SU-E-T-458: Radiobiological Comparison of Single and Dual-Isotope Prostate Seed Implants

Area of Science:

• Medical Physics
• Radiation Oncology
• Biophysics

Background:

• Low dose-rate brachytherapy for prostate cancer commonly uses single isotopes.
• Dual-isotope implants may offer combined benefits like extended half-life and varied relative biological effectiveness.
• However, dual-isotope use complicates treatment planning and quality assurance.

Purpose of the Study:

• To compare single and dual-isotope prostate brachytherapy implants using a linear-quadratic model.
• To determine if the benefits of dual-isotope implants outweigh the increased complexity in treatment planning and quality assurance.

Main Methods:

• Six treatment plans were created for ten patients using single or dual-isotope combinations (125I, 103Pd, 131Cs).
• Physicians contoured the prostate, urethra, rectum, and bladder for each plan.
• Biologically effective dose was used to calculate tumor control and normal tissue complication probabilities, evaluating plans with favorable, intermediate, and unfavorable radiobiological parameters.

Main Results:

• Iodine-125 implants showed the most sensitivity to tumor aggressiveness.
• Significant organ response differences were observed at common dose levels, likely due to suboptimal seed strengths.
• Optimizing initial seed strength to maximize complication-free tumor control minimized differences between isotope combinations, even for aggressive tumors.

Conclusions:

• Radiobiologically based comparison of single and dual-isotope prostate seed implants was performed.
• Varying initial seed strength improved all isotope combinations.
• Minimally optimized treatment plans showed no substantial differences in predicted outcomes among various isotope combinations.