Planning target volumes for radiotherapy: how much margin is needed?

Area of Science:

• Radiotherapy physics and planning
• Medical imaging and uncertainty quantification
• Radiation oncology

Background:

• Radiotherapy planning target volume (PTV) margins account for uncertainties like patient positioning and organ motion.
• Current PTV margins often consider doses to adjacent healthy tissues, deviating from ideal physical uncertainty-based margins.
• This study focuses on calculating PTV margins based solely on physical position uncertainties.

Purpose of the Study:

• To calculate the ideal CTV-PTV margin size considering only physical position uncertainties.
• To evaluate different strategies for determining CTV-PTV margins based on probability.
• To provide a data-driven recommendation for PTV margin selection in radiotherapy planning.

Main Methods:

• Modeled clinical target volume (CTV) position using independent Gaussian distributions in three Cartesian directions.
• Analyzed three margin strategies: complete CTV enclosure, beam's eye view (BEV) projection enclosure, and edge point inclusion.
• Derived cumulative probability distributions for each margin strategy.

Main Results:

• A 1 SD margin resulted in 24% CTV enclosure, 39% BEV projection enclosure, and 84% edge point inclusion.
• A 95% probability of complete CTV enclosure requires a 2.8 SD margin.
• A 95% probability for BEV projection enclosure requires 2.45 SD; for edge point inclusion, it requires 1.65 SD.

Conclusions:

• The third strategy (edge point inclusion) correlates margin size with minimum target dose.
• A margin of 1.65 SD is recommended for calculating PTV from CTV, ensuring >95% minimum CTV dose.
• Modifying beam portals, not PTV margins, should be used for sparing healthy structures, improving dose distribution accuracy.