Individual volume-based 3D gamma indices for pretreatment VMAT QA

Area of Science:

• Radiation Oncology
• Medical Physics
• Radiotherapy Quality Assurance

Background:

• Gamma analysis is a standard for patient-specific Quality Assurance (QA) in intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT).
• The clinical relevance of 2D gamma passing rates (%GP) and their correlation with actual dosimetric differences has been increasingly questioned.
• There is a need for more accurate and clinically relevant QA metrics in VMAT.

Purpose of the Study:

• To investigate the feasibility of using individual volume-based 3D gamma indices for pretreatment VMAT QA.
• To compare the correlation between individual volume %GP and global %GP with dosimetric errors.
• To assess the sensitivity and correlation of these gamma indices with dose-volume histogram (DVH) metrics.

Main Methods:

• Evaluated percentage dosimetric errors (%DE) of DVH metrics for target volumes and organs at risk.
• Calculated %GPs for individual volumes and global gamma indices for one- and two-arc VMAT plans.
• Analyzed correlations and sensitivities between %GPs and %DE metrics across different gamma criteria (2%/2 mm, 3%/3 mm, 4%/4 mm).

Main Results:

• Individual volume %GPs demonstrated a higher correlation with individual %DE metrics compared to global %GPs.
• For two-arc VMAT, individual volume %GPs correlated with significantly more %DE metrics (9-12) than global %GPs (1-2) across tested criteria.
• Similar trends were observed for one-arc VMAT, with individual volume %GPs showing stronger correlations with a larger number of %DE metrics.

Conclusions:

• Individual volume-based %GP is a feasible and potentially superior metric for VMAT QA.
• This approach offers a better correlation with DVH-based dosimetric errors compared to traditional global %GP.
• Individual volume gamma analysis provides a more refined and clinically relevant assessment of VMAT plan accuracy.