SU-E-T-201: Comparison Between Two Immobilization Systems for Cranial Stereotactic Radiosurgery Using The ExacTrac Image-Guidance System

Purpose: The ExacTrac Image-Guidance system enables frameless stereotactic radiosurgeries with real-time image guidance that allows repositioning on a sub-millimetric scale. In order for such treatments to be effective, the patient’s head has to be immobilized using a custom mask created to conform to the patient’s anatomy. There are two cranial immobilization systems used in our institution: masks manufactured by Brainlab and Civco. For both systems, it is not evident what the achievable level of intrafraction immobilization is for each system in our patient population. The ExacTrac system makes it possible to quantitatively assess the immobilization the two mask systems offer. Methods: The ExacTrac imaging system was used to assess intrafraction geometric displacement in the three Cartesian directions for each of these masks multiple times during Stereotactic Radiosurgery, SRS or hypofractionated treatment. Intrafraction motions were considered uncorrelated random movements away from isocenter. The displacement from isocenter, excluding initial correction, was calculated in over 100 instances for each mask system. The distances from isocenter were compared with histograms, average, median, and by percentile. Results: It was found that the Brainlab mask had mean distance from isocenter of 0.5 mm (SD 0.45 mm, median 0.37 mm) compared to 1.0 mm (SD 1.25, median 0.61 mm) for Civco. The distributions were compared with a Mann-Whitney U test and were found to be significantly different (P 3 mm and 0.6 mm and <1.5 mm, for the Civco and Brainlab systems respectively. As a rule of thumb, the averages, medians, and percentiles for the Civco system were approximately twice that of the Brainlab system. Conclusion: Given the results, the Brainlab mask system should be used to minimize risk of geometric error in delivery of SRS or high dose gradient techniques.