Positional Variability of a Tandem Applicator System in HDR Brachytherapy for Primary Treatment of Cervix Cancer

Evaluation of the inter- and intraindividual applicator variability of multiple high-dose-rate (HDR) brachytherapy applications for primary treatment of cancer of the uterine cervix. Retrospective analysis of 460 pairs of orthogonal X-ray films for conventional treatment in 92 patients with five intrauterine applications using an HDR tandem applicator. Measurement of the position of the applicator origin relative to a bony reference system in three dimensions. Evaluation of the differences of the applicator position in all 460 applications (interindividual variability), of the five applications in a single patient (intraindividual variability) and of the intraindividual variability relative to the applicator position at the first application. The position of the applicator origin in the pelvis ranged from 23 mm cranial and 55 mm caudal to the top of femoral heads, 23 mm right and 27 mm left to the pelvic midline, and 6–53 mm dorsal to the mid of the femoral heads. Standard deviation (SD) of interindividual applicator variability was 12.9 mm (minimum/maximum –55/+23 mm, mean –13.6 mm) in longitudinal, 5.1 mm (–27/+23 mm, mean 1.6 mm) in lateral, and 7.6 mm (6/53 mm, mean 26 mm) in anterior-posterior [AP] direction. SD of intraindividual variability was 5.5 mm (–21/+23 mm, mean 0 mm) in longitudinal, 2.5 mm (–17/+19 mm, mean 0 mm) in lateral, and 4.2 mm (–15/+18 mm, mean 0 mm) in AP direction compared to intraindividual variability relative to the first insertion with an SD of 8.9 mm (–23/+36 mm, mean 2.8 mm) in longitudinal, 4.0 mm (–11/+23 mm, mean 0 mm) in lateral, and 6.8 mm (–27/+17 mm, mean –0.8 mm) in AP direction. Intraindividual applicator variability is significantly smaller than interindividual variability. Applicator-related procedures such as midline shielding or dose matching of tele- and brachytherapy should be performed with information on at least one individual applicator position.