Dose variation due to differences in applicator placement used for intracavitary brachytherapy of cervical cancer

The prognosis for women with early stage cancers of the cervix is excellent. The cervix and proximal vagina are relatively radioresistant structures; dose limiting critical structures for radiation treatment in this area are tissues such as the bladder and rectum. The high dose rate gradient of brachytherapy allows high doses to the tumor volume, while doses to the neighboring structures are lower. Intracavitary treatment of cervical cancer is performed by insertion of tandem and ovoid applicators. Correct tandem and ovoid placement is verified and documented with orthogonal radiographs. If placement of the tandem and ovoid is not acceptable, the packing is removed, and the tandem or ovoid is repositioned or exchanged as necessary. To examine the difference in dose to specific reference points when tandem or ovoid placement was changed by the physician, three patients' initial insertion radiographs were compared to those that were accepted to treat. Points of comparison selected for the patients on each of the insertions were: point A (L and R), external iliac nodes (L and R), bladder, and rectum. Each of the placement changes for the three patients resulted in differences in distance, and therefore, differences in dose. Changes as high as 823 cGy resulted from the adjustment of only one component. In the cases examined, repositioning improved dose distribution to the cervix, and lowered the dose to adjacent critical structures.