R J Yaes1. 1. Division of Medical Imaging and Radiopharmaceutical Drug Products, Center For Drug Evaluation and Research, U.S. Food and Drug Administration, Rockville, MD, USA. yaesr@cder.fda.gov
Abstract
PURPOSE: To develop a simple method of calculating biologically effective doses in high-dose regions of permanent interstitial implants. METHODS AND MATERIALS: The incomplete repair model is used to clarify the relationship between dose, D, and biologically effective dose (BED), for permanent interstitial implants. The relationship is used to ascertain the BED at high-dose regions that may occur in (125)I, (103)Pd, and (198)Au prostate implants. RESULTS: The relationship between D and BED is nonlinear and is given by BED(D) = D + D(2)/D(lambda), where D(lambda) = [(t(lambda)/t(mu)) + 1](alpha/beta), t(lambda) and t(mu) are the half-lives of the isotope and of sublethal damage repair respectively, and alpha/beta is the alpha:beta ratio. Idealized geometrically identical (125)I, (103)Pd, and (198)Au prostate implants with minimum target dose (MTD) of 160 Gy, 120 Gy, and 64 Gy, respectively, are considered. The BED for (103)Pd and (198)Au will be less than the BED for (125)I, for doses up to about 2.5 times the MTD. For higher doses, the BED for (103)Pd may be significantly higher than for (125)I. CONCLUSION: Permanent interstitial implants using short-lived isotopes may have regions with very high biologically effective doses.
PURPOSE: To develop a simple method of calculating biologically effective doses in high-dose regions of permanent interstitial implants. METHODS AND MATERIALS: The incomplete repair model is used to clarify the relationship between dose, D, and biologically effective dose (BED), for permanent interstitial implants. The relationship is used to ascertain the BED at high-dose regions that may occur in (125)I, (103)Pd, and (198)Au prostate implants. RESULTS: The relationship between D and BED is nonlinear and is given by BED(D) = D + D(2)/D(lambda), where D(lambda) = [(t(lambda)/t(mu)) + 1](alpha/beta), t(lambda) and t(mu) are the half-lives of the isotope and of sublethal damage repair respectively, and alpha/beta is the alpha:beta ratio. Idealized geometrically identical (125)I, (103)Pd, and (198)Au prostate implants with minimum target dose (MTD) of 160 Gy, 120 Gy, and 64 Gy, respectively, are considered. The BED for (103)Pd and (198)Au will be less than the BED for (125)I, for doses up to about 2.5 times the MTD. For higher doses, the BED for (103)Pd may be significantly higher than for (125)I. CONCLUSION: Permanent interstitial implants using short-lived isotopes may have regions with very high biologically effective doses.