Effect of respiratory trace shape on optimal treatment margin

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Accurate radiation delivery requires accounting for organ motion during treatment.
• Optimizing treatment margins is crucial to balance target coverage and minimize dose to organs at risk.

Purpose of the Study:

• To investigate how the shape of a target's trajectory during respiration affects the optimal safety margin in radiation therapy.
• To determine if different respiratory motion patterns necessitate varying treatment margins.

Main Methods:

• Intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) plans were created for spherical targets of varying sizes.
• Simulated respiratory motion using different sinusoidal and patient-specific traces.
• Delivered dose was measured using a 2D ion chamber array, and optimal margins were determined based on target coverage.

Main Results:

• The optimal margin was consistently smaller than the peak-to-peak motion range.
• Respiratory traces with less time spent in inhale phases resulted in smaller optimal margins.
• Target size and radiation therapy technique (IMRT vs. VMAT) also influenced the optimal margin size.

Conclusions:

• The shape of the respiratory motion, specifically the time spent in different breathing phases, is a critical factor in determining the optimal treatment margin.
• Targets with motion patterns spending less time in exhale phases (e.g., sin6 trace) require smaller margins (2-4 mm less) compared to those with equal inhale/exhale times (e.g., sin trace).