Development of an optical-based image guidance system: technique detecting external markers behind a full facemask

Area of Science:

• Medical Physics
• Radiation Oncology
• Image-Guided Therapy

Background:

• Optical image-guided systems offer radiation-free patient monitoring in cancer treatment.
• Current systems often require modifications to full facemasks for head and neck (H&N) cancer and brain tumor patients.
• Accurate patient positioning is crucial for effective radiation delivery and minimizing off-target exposure.

Purpose of the Study:

• To develop and evaluate an optical-based guidance system for managing interfractional and intrafractional setup errors in H&N and brain tumor patients.
• To enable real-time patient monitoring while utilizing a full facemask for immobilization.
• To assess the accuracy and consistency of the developed system compared to commercial alternatives.

Main Methods:

• Infra-red (IR) reflecting markers were attached to a head phantom immobilized by a full face thermoplastic mask.
• A stereo camera system calibrated using the direct linear transform (DLT) algorithm reconstructed 3D marker coordinates.
• Real-time phantom positioning was acquired by detecting IR markers through the facemask, with 6-DOF errors calculated via rigid-body registration against planning CT images.

Main Results:

• The system's calibration accuracy was submillimeter (0.33 mm +/- 0.27 mm).
• Mean marker position deviation between optical images and planning CT was 0.50 mm +/- 0.67 mm.
• Maximum 6-DOF registration deviations were under 1 mm for translation and 1 degree for rotation.

Conclusions:

• The developed optical guidance system demonstrates accuracy and consistency comparable to commercial systems.
• The system effectively manages setup errors by tracking markers through a full facemask.
• This approach allows for simultaneous patient immobilization and precise image guidance in H&N and brain tumor radiotherapy.