Tissue composition and density impact on the clinical parameters for (125)I prostate implants dosimetry

Area of Science:

• Medical Physics
• Radiotherapy Dosimetry
• Nuclear Medicine

Background:

• The TG-43U1 formalism is standard for brachytherapy dose calculations.
• Accurate dosimetry is crucial for effective prostate cancer treatment with Iodine-125 (125I) implants.
• Monte Carlo (MC) simulations offer detailed dosimetric analysis.

Purpose of the Study:

• To quantify the dosimetric impact of using prostate tissue parameters versus water-based parameters in treatment planning systems for 125I prostate implants.
• To evaluate uncertainties associated with MC calculations for TG-43U1 parameters in prostate tissue.
• To assess the clinical relevance of these dosimetric differences in 30 patients.

Main Methods:

• MCNPX code was used to calculate TG-43U1 parameters in water and prostate tissue.
• Prostate density was derived from CT scans of 100 prostate cancer patients.
• Dose distributions were compared using water-based and prostate-tissue-based parameters.

Main Results:

• Significant deviations were observed for prostate V100 (-2.6%), V150 (-13.0%), and D90 (-5.8%) when using prostate tissue parameters.
• Rectal dosimetry also showed notable differences (V100 -2.0%, D0.1 -5.1%).
• Differences between water and prostate calculations for the same medium were minimal (<0.3%), indicating the impact of tissue properties.

Conclusions:

• Replacing water-based TG-43U1 parameters with prostate tissue parameters can considerably affect dose distributions in 125I prostate brachytherapy.
• Uncertainties in MC calculations, particularly from seed geometry (up to 13.4% for F(r,θ)), are significant.
• Careful selection of TG-43U1 parameters in planning systems is essential for accurate patient dosimetry, and seed internal geometry uncertainties are a major limitation.