Changes in applicator position with fractionated high dose rate gynaecological brachytherapy

The reproducibility of high dose rate gynaecological brachytherapy applicator position in the same patient at repeated treatments has been studied. Measurements on antero-posterior and lateral radiographs to detect relative positions of tube and ovoid to the bony pelvis, bladder and rectum were made. Major variations within the same patient in craniocaudal positioning of the applicators and in antero-posterior movement was seen. The position of the anterior rectal wall was found to move a median of 10.5 mm (range, 0-22) in relation to the ovoid source between first and second insertions. These changes can be accounted for in differences in technique between first and second insertion and in particular the use of posterior vaginal packing and exterior clamping. The importance of individual dosimetry for each high dose rate insertion is clear with major differences in dosimetry to critical normal structures having been found.