A symmetric probabilistic γ-index for Monte Carlo dose comparisons

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• The gamma-index (γ-index) is a standard metric for comparing radiation dose distributions.
• It combines dose difference and distance-to-agreement into a single value.
• The γ-index passing rate is commonly used to quantify agreement between dose distributions.

Purpose of the Study:

• To analyze the asymmetry and sensitivity of the γ-index to statistical fluctuations in proton dose distributions.
• To introduce a probabilistic and symmetric γ-index for improved robustness.

Main Methods:

• Analysis of realistic clinical proton dose distributions.
• Investigation of the impact of statistical fluctuations on γ-index passing rates.
• Development and introduction of a probabilistic and symmetric γ-index.

Main Results:

• The standard γ-index is not symmetric regarding reference and evaluation dose distributions.
• Statistical fluctuations in dose data significantly affect γ-index passing rates.
• Fluctuations have opposing effects depending on whether they are in the reference or evaluation distribution.
• The proposed probabilistic and symmetric γ-index demonstrates increased robustness against these fluctuations.

Conclusions:

• The standard γ-index and its passing rate are susceptible to statistical noise in proton therapy dose data.
• A probabilistic and symmetric γ-index offers a more reliable metric for dose comparison in the presence of fluctuations.
• This enhanced metric improves the accuracy and reliability of quality assurance in proton therapy.