SU‐FF‐T‐15: On the Prescription Dose in Permanent Cs‐131 Seed Prostate Implants

Abstract

Purpose: Recently, 131Cs seeds have been introduced for prostate permanent seed implants. This type of seed has a relatively short half‐life and has its most prominent emitted photon energy peaks in the 29 keV to 34 keV region. Traditionally, 145 Gy and 125 Gy have been prescribed for 125I and 103Pd seed prostate implants, respectively. Since both the half‐life and dosimetry characteristics of 131Cs seed are quite different from those of 125I and 103Pd, the appropriate prescription dose for 131Cs seed prostate implant may well be different. This study was to determine an appropriate dose prescription scheme for permanent 131Cs seed prostate implants. Method and Materials: A linear quadratic radiobiological model was used in this study. Prostate edema of different durations and sizes was taken into consideration in the dose and tumor cell survival fraction calculations. The tumor cell survival fractions in 103Pd and 125I permanent seed implants were taken as reference values to derive the appropriate dose prescription value for 131Cs seed implants. Calculations were also performed for tumors of different tumor doubling times. Results: As expected, the derived prescription dose values were dependent on type of tumors and types of edema. For fast growing tumors, the derived prescription doses were different depending on the type of reference implant was used: 125I or 103Pd seed implant. However, for slow growing tumors such as prostate cancer, the derived prescription dose values were very similar and it was determined that 124 Gy was an appropriate prescription dose for permanent 131Cs seed prostate implants. Conclusion: An appropriate prescription dose has been derived for 131Cs permanent seed prostate implants. The dose value was determined to be 124 Gy.