Fast intra-fractional image-guidance with 6D positioning correction reduces delivery uncertainty for stereotactic radiosurgery and radiotherapy.

The purpose of this study is to analyze intra-fractional positioning uncertainty for stereotactic radiotherapy and radiosurgery of cranial tumors. Specifically, we wish to determine the use of intra-fractional image guided patient positioning verification is necessary during delivery of "frameless" stereotactic radiotherapy and radiosurgery (SRT/SRS) and non-coplanar radiation beams, and if positioning uncertainty is associated with overall treatment time. Orthogonal radiographic treatment verification data was extracted for 288 patients and 1344 fractions, and were analyzed with respect to 3D translational and angular corrections once during treatment delivery of SRT/SRS. We find that positioning corrections greater than 2 mm are required for approximately 6% of beams, and that the magnitude of the translational corrections was significantly associated with the delay time between beams (p=0.003). Further, we find that the maximum angular and translational deviations were associated (p<0.001). We conclude that a subgroup of SRT/SRS patients may have considerable positioning error unless this is monitored and corrected during treatment, and that keeping the imaging and delivery times below approximately 5 min is beneficial towards clinically relevant geographical errors. In case longer time-delays than 5 min occurs, the treatment staff should consider acquiring a new set of radiographs in order to verify the patient's position, assuming this technically feasible to be performed quickly.