Optimized point dose measurement: An effective tool for QA in intensity-modulated radiotherapy

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Significant deviations (> ±3%) between calculated and measured doses at the isocenter occur in Intensity-modulated radiotherapy (IMRT) point dose measurements.
• These discrepancies stem from complex IMRT field profiles, lack of lateral electronic equilibrium in small fields, and scatter/leakage contributions.

Purpose of the Study:

• To identify an optimal measurement point for ion chambers to minimize dose calculation and measurement discrepancies in IMRT.
• To improve the accuracy of point dose verification in IMRT.

Main Methods:

• Measurements were performed using a 0.125-cc ion chamber and a Universal IMRT phantom.
• Beam profiles were analyzed to identify steep-gradient regions where the ion chamber was positioned.
• Treatment Planning System (TPS) calculated doses were used to determine optimal chamber displacement for improved accuracy.

Main Results:

• Optimizing ion chamber placement by adjusting its position based on TPS calculations reduced deviations to within ±3% for 20 studied cases.
• The mean percentage deviation after optimization was 2.10% ± 1.14 (standard deviation 2.33, 95% confidence interval).
• Cases with persistent significant variations were further investigated using film dosimetry.

Conclusions:

• Adjusting ion chamber position based on TPS data is an effective method to resolve discrepancies in IMRT point dose measurements.
• This optimization strategy ensures that dose verification meets established tolerance criteria, enhancing treatment safety and efficacy.
• Film dosimetry can be employed for further analysis in challenging cases.