Enhancing intrafraction position monitoring for prostate radiotherapy on a conventional linear accelerator: an optimization study.

To compare the intrafraction prostate motion monitoring capabilities between intrafraction Cone Beam Computed Tomography (IF-CBCT) and SeedTracker-based real-time monitoring, and to optimize imaging doses in real-time monitoring using the IF-CBCT image acquisition method. Simulations of static and dynamic intrafraction prostate motions were conducted on a phantom using a robotic arm. The study utilized the XVI imaging system of the Elekta linear accelerator for IF-CBCT and SeedTracker-based monitoring during hypofractionation and Stereotactic Body Radiation Therapy (SBRT). The optimal imaging frequency for real-time monitoring was determined by calculating VMAT gantry traverse times. The effective dose resulting from IF-CBCT and SeedTracker-based monitoring approaches were compared. IF-CBCT showed static offsets as seed duplications and the offsets calculated using 'Seed' automatic image registration available XVI system depend on the initial position of the seeds in verification and localisation image sets. This dependency resulted in large differences (up to 4.9 mm) between actual and calculated position offsets. Dynamic offsets resulted in blurring or duplication of seeds in IF-CBCT images depending on the type of the dynamic motion. SeedTracker-based real-time monitoring successfully identified position deviation events as they occurred during treatment. For hypofractionation and SBRT treatments, IF-CBCT imaging resulted in an effective dose of 54.3 mSv and 13.6 mSv, respectively. Optimized imaging frequency for real-time monitoring led to a dose reduction of up to 86.2% and 97.2% for hypofractionation and SBRT regimens, respectively, compared to the IF-CBCT approach. SeedTracker real-time monitoring effectively identified target position deviations in real-time, surpassing the capabilities of the IF-CBCT approach. Moreover, the SeedTracker imaging approach significantly reduced imaging doses compared to IF-CBCT.