P<0.05)(table 7,8).A change of more than 30ml(35%)made the rectal irradiated dose significantly exceed the prescription limit(P<0.05)(table 8).3.The effect of IGRT on the amount of bladder and rectumThe amount of bladder in Plan2 was higher than that of Planl,especially at the first,11-15 and 21-23 times(P<0.05)(table 9).The rectal dose of Plan2 was also significantly higher than that of Planl.The V50 of Plan2 from tenth times to the end was significantly higher than that of Plan 1.Dmean and V45 were significantly higher than those of Planl at 8-20 times(P<0.05)(Table 9,10)4.Single factor and multifactor analysis on the influence of bladder and rectumUnivariate and multivariate analysis showed that tumor shrinkage,image guided and organs is filling factor independent of bladder and rectum is affected by the amount of,without image guidance case,position deviation caused by the difference in dose of far more than other factors,are independent factors affecting the amount of bladder and rectum(P<0.05)(table 11～13).Conclusion1.IGRT is necessary in the course of radiotherapy,especially in the first or tenth times of radiotherapy,which can significantly reduce the amount of bladder and rectum.2.In the absence of image guidance,the dose difference caused by position deviation is far greater than bladder and rectum filling and tumor shrinking.Based on the image guide position registration,proper filling of bladder without demanding to hold back,and in the process of radiotherapy for rectal emptying can reduce the dose of bladder and rectum.3.When the tumor is most obvious(about 10 times),the timely constriction and replan can provide a guarantee for accurate radiotherapy,and reduce the dose of bladder and rectum.