Comprehensive radiation and imaging isocenter verification using NIPAM kV-CBCT dosimetry

Area of Science:

• Medical Physics
• Radiotherapy Quality Assurance

Background:

• Accurate radiation isocenter alignment is critical for stereotactic radiosurgery (SRS) precision.
• Existing quality assurance (QA) methods may not fully assess coincidence with imaging coordinate systems.

Purpose of the Study:

• To develop and demonstrate a comprehensive method for directly measuring radiation isocenter uncertainty.
• To evaluate the coincidence of the radiation isocenter with the cone-beam computed tomography (kV-CBCT) imaging coordinate system.
• To ensure the method can be performed within a typical QA time slot.

Main Methods:

• Utilized a three-dimensional (3D) N-isopropylacrylamide (NIPAM) dosimeter where dose increases electron density, visible in kV-CBCT.
• Irradiated the dosimeter at eight couch/gantry combinations with unique orientations using MLCs and cones.
• Quantified displacement from the imaging isocenter by analyzing CBCTs and calculating contrast-to-noise ratio (CNR).

Main Results:

• The entire QA process, including setup, irradiation, and CBCT readout, was completed within 38 minutes.
• The minimum radius encompassing all beams was found to be as low as 0.38 mm for MLCs.
• The method demonstrated sensitivity to alignment errors, with a 0.5 mm MLC shift increasing the radius to 0.90 mm.

Conclusions:

• A feasible method for comprehensive isocenter verification using NIPAM dosimeters with sub-millimeter accuracy was demonstrated.
• The technique effectively evaluates coincidence with the imaging coordinate system for both MLCs and SRS cones.
• This approach enhances SRS quality assurance by providing direct and efficient isocenter uncertainty measurements.