Limitations of the minimum peripheral dose as a parameter for dose specification in permanent 125I prostate implants

Area of Science:

• Medical Physics
• Radiation Oncology
• Brachytherapy

Background:

• Permanent prostate brachytherapy utilizes Iodine-125 (125I) seeds for localized radiation delivery.
• Accurate dose delivery is crucial for treatment efficacy and minimizing side effects in prostate cancer.
• Dose specification parameters are essential for planning and evaluating brachytherapy implants.

Purpose of the Study:

• To assess the practicality of the minimum peripheral dose as a dose specification parameter for permanent 125I prostate implants.
• To determine if the minimum peripheral dose can be reliably achieved in clinical scenarios.

Main Methods:

• A computer model of ellipsoidal 125I prostate implants was developed.
• Variations in implant geometry (dimension, elongation) and seed placement (ideal vs. non-ideal) were simulated.
• The impact of seeds placed outside the target volume was also investigated.

Main Results:

• Minimum peripheral dose is highly sensitive to seed placement accuracy.
• Implant irregularities typically reduce the minimum peripheral dose unpredictably, often by over 25%.
• Despite dose variations, less than 10% of the target volume received less than the prescribed minimum peripheral dose in most cases.

Conclusions:

• Prescribed minimum peripheral doses are rarely achieved in permanent 125I prostate implants due to seed placement imperfections.
• Achieving target dose coverage necessitates either a relaxed coverage requirement (90%) or a 20% increase in total source strength.
• Placing seeds outside the target volume improves dose uniformity but not coverage and increases normal tissue irradiation.