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Source position verification and dosimetry in HDR brachytherapy using an EPID.
R L Smith1, M L Taylor, L N McDermott
1William Buckland Radiation Oncology, Alfred Health, Melbourne 3004, Australia and School of Applied Sciences and Health Innovations Research Institute, RMIT University, Melbourne 3001, Australia.
Medical Physics
|December 11, 2013
Summary
Electronic portal imaging devices (EPIDs) can verify high dose rate (HDR) brachytherapy accuracy. This study shows EPIDs provide crucial source position and dose distribution data for quality assurance.
Area of Science:
- Medical Physics
- Radiotherapy
- Brachytherapy
Background:
- Accurate high dose rate (HDR) brachytherapy delivery relies on precise source positioning and timing.
- Treatment inaccuracies stem from anatomical changes or human errors in plan implementation.
- Electronic portal imaging devices (EPIDs) are used for patient positioning and dosimetric verification in external beam radiotherapy.
Purpose of the Study:
- To characterize the response of an EPID for use with an iridium-192 (192Ir) brachytherapy source.
- To demonstrate EPID's utility as a verification tool for both source position and dose distribution in HDR brachytherapy.
- To assess EPID's potential for quality assurance in HDR brachytherapy.
Main Methods:
- Investigated EPID response to an 192Ir source, assessing reproducibility, linearity with dose rate, energy dependence, and charge build-up.
- Determined three-dimensional source position resolution.
- Delivered a phantom treatment plan and compared measured EPID dose distribution with the planned dose.
Main Results:
- Mean absolute source position error was 0.26 mm in the plane parallel to the EPID.
- Z-coordinate resolution varied with source-detector distance (SDD), with 95% confidence intervals of ±0.1 mm (50 mm SDD) to ±2.0 mm (150 mm SDD).
- EPID response was linear to dose rate, with a developed calibration procedure accounting for spectral dependence and nonlinearity. Measured dose differed by <2% for 98% of pixels at 100 mm SDD.
Conclusions:
- EPID dosimetry offers a novel quality assurance method for HDR brachytherapy.
- This technique verifies both the geometrical distribution of delivered dose and source positions.
- It surpasses current HDR brachytherapy verification systems by providing comprehensive QA data.
