A study of the interplay effect in radiation therapy using a Monte-Carlo model

Area of Science:

• Medical Physics
• Radiotherapy
• Computational Modeling

Background:

• Breathing motion in radiotherapy can cause Interplay Effects (IE) and Blurring Effects (BE), altering the delivered radiation dose.
• Accurate dose calculation considering motion is essential for effective modulated radiotherapy.

Purpose of the Study:

• To implement and validate an open-source Monte Carlo (MC) model for calculating delivered dose in modulated radiotherapy, incorporating breathing motion effects.
• To assess the impact of Interplay Effects (IE) on dose distribution.

Main Methods:

• A Varian TrueBeam Monte Carlo model was developed using GATE software.
• Dose calculations were performed for various modulated plans and breathing patterns.
• Model predictions were validated against phantom measurements using a programmable motion platform.

Main Results:

• The MC model achieved a 97.5% (3%/3 mm) 3D global gamma index pass rate upon validation.
• Interplay Effects (IE) showed significant correlation with motion period and anteroposterior amplitude.
• IE increased the D2% by +6.9% and decreased the D98% of the clinical target volume (CTV) by -3.3%.

Conclusions:

• The feasibility of assessing Interplay Effects (IE) using a Monte Carlo model was successfully validated.
• Limiting IE requires more than 20 breathing cycles per treatment arc.