Feasibility of closed-MLC tracking using high sensitivity and multi-layer electronic portal imagers

Area of Science:

• Medical Physics
• Radiation Oncology
• Imaging Technology

Background:

• Radiation therapy treatment conformity is often limited by target tissue movement.
• Tumor tracking using beam's-eye-view (BEV) is explored, but fiducial markers can exit the linear accelerator (LINAC) multi-leaf collimator (MLC) field-of-view (FOV).
• Radiation leakage through the MLC is approximately 1%-2%, necessitating improved methods for visualizing markers outside the FOV.

Purpose of the Study:

• To present a proof-of-concept for tracking fiducial markers outside the MLC FOV.
• To investigate the use of high-sensitivity prototype detectors, specifically LKH-5 scintillating glass, and multi-layer imager (MLI) architecture.
• To assess the impact of detector efficiency and image correction on fiducial marker visibility.

Main Methods:

• Employed high-efficiency, prototype LKH-5 scintillating glass detectors.
• Investigated multi-layer imager (MLI) architecture to enhance detector sensitivity.
• Utilized a rectangular median filter for image correction and quantified visibility using peak-to-sidelobe ratio (PSR) of normalized cross-correlation (NCC).

Main Results:

• Improved detector efficiency, via LKH-5 or MLI, reduced the dose required for fiducial marker visibility.
• Minimum doses for visualization with gadolinium oxysulfide (GOS) detectors were 20 MU (single-layer) and 10 MU (4-layer).
• With LKH-5 detectors, minimum doses decreased to 4 MU (single-layer) and 2 MU (4-layer), with a maximum frame rate of ~12 fps at 6 MV FFF.

Conclusions:

• High-sensitivity detectors, like LKH-5, and MLI architecture can effectively track fiducial markers outside the MLC FOV.
• Enhanced detector efficiency significantly reduces the required dose for fiducial marker visualization.
• Image correction techniques improve fiducial marker representation in blocked images, aiding in radiation therapy precision.