Patient specific evaluation of breathing motion induced interplay effects.

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Interplay effects between target motion and dynamic beam parameters in lung Stereotactic Ablative Radiotherapy (SABR) can compromise target coverage.
• Accurate estimation of these effects is vital for determining the necessity of motion management techniques.

Purpose of the Study:

• To implement and validate a comprehensive methodology for pre-treatment estimation of interplay effects in lung SABR.
• To assess the potential need for motion management techniques based on estimated interplay effects.

Main Methods:

• A virtual, motion-simulated 3D dose distribution was calculated using measurement data from a stationary phantom and time-dependent target motion patterns.
• In-house software enabled user-defined motion patterns (simplistic or real patient breathing) with defined beam starting phases.
• The method was validated with programmed phantom motion and analyzed using Gamma analysis (2%/2mm) for 14 lung SABR plans under various breathing conditions.

Main Results:

• Validation measurements achieved >98% pass rates.
• Gamma pass rates below 92% were observed for sinusoidal breathing patterns with <25 breaths per delivery time and realistic patterns with <18 breaths per delivery time, indicating significant interplay effects.

Conclusions:

• The influence of interplay effects on target coverage is significantly dependent on individual patient breathing patterns.
• The presented moving-platform-free approach effectively verifies Intensity Target Volume (ITV)-based treatment plans.
• This method aids in identifying whether clinical goals can be met without explicit respiratory management techniques.