A novel approach to accurate portal dosimetry using CCD-camera based EPIDs

Area of Science:

• Medical Physics
• Radiotherapy Technology
• Image Processing

Background:

• Accurate portal dosimetry is crucial for radiotherapy quality assurance.
• Existing methods for electronic portal imaging devices (EPIDs) often rely on a priori assumptions about cross-talk.
• CCD camera-based EPIDs (CEPIDs) offer high signal-to-noise ratios for improved imaging.

Purpose of the Study:

• To develop and validate a new portal dosimetry method using CEPIDs.
• To determine and correct for non-symmetrical and position-dependent cross-talk in CEPIDs.
• To enable fast and accurate derivation of portal dose images (PDIs) from electronic portal images (EPIs).

Main Methods:

• Directly imaged cross-talk kernels from small fields ('pencil beams') using a cooled CCD camera.
• Signal calibration using two reference fields.
• Iterative deconvolution to derive PDIs from EPIs, accounting for determined cross-talk.

Main Results:

• The method achieved high accuracy (0.2+/-0.7% 1 SD) across various field configurations and phantom types.
• Validated against scanning diode measurements for diverse clinical scenarios, including IMRT fields.
• Identified back-scattered radiation as a significant contributor to CEPID cross-talk signals.

Conclusions:

• The proposed method provides a fast and accurate approach to portal dosimetry using CEPIDs.
• It overcomes limitations of previous methods by directly characterizing cross-talk.
• Offers a robust solution for real-time quality assurance in radiotherapy.