Comparison between CT-based volumetric calculations and ICRU reference-point estimates of radiation doses delivered to bladder and rectum during intracavitary radiotherapy for cervical cancer

Purpose: To compare CT-based volumetric calculations and International Commission on Radiation Units and Measurements (ICRU) reference-point estimates of radiation doses to the bladder and rectum in patients with carcinoma of the uterine cervix treated with definitive low-dose-rate intracavitary radiotherapy (ICRT). Methods and Materials: Between November 2001 and March 2003, 60 patients were prospectively enrolled in a pilot study of ICRT with CT-based dosimetry. Most patients underwent two ICRT insertions. After insertion of an afterloading ICRT applicator, intraoperative orthogonal films were obtained to ensure proper positioning of the system and to facilitate subsequent planning. Treatments were prescribed using standard two-dimensional dosimetry and planning. Patients also underwent helical CT of the pelvis for three-dimensional reconstruction of the radiation dose distributions. The systems were loaded with ¹³⁷Cs sources using the Selectron remote afterloading system according to institutional practice for low-dose-rate brachytherapy. Three-dimensional dose distributions were generated using the Varian BrachyVision treatment planning system. The rectum was contoured from the bottom of the ischial tuberosities to the sigmoid flexure. The entire bladder was contoured. The minimal doses delivered to the 2 cm³ of bladder and rectum receiving the highest dose (D_BV2 and D_RV2, respectively) were determined from dose-volume histograms, and these estimates were compared with two-dimensionally derived estimates of the doses to the corresponding ICRU reference points. Results: A total of 118 unique intracavitary insertions were performed, and 93 were evaluated and the subject of this analysis. For the rectum, the estimated doses to the ICRU reference point did not differ significantly from the D _RV2 (p = 0.561); the mean (± standard deviation) difference was 21 cGy (± 344 cGy). The median volume of the rectum that received at least the ICRU reference-point dose was 2.1 cm³. In 66 (71%) of 93 cases, <5 cm³ was treated to this dose. However, for the bladder, the estimated doses to the ICRU reference point were significantly lower than the D_BV2 (p <0.001); the mean difference was 680 cGy (± 543 cGy). The median volume of the bladder that received at least the ICRU reference-point dose was 13.0 cm³. Conclusions: Our data suggest that the estimated dose to the ICRU rectal point may be a reasonable surrogate for the D_RV2. However, this result may not be applicable to other treatment guidelines and ICRT applicator systems. In contrast, the dose to the ICRU bladder point does not appear to be a reasonable surrogate for the D _BV2. Correlation with late complications are needed to define the role of three-dimensional dosimetry in treatment planning.