Clinical Study On Displacement Of Applicator In Accelerated Fractionation Brachytherapy For Locally Advanced Cervical Cancer

Purpose:In this study,we will discuss the changes of the position of Utrecht intracavitary and interstitial applicator in the two successive fractions within one application in the accelerated fractionation high-dose-rate three-dimensional brachytherapy of locally advanced cervical cancer,and its impact on the dose of the target area and organs at risk.Material/Method（s）:The clinical data of external beam radiation（EBRT）combined with accelerated fractionation brachytherapy in patients with locally advanced cervical cancer from May 2021 to November 2022 was retrospectively analyzed.Patients suffered from cervical cancer in IIA2,IIB,IIIA,IIIB,IIIC and IVA（2018 FIGO stage）were enrolled in the study.They all received EBRT with or without concurrent platinum-containing chemotherapy combined with brachytherapy,of which the dose of EBRT was 44～50.4Gy,1.8～2Gy/f,and the dose of brachytherapy was 28 Gy/4f,using MRI guided combined intracavitary and interstitial（IC/IS）brachytherapy.The dosimetric parameters for three-dimensional brachytherapy of cervical cancer recommended by（GYN）GEC-ESTRO working group were used.The accelerated fractionation brachytherapy mode is two fractions within one application,with the interval of more than 12 hours.The displacement of the position of the applicator between the two fractions was measured on image scanned before treatment.The distance of the applicator moving along the axis was used as the displacement data of Utrecht applicator.Based on the original plan,the displacement data was used to modify the offset for corresponding channel to simulate the plan for displacement.Compared the changes of HR-CTV D₉₀,bladder D_2cc,rectum D_2cc,sigmoid D_2cc,small intestine D_2cc,as well 2Gy/fraction equivalent biological dose（EQD2）between the original plan and the plan of displacement.Results:A total of 20 patients were enrolled,40 applications of applicator implantation,a total of 80 treatment plans.Among them,there were 2 patients in stage IIA2,4patients in stage IIB,1 patient in stage IIIA,2 patients in stage IIIB,10 patients in stage IIIC and 1 patient in stage IVA.In the first application of IC/IS applicator,the median displacement distance of the intracavity applicator（tandem and ovoids）was1.0（interquartile range（IQR）0.5～1.3）mm,85.0%positive,15.0%negative,with maximum 1.9 mm,the median displacement distance of the interstitial needles was0.8（IQR-0.1～1.7）mm,81.9%positive,18.1%negative,with maximum 3.8 mm;in the second application of IC/IS applicator,the median displacement distance of the intracavity applicator（tandem and ovoids）was 0.9（IQR-0.2～1.3）mm,80.0%positive,20.0%negative,with maximum 1.7 mm,the median displacement distance of the interstitial needles was 0.6（IQR-0.6～1.7）mm,71.4%positive,28.6%negative,with maximum 4.5 mm.The physical doses of HR-CTV D₉₀,bladder D_2cc,rectum D_2cc,sigmoid D_2cc,and small intestine D_2ccfor original plan were 7.51（IQR 7.30～7.82）Gy,5.67（IQR5.01～6.07）Gy,3.67（IQR 2.67～4.18）Gy,4.41（IQR 3.94～4.69）Gy,4.15（IQR3.21～4.68）Gy,respectively;while 7.55（IQR 7.26～7.69）Gy,5.60（IQR 4.94～6.13）Gy,3.63（IQR 2.63～4.10）Gy,4.40（IQR 4.04～4.61）Gy,4.23（IQR 3.26～4.67）Gy for the plan of displacement,respectively.The dose difference of bladder D_2cc,rectum D_2ccand small intestine D_2ccbetween original plan and displacement plan was statistically significant.The median dose change were-0.030（IQR-0.078～-0.008）Gy,-0.045（IQR-0.093～-0.005）Gy,0.045（IQR 0.013～0.095）Gy,P values were0.033,0.029 and 0.023,respectively.The median percentage of dose change were-0.60（IQR-0.90%～-0.05%）,-1.41%（IQR-2.80%～-0.15%）,1.13%（IQR-0.29%～2.73%）,respectively.The dose difference between HR-CTV D₉₀and sigmoid D_2ccwas not statistically significant.The cumulative EQD2 of EBRT combined with BT of HR-CTV D90,bladder D_2cc,rectum D_2cc,sigmoid D_2cc,and small intestine D_2ccfor original plan were 88.7（IQR 87.32～91.15）Gy,81.2（IQR 74.22～83.42）Gy,64.1（IQR 57.70～67.07）Gy,69.8（IQR 64.77～72.07）Gy,68.3（IQR 64.05～70.55）Gy,respectively;while 88.3（IQR 87.27～90.72）Gy,80.7（IQR 73.77～83.70）Gy,64.0（IQR 57.30～66.90）Gy,68.9（IQR 64.65～72.10）Gy,68.5（IQR 64.27～70.97）Gy for the plan of displacement,respectively.There were statistically significant differences in bladder D_2cc,rectum D_2ccand small intestine D_2ccbetween the original plan and displacement plan.The median dose changes before and after transfer were-0.2（IQR-0.4～-0.1）Gy,-0.1（IQR-0.3～-0.1）Gy,and 0.1（IQR 0.0～0.3）Gy,respectively.The P values were 0.006,0.018 and 0.015,respectively.There was no statistically significant difference in HR-CTV D₉₀and sigmoid D_2cc.Conclusion:1.In accelerated fractionation brachytherapy for patients with locally advanced cervical cancer,the displacement direction is mainly positive along the axis.The change in dose is related to the direction of displacement,and the dose to rectum is most affected by applicator displacement.2.The accelerated fractionation brachytherapy for patients with locally advanced cervical cancer,with limited absolute displacement of applicator and dose changes to target volumes and OARs during the interval,and no need for secondary planning design,is a safe and feasible treatment model.