The influence of angle of cervical cancer’s applicators on dosimetry and positions of reference points and critical organs

Abstract

The clamps used for fixation of the applicators for high-dose-rate intracavitary brachytherapy of carcinoma of the cervix during treatment time can adjust the applicators’ angle away from critical organs, bladder and rectum, to reduce radiation dose to these organs additional to gauze’s packing. However, such adjustment can vary the positions of reference points and also critical organs. Objective: To study the influence of applicators’ angle on bladder and rectal doses and positional variations of reference points including critical organs. Materials and Method: Fifty pairs of orthogonal radiographs using reconstruction jig, before and after adjustment of applicators’ angle, of 50 applications for 29 patients with cervical cancer which were inserted with both tandem and ovoids were reviewed. Treatment plannings were performed with the BrachyVision version 7.3.10 software. Prescribed dose at point A was 6 Grays (Gy), without dwell time optimization. Variations of maximum bladder and rectal doses influenced by angle’s adjustments were statistically tested by Wilcoxon matched pair test. The displacement of reference points and changes of distance between nearby organs were studied using the vector analysis. The correlations of the changes of applicators’ angle with positional variations of reference points and the changes of distance between organs were analysed with Spearman’s rho test. Results: The averaged change of applicators’ angle was 7.81±4.08 degree and it resulted the significant variations of maximum bladder and rectal dose (both p<0.001) with mean+ SD of 39.78±36.20 cGy for bladder and 38.00±31.28 cGy  for rectum. The correlation with the change of applicators’angle of the variation of maximum bladder dose was more than that of maximum rectal dose with correlation coelticients of 0.751 for bladder and 0.575 for rectum. The averaged displacement of cervical os, point A, B and cervix were 0.62±0.31, 0.89±0.44, 0.88±0.45 and 0.76±0.38) cm, respectively. The averaged changes in distance from bladder to rectum, cervical os to bladder and cervical os to rectum were 0.11±0.09, 0.13±0.11 and 0.17±0.10 cm, respectively. The variations of positions of reference points correlated with the changes of applicators’ angles more than those of the distances between organs (Correlation coeltients were 0.867, 0.941, 0.926 and 0.918 for cervical os, point A, B and cervix, respectively, and were 0.561, 0.600 and 0.631 for bladder to rectum, cervical os to bladder and cervical os to rectum, respectively). Conclusion: The applicators’ angle significantly influenced on the maximum bladder and rectal doses resulted from positional variations of points of prescribed dose. All of positional variations of reference points increased with the change of applicators’ angle. All nearby organs were moved together with the cervical os with direction of applicators’ angle adjustment but less affected than positions of reference points. However, the averaged positional variations of reference points were not more than those in the previous researches which studied in multiple applications of same patients.