A CT-based software tool for evaluating compensator quality in passively scattered proton therapy.

Area of Science:

• Medical Physics
• Radiotherapy Technology
• Quality Assurance

Background:

• Passively scattered proton therapy (PSPT) relies on custom compensators to shape the proton beam.
• Accurate manufacturing of these compensators is critical for precise dose delivery.
• Existing quality assurance (QA) methods may not fully capture geometric inaccuracies.

Purpose of the Study:

• To develop and validate a quantitative computed tomography (CT)-based QA tool for assessing the accuracy of manufactured PSPT compensators.
• To compare measured compensator thickness with planned thickness using advanced image processing techniques.

Main Methods:

• A CT-based QA tool was developed to measure compensator thickness from CT images.
• Euclidean distance transformation and kd-tree search were used to compare measured and planned thicknesses.
• Accuracy was evaluated using parameters like mean distance, maximum distance, global thickness error, and central axis shifts.

Main Results:

• All evaluated compensators showed mean distances, global thickness errors, and central axis shifts within 1 mm.
• Maximum distances ranged from 1.1 to 3.8 mm.
• A strong negative correlation (-0.93) was found between mean depth gradient and pixels within 1 mm, indicating gradient as a complexity indicator.

Conclusions:

• The CT-based QA tool provides quantitative evaluation of manufactured compensator accuracy in PSPT.
• The tool can identify subtle inaccuracies, even when manual verification passes.
• Compensator complexity, indicated by depth gradients, influences accuracy and requires careful assessment.