| Objectives: On the basis of CT images,we compared the differences of DVH parameters of post-operative brachytherapy in early stage endometrial carcinoma,using different normalization methods and the optimization after normalizing between different groups.Considering the doses delivered to the target volume and organs at risk(OAR),we provided a more reasonable method to target volume delineation for postoperative brachytherapy in early endometrial carcinoma Moreover,we evaluated the incidence and location of air pockets around the applicator,and measured the distance of vaginal mucosa displaced by air pockets and assessed their dosimetric effect on the vaginal mucosa during post-operative brachytherapy of endometrial carcinoma.Methods: We reviewed the data of 29 patients with early stage(I-II)endometrial carcinoma,treated between January 2015 and January 2016 by iridium-192 high dose rate postoperative brachytherapy in our department of radiotherapy.16 patients were treated with external beam radiotherapy and brachytherapy.External beam radiotherapy was performed by three-dimensional(3D)conformal intensity modulated technique with linear Accelerator(Trilogy)using 6-MV X-ray.A detailed pelvic examination was performed before brachytherapy,to determine the most appropriate cylinder applicator size to be used.All the pelvic CT scans were transferred to the Oncentra treatment planning system.The following normalization methods and optimization after normalize were used.Normalization:1)doses normalized to the reference dose points outside of dwell points;2)doses normalized to the clinical target volume(CTV).Optimization: Proper treatment scope of rectum and bladder to dodge,reduce the doses to OAR.The dose prescribed was 5Gy for three fractions in patients treated with external beam radiotherapy and brachytherapy,and 5Gy for six fractions for patients treated with brachytherapy only.Treatment plan to group according to the different normalization methods and optimization,normalized to the reference dose points outside of dwell points for R.N group,further optimized for R.O group;normalized to target volume for T.N group,further optimized for T.O group.We recorded the following the DVH parameters of each group: CTV-D90,D2 cc for rectum and bladder.At the same time,using the CT images of TPS and the isodose curves to evaluate the incidence and location of air pockets around the vaginal cylinder applicator,and to measure the distance of mucosa displaced by air pockets and assess their dosimetric effect on the vaginal mucosa.Statistical analysis was performed using SPSS17.0 software.The paired t test was applied to compare the differences of DVH parameters between groups.Results:1.DVH parameters in each groupThe mean value of DVH parameters in R.N group were: 546.90±23.51cGy、405.17±88.05cGy、407.55±72.86cGy;The mean value of DVH parameters in R.O group were: 500.64±0.90 c Gy、372.27±76.89cGy、370.85±63.05cGy;The mean value of DVH parameters in T.N group were: 487.87±22.98cGy、363.69±85.24cGy、362.07±59.57cGy;The mean value of DVH parameters in T.O group were: 500.40±0.55 cGy 、376.35±78.18cGy、370.09±60.55 cGy.2.Comparison of DVH parameters in different normalization methods and the optimization after normalize between groups.2.1 Comparison between R.N group and T.N groupThere was a significant difference between the two groups about CTV-D90,D2 cc for rectum and bladder,and the mean value of DVH parameters in R.N group was greater than T.N group.There was no significant difference between the two groups about D2 cc for rectum and bladder.3.Comparison the DVH parameters before and after optimization3.1 Comparison between R.N group and R.O groupThere was a significant difference between the two groups about CTV-D90,D2 cc for rectum and bladder,and the mean value of DVH parameters in R.N group was greater than R.O group.3.2 Comparison between T.N group and T.O groupThere was no significant difference between the two groups about CTV-D90 and D2 cc for bladder.But it showed significant difference about D2 cc for rectum,and the mean value in T.N group was greater than T.O group.4.Comparison between R.O group and T.N groupThere was no significant difference between the two groups about all DVH parameters.5.The study about air pockets around applicator5.1 Incidence and location of air pocketsAir pockets around the vaginal cylinder applicator were seen in 20 patients(70%).7 patients(35%)had one air pocket,10 patients(50%)and 3 patients(15%)had two and two more air pockets,respectively.The air pockets were located in the apical part of the applicator in 2 patients(10%)and in the lateral sides of the applicator in 16 patients(80%),while 2 patients(10%)had air pockets located both at the apex and lateral sides of applicator.5.2 The displacement of mucousThe mean average distance of the mucosa displaced by air pockets was 0.27±0.09cm(range,0.15-0.40cm).5.3 Dosimetric effects of air pocketsThe range of dose reduction to vaginal mucosa over the air pocket from 180-350cGy(mean: 245±50.21cGy)。 2.2 Comparison between R.O group and T.O group about D2ccConclusion:1.The study compared the differences of DVH parameters in different normalization methods and the optimization after normalizing between groups,and the following results were obtained: the method of doses normalized to the target volume is better than to the reference dose points outside of the dwell points.If doses normalized to the latter,it is better to further optimize while the former don’t need to be optimized additionally.2.Taking a comprehensive comparison of the doses to the target volume and OAR,both doses normalized to the reference dose point outside of the dwell point and then further optimize and doses normalized to the target volume are reasonable target delineation methods.3.In early endometrial carcinoma post-operative brachytherapy,air pockets around the applicator remain a common phenomenon.Doses reductions caused by air pockets may increase the recurrence risk of vagina. |
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