The spatial accuracy of two frameless, linear accelerator-based systems for single-isocenter, multitarget cranial...

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiosurgery Technology

Background:

• Single-isocenter, multitarget cranial stereotactic radiosurgery (SRS) offers efficiency but poses challenges with spatial accuracy, particularly for targets distant from the isocenter.
• Frameless, linac-based SRS systems are increasingly used for multitarget treatments, necessitating precise spatial positioning.
• Understanding the impact of distance from the isocenter on positioning accuracy is crucial for treatment planning and safety.

Purpose of the Study:

• To evaluate and compare the spatial accuracy of two frameless, linac-based SRS systems for single-isocenter, multitarget treatments.
• To assess the accuracy as a function of distance from the isocenter.
• To inform the selection of appropriate planning target volume (PTV) margins for multitarget SRS.

Main Methods:

• Two SRS systems were evaluated: one using ExacTrac for image guidance and another using cone beam computed tomography (CBCT).
• A phantom with 12 targets (BBs) was positioned at varying distances (up to 13.8 cm) from the isocenter.
• Data were collected after initiating alignment from five different offsets and imaging at seven gantry and couch angles.

Main Results:

• Positioning errors varied with distance from the isocenter for both systems.
• A 1-mm PTV margin was deemed adequate for targets up to approximately 7-8 cm from the isocenter.
• For targets 10 cm or more from the isocenter, CBCT-based alignment errors were within 1.1 mm, while ExacTrac-based errors reached up to 2.2 mm.

Conclusions:

• CBCT-based image guidance demonstrated superior spatial accuracy for targets farther from the isocenter in single-isocenter, multitarget cranial SRS.
• The findings support the use of CBCT for improved precision in complex SRS cases.
• Careful consideration of PTV margins is necessary, especially for targets located at greater distances from the treatment isocenter.