Reserch On Applicator Shifts And Dose Distributions In3D CT-based Brachytherapy For Cervical Cancer

Purpose: To investigate Utrecht intertitial applicator shifts,and the changes ofdose distribution of target and OAR during3D CT-based HDR brachytherapyof cervical cancer.Methods:40cervical cancer patients underwent brachytherapy after EBRT（external beam radiotherapy） for total dose to45Gy/25fractions. Theprescribed dose of brachytherapy was7Gy×4fractions. The dose of EBRTand brachytherapy were calculated and transformed to EQD2values in Gyα/β,biologically weighted dose normalized to2Gy fractionation. The used α/βvalues for target volumes and normal organs were10and3Gy, respectively.After the Utrecht intertitial applicator implanted, CT simulation was achievedfor oncologist to contour the target volume and OAR,including high riskCTV(CTVhr), intermediate CTV(CTVir), which was performed according tothe GYN GEC ESTRO recommendations,and the normal organs,includingbladder, rectum, sigmoid colon and small intestines.The target volumeimagines were imported into Oncentra Master Plan V4.3. The treatment planwas optimized according to the CTVhr, CTV-hr D90controlled by80to85Gyα/β，and executed in the case of total dose to normal organs constrained by90Gyα/β3for bladder,75Gyα/β3for rectum, sigmoid colon,small intestines.Then，the plan was transported into the afterloader. After the treatment, CTsimulation was repeated for oncologist to contour the target volume and OARagain. The time for the two simulation was controlled under2hs.We matched the two CT imaging by pelvic bone structures and erect the3D coordinatesystem,including X (left and right defined as positive and negativerespectively),Y(head and foot defined as positive and negativerespectively)and Z(ventral and dorsal defined as positive and negativerespectively).In both imagings, we defined the tandem by the tip and the baseas the marker points, and evaluated applicator shift according to coordinate ofmarker points in3D CT-based imaging.Based on the repeated CT imaging,oncologist contoured the target volume and normal organs again according tothe request above.We transport the treatment plan into the repeated CTimaging and evaluated the dose distribution changes of the target volume andnormal organs between the two simulation.Results: The average applicator shift,was0.095mm to left,1.49mm to head,and2.3mm to ventral for the tip of the tandem. The average applicator shift,was0.16mm to right,2.14mm to head, and1.9mm to ventral for the base ofthe tandem. The EQD2values in Gyα/β of CTV-hr D90decreased by3.5%Gyα/β10, bladder D2cc decreased by8.77%Gyα/β3, rectum D2cc decreasedby4.0%Gyα/β3,sigmoid colon D2cc decreased by3.72%Gyα/β3,smallintestines D2cc increased by10.94%Gyα/β3. The average physical doses ofCTV-hr D90decreased by2.55%,bladder D2cc decreased by5.94%,rectumD2cc decreased by2.94%,sigmoid colon D2cc decreased by3.38%,and smallintestines D2cc increased by3.72%.Conclusions:1、Applicator shifts on CT data evaluation was available, and the change in head-foot direction along with abdomen-backdirection is more obviously than that in left-right direction.2、The D90of high risk clinical target volume and D2cc ofbladder、rectum、sigmoid colon、small intestine variated,and thatmay be related with applicator shifts.