γ+ index: A new evaluation parameter for quantitative quality assurance.

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• Accurate dose delivery is crucial in Intensity-Modulated Radiation Therapy (IMRT).
• Existing gamma index methods primarily assess physical dose differences, potentially overlooking radiobiological impact.
• There is a need for advanced quality control metrics that integrate radiobiological information.

Purpose of the Study:

• To introduce and validate a novel quality assurance index, gamma plus (γ+), which extends the conventional gamma index by incorporating radiobiological data.
• To evaluate the robustness of the γ+ index in quality control analysis for IMRT treatment plans.
• To assess the utility of γ+ using radiobiological measures like biologically effective uniform dose (D) and complication-free tumor control probability (P+).

Main Methods:

• Developed an in-house software for γ+ calculation, utilizing biological effective dose (BED) instead of solely physical dose difference.
• Tested the γ+ index using 2D dose distributions from a test pattern, a theoretical clinical case, and a patient's lung cancer IMRT plan.
• Compared planned and delivered dose distributions (film) using the γ+ index and radiobiological measures (P+, D).

Main Results:

• The γ+ index distributions differ from the standard gamma index, reflecting the inclusion of radiobiological parameters.
• γ+ values varied with different treatment parameters (e.g., dose per fraction) in the theoretical clinical case.
• For a lung cancer case, planned and delivered doses corresponded to P+ values of 50.8% and 49.0%, respectively, with target doses of 54.0 Gy and 53.3 Gy.

Conclusions:

• The γ+ index offers enhanced quantitative evaluation for IMRT dose delivery and quality control.
• By integrating radiobiological dose information, γ+ provides insights into expected tissue responses.
• This new index demonstrates advantageous properties for improving radiotherapy treatment accuracy and patient safety.