Dosimetric analysis in brachytherapy of carcinoma of the cervix

Brachytherapy plays an essential role in the definitive radiation treatment of cervical carcinoma. The dosimetry of intracavitary irradiation is complex in that the optimum doses that can be delivered are dictated not only by the volume and extent of tumor but also by the close vicinity of dose-limiting structures, such as the small and large intestines, rectum, and bladder. To facilitate understanding of the relationships between the various dosimetric parameters involved, a retrospective analysis of 50 randomly chosen intracavitary insertions with Cesium-137 in 41 patients performed at our institution between 1975 and 1985 was carried out. All 50 cases utilized Fletcher-Suit-Delcos applicators and only the insertions using three sources in the tandem and one in each of the ovoids were included in this analysis. Using the AP and lateral radiographs and the lymphatic trapezoid, the reference points were obtained and transferred digitally to the treatment planning computer, and computerized dosimetry performed. In addition to the specified reference points, points were added and modified to obtain more complete information. The doses at the specified points were normalized to the average dose at AT, a reference point 2 cm superior to external os and 2 cm lateral to the tandem, and expressed as a percentage. It was noted that the average dose at the closest bladder point was 103 +/- 41% of the dose at AT, the maximum rectal dose 77 +/- 29% of the dose at AT and the maximum small bowel dose 65 +/- 16% of that at AT. The analysis of percent contribution of various sources to different reference points revealed that the dose to point AT was equally contributed to by all sources; bladder and rectal doses were mainly contributed to by the lowermost uterine and ovoid sources. Our analysis may provide a model for optimizing brachytherapy in cervical carcinoma.