The use of external beam radiotherapy in the salvage of ¹³¹Cs seed prostate implants compromised by edema

Ning Yue, Ph.D.,

Cancer Institute of New Jersey

New Brunswick, New Jersey

Purpose: ¹³¹Cs seeds have been used in the prostate seed implants. The short half life of ¹³¹Cs may proliferate the negative impacts caused by edema, compromising the treatment efficacy further than ¹²⁵I and ¹⁰³Pd seeds implants. One solution is to use external beam radiotherapy to salvage the compromised implants. This study is to investigate the proper fractionation in the external beam salvage of the compromised ¹³¹Cs seed prostate implants.

Method and Materials: The LQ radiobiological model was used to calculate the prostate tumor cell survival fractions for both external beam radiotherapy and ¹³¹Cs seed prostate implant therapy. In the calculations for ¹³¹Cs seed prostate implants, the edema induced dosimetric changes were incorporated by simulating the edema resolving process. This dynamic process was dependent on edema duration and size, and their impacts on the tumor cell surviving fractions were also taken into account. The external beam fractionation was determined by cell surviving fraction increase induced by the edema. The roles of prescription dose and other radiobiological parameters were investigated.

Results: As edema duration and size increased, the fraction number needed in the external beam boosts increased. For edema of 30-day duration and 90% magnitude, the fraction number can be as high as 15. For an average edema, the needed fraction number is about 7 with 180 cGy per fraction.

Conclusion: External beam fractionation sizes have been determined to neutralize the negative impacts caused by edema in ¹³¹Cs seed implants. There is no significant dependence of the fractionation on implant prescription dose.