The contribution from transit dose for (192)Ir HDR brachytherapy treatments

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Brachytherapy treatment planning systems increasingly use model-based dose calculation algorithms, moving beyond TG43-U1 to TG-186 recommendations.
• Current systems lack clear guidelines for calculating transit dose, which occurs as the brachytherapy source moves within the patient.

Purpose of the Study:

• To develop and validate a methodology for calculating the transit dose in high-dose-rate (HDR) brachytherapy.
• To assess the significance of transit dose in various clinical scenarios, including prostate and gynecological treatments.

Main Methods:

• A methodology was developed to calculate transit dose using treatment planning system (TPS) data and source speed information (instantaneous and average).
• The trajectory of an (192)Ir HDR source was defined using applicator contour points and dwell positions from the TPS.
• Monte Carlo simulations were employed to calculate transit dose distribution, incorporating source speed profiles (maximum, average, and uniform acceleration).

Main Results:

• The transit dose component's significance varies with the adopted speed profile, which is often not well-defined in clinical practice.
• Interstitial brachytherapy treatments showed the most significant transit dose effects.
• In worst-case scenarios, transit dose reached 3% in gynecological cases and up to 11.1% in prostate cases, increasing with more catheters or shorter dwell times.

Conclusions:

• Transit dose can be a significant factor in HDR brachytherapy, potentially exceeding 5% if not accounted for.
• Simple dwell time corrections are insufficient for compensating the non-uniform transit dose distribution.
• Accurate source acceleration and speed data, ideally provided by manufacturers or measured clinically, are essential for precise transit dose calculation.