Comparison of methods for calculating rectal dose after 125I prostate brachytherapy implants

Abstract

Purpose : To compare several different methods of calculating the rectal dose and examine how accurately they represent rectal dose surface area measurements and, also, their practicality for routine use. Methods and Materials : This study comprised 55 patients, randomly selected from 295 prostate brachytherapy patients implanted at the Vancouver Cancer Center between 1998 and 2000. All implants used a nonuniform loading of 0.33 mCi (NIST-99) 125I seeds and a prescribed dose of 144 Gy. Pelvic CT scans were obtained for each patient ∼30 days after implantation. For the purposes of calculating the rectal dose, several structures were contoured on the CT images: ( 1) a 1-mm-thick anterior rectal wall, ( 2) the anterior half rectum, and ( 3) the whole rectum. Point doses were also obtained along the anterior rectal surface. The thin wall contour provided a surrogate for a dose-surface histogram (DSH) and was our reference standard rectal dose measurement. Alternate rectal dose measurements (volume, surface area, and length of rectum receiving a dose of interest [DOI] of ≥144 Gy and 216 Gy, as well as point dose measures) were calculated using several methods (VariSeed software) and compared with the surrogate DSH measure ( SA DOI ). Results : The best correlation with SA 144 Gy was the dose volumes (whole or anterior half rectum) ( R = 0.949). The length of rectum receiving ≥144 Gy also correlated well with SA 144 Gy ( R ≥0.898). Point dose measures, such as the average and maximal anterior dose, correlated poorly with SA 144 Gy ( R ≤0.649). The 216-Gy measurements supported these results. In addition, dose-volume measurements were the most practical (∼6 min/patient), with our surrogate DSH the least practical (∼20 min/patient). Conclusion : Dose-volume measurements for the whole or anterior half rectum, because they were the most practical measures and best represented the DSH measurements, should be considered a standard method of reporting the rectal dose when calculating the DSH is not practical. Average or maximal anterior rectal doses are not reliable indicators of surface area dosimetry.