Estimation of optimal margin for intrafraction movements during frameless brain radiosurgery.

PURPOSE

To examine the dosimetric effect of intrafraction movements occurred during image-guided frameless brain radiosurgery and to derive the optimal margin recipe to compensate the movement.

METHODS

The patients' movements during image-guided radiosurgeries were measured using skull-tracking method incorporated in the CyberKnife system. The dosimetric changes with the movements were computed using the six different dynamic-arc treatment plans based on the dose-grid analysis method. The authors extensively searched the proper relationship between the dose variations and the intrafraction geometric errors. The optimal margin for intrafraction movement was estimated via statistical analysis of the dosimetric changes with 262 actual patients' data.

RESULTS

The overall geometric effect of intrafraction movements was approximated as 1.0 r+0.2σ, where r and σ are the average and standard deviation of the movements, respectively. The authors computed the required margins to compensate the movements with various confidence levels and with various estimated times for completing the treatments. The computed optimal margins were calculated as 2.1, 3.2, and 4.2 mm at 90% confidence level when the authors assumed the estimated treatment times of 10, 20, and 30 min, respectively.

CONCLUSIONS

The authors provide a quantitative relationship for dosimetric change with the intrafraction movement and derived appropriate margin recipes to ensure the prescribed dose delivery to targeted area for frameless brain radiosurgery.