Shift And Dosimetric Analysis Of Vaginal Stump For Postoperative Cervical Cancer By Volumetric Modulated Arc Therapy

Objective:To study the displacement of vaginal stump in different time intervals and in the same time interval when patients received volumetric modulated arc therapy(VAMT)for for postoperative cervical cancer;and to analyze the dosimetric changes of vaginal stump in treatment.To study the positioning error between treatments and the reasonable external boundary of PTV.Methods:1.From April 2019 to July 2019,15 patients of postoperative cervical cancer were selected from the Department of radiotherapy,whose FIGO stage was IA-IIA.The patients were 28-65 years old,the median age was 38 years old,and the BMI index was less than 24kg/m~2.The target was mapped by the clinician according to the RTOG guidelines.The prescription dose of external radiation plan was PTV 50Gy/25F/2Gy(prevention of radiation).95%PTV was designed to reach the prescribed dose.In the course of treatment,CBCT was performed once a week before and after radiotherapy for all patients.The vaginal stump was delineated on the CBCT images.The Inter-frational motion of the vaginal stump was obtained by measuring CBCT before radiotherapy and initial positioning CT after bony registration.Similarly,The Inter-frational motion of the vaginal stump was obtained by measuring CBCT obtained before radiotherapy and CBCT obtained before radiotherapy after bony registration.The CBCT image of vaginal stump target,which were obtained before radiotherapy,were introduced into the planning system,and D98,D95,D90,Dmean,D50 and D2 of vaginal stump in the actual treatment were calculated.The difference of dosimetry parameters between the planning and the actual treatment was compared.The dose of 20GY/10F/2Gy(added dose locally)was added to the vagina stump in sequence.And the difference of dosimetry parameters between the treatment plan and the actual treatment was compared when the tumor change was not considered and the single dose was increased.2.50 cases of postoperative cervical cancer were collected with FIGO stage of IA-IIA in our department from July 2018 to July 2019,and then conducted CBCT verification before treatment for the first time and once a week Scanning,a total of 206 CBCT images were obtained to analyze the positioning errors of head and foot,abdomen and back,left and right directions and the boundary of planned target volume.Results:1.The mobility of the vaginal stump was 2.5±1.8 mm,3.4±3.6 mm and 3.8±3.8 mm in the left and right,head and foot,abdomen and back directions,respectively.The10.6 mm,6.8 mm and 11.4 mm expansion respectively in these three directions could contain 95%of the position change of the vaginal stump.The mobility of the vaginal stump was 0.5±0.9 mm,1.1±1.4 mm and 1.2±1.6 mm in the head and foot,abdomen and back,left and right directions,respectively.In these three directions 3.9mm,2.3mm and 4.4respectively can contain 95%of the position change of the vaginal stump;In the prevention group,there was no significant difference of the D98,D95,D90,Dmean,D50,D2between actual treatment and the planning(P>0.05);When added dose to vaginal stump locally,D98,D95,D90 in the actual treatment were significantly smaller than D98,D95,D90 in the plan(P<0.05).2.In the course of treatment,the positioning errors of 50 patients were-0.77±1.65mm,1.12±2.08mm and0.63±1.82mm in the left and right,head and foot,abdomen and back directions,respectively.Thus,the PTV expansion figure in these three directions were 6.0mm,6.9mm and 8.2mm.Conclusions:For postoperative cervical cancer,the vaginal stump moved between Inter-frational treatments and within Intra-frational treatments.The displacement in the direction of head and foot,abdomen and back was larger than that in the left and right directions.Although we can ensure the consistency of the filling of bladder and rectum between the different treatments.When the vaginal stump needed to added dose locally,the external expansion boundary of the vaginal stump should be appropriately increased,so as to ensure that the dosage of the vaginal stump could be reached to the prescription dose.In this study the PTV expansion figure obtained can be used as a reference for the PTV expansion figure of the treatment center.But a larger sample size is still needed to provide a more accurate figure.