The variability of applicator position among high dose rate (HDR) intracavitary brachytherapy applications in locally advanced cervical cancer patients who were treated with ring & tandem applicators

Aim: HDR applicators are frequently used in the brachytherapy of cervical cancer. The reproducibility of brachytherapy applications among fractions is very important in terms of optimisation of treatment. In this study, the interindividual and intraindividual applicator position variability was evaluated retrospectively in HDR intracavitary brachytherapy applications performed in locally advanced cervical cancer treatment using ring and tandem (R & T) applicators. Patients and Method: Eight patients with locally advanced cervical cancer formed the study population who had been treated in Dokuz Eylül University Department of Radiation Oncology between the years 2000 and 2005 with HDR brachytherapy using R & T applicators. Patients were treated with 45 Gy external radiotherapy (RT) using pelvic box technique and then they were given intracavitary brachytherapy in 3 fractions of 8 Gy (to point A). The 3–dimensional geometrical variation of applicator center in craniocaudal, mediolateral and anteroposterior directions was determined on the basis of bony reference points in 24 pairs of orthogonal films obtained in the conventional simulator. Then the following evaluations were performed: (1) The applicator position variability in all applications (interindividual variability), (2) The applicator position variability of other two applications relative to the 1st application in a single patient (intraindividual variability relative to the 1st application), (3) The applicator position variability of 3 applications relative to the mean of 3 applications in a single patient (intraindividual variability relative to the mean of 3 applications). In the lateral simulation films, the distances between applicator center and bladder and rectum reference points were also measured. Among the potential factors that could influence the reproducibility of R & T applications, age, stage, the period between external RT and brachytherapy were evaluated with univariate analysis. Results: The range of R & T applicator center's position was 13,08 mm, 1,66 mm, and 15 mm in craniocaudal, mediolateral and anteroposterior directions, respectively. Standard deviation (SD) of interindividual applicator variability was 3,83 mm in craniocaudal, 0,39 mm in mediolateral and 2,86 mm in anteroposterior directions. SD of intraindividual variability relative to 1st application was 1,91 mm in craniocaudal, 0,4 mm in mediolateral, and 4,26 mm in anteroposterior directions. And SD of intraindividual variability relative to the mean of 3 applications was 0,95 mm in craniocaudal, 1,86 mm in mediolateral, and 1,24 mm in anteroposterior directions. The lateral angulation of R & T applicator (α) was 10° to the right and 0° to the left while anteroposterior angulation (β) was 30° to the anterior direction and 0° to the posterior direction. SD of the distance between applicator center and bladder and rectum reference points was 1,19 mm and 0,5 mm, respectively. In the univariate analysis, there was no factor influencing applicator position variability. Conclusion: According to the results of this study, the applicator position variability in R & T applications was relatively less compared to other studies in the literature. In order to extract definitive conclusions about factors affecting positional reproducibility of R & T applicators, studies are needed that evaluate more parameters and that include higher number of patients. Key words: cervical cancer, radiotherapy, brachytherapy, ring and tandem, applicator position variability, reproducibility of brachytherapy applications.