A method for quantitative evaluations of scanning-proton dose distributions

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• Patient-Specific Quality Assurance (PSQA) is crucial for verifying the accuracy of radiation therapy dose calculations and delivery.
• The established gamma test is widely used but may not optimally parameterize errors for advanced techniques like proton therapy.
• Discrete spot scanning proton therapy allows for precise modulation of individual dose voxels, necessitating improved quality assurance metrics.

Purpose of the Study:

• To develop and evaluate a novel dose plane comparison approach for PSQA in proton therapy.
• To create a more intuitive error parameterization compared to the existing gamma test.
• To ensure the new method maintains the capabilities of the gamma test while improving relevance to machine performance.

Main Methods:

• A dose difference minimum (DDM) metric was developed, calculating the minimum dose deviation within a defined search radius for each pixel.
• Analysis involved 300 dose planes, comparing calculated points to measured pixels.
• The DDM metric was aggregated into histograms, and pass rates were determined based on a <3% dose difference criterion. Search radius was based on beamlet spatial deviation.

Main Results:

• The mean beamlet radial deviation was 0.38mm, with 99.9% of beams within 1mm of nominal.
• The DDM analysis (3%/1mm) showed a pass rate of 97.6% +/- 3.6%.
• This was comparable to the gamma test (3%/2mm) pass rate of 97.7% +/- 3.2%, indicating similar agreement levels.

Conclusions:

• The DDM method, using a cylindrical search shape, is proposed as a more relevant metric for PSQA in proton therapy.
• DDM avoids false positives by considering machine performance limits, unlike gamma tests with overly restrictive spatial thresholds.
• This new algorithm provides a straightforward evaluation of plan quality by accepting pixels within the dose and search radius thresholds.