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

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) I-125 seeds and a prescribed dose of 144 Gy. Pelvic CT scans were obtained for each patient similar to30 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 greater than or equal to144 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(144Gy) was the dose volumes (whole or anterior half rectum) (R = 0.949). The length of rectum receiving greater than or equal to144 Gy also correlated well with SA(144Gy) (R greater than or equal to0.898). Point dose measures, such as the average and maximal anterior dose, correlated poorly with SA(144Gy) (R less than or equal to0.649). The 216-Gy measurements supported these results. In addition, dose-volume measurements were the most practical (similar to6 min/patient), with our surrogate DSH the least practical (similar to20 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. (C) 2002 Elsevier Science Inc.