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ExacTrac Setup Uncertainties in Intracranial and HN Patients: Comparison with CT-On-Rail

X Wang

X Wang*, Z Zhao, H Wang, J Yang, MD Anderson Cancer Center, Houston, TX

SU-E-J-11 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

To evaluate the accuracy of ExacTrac system based setup in intracranial and HN patients, and provide quantitative data to reduce PTV margin.

The accuracy of ExacTrac system based patient setup depends on both the excellence of its image registration and the reliability of the congruence of its image isocenter with the linac radiation isocenter. The accuracy of ExacTrac system's image registration was studied by marking the ExacTrac determined isocenter (< 1mm residual translation, and < 2 degree residual rotation) with 2 mm in diameter BBs using the room laser and comparing it with CT determined isocenter. CT-on-rail was used to acquire the 3D-data using the same scan parameters as the planning CT. An in-house software registered the daily CT with the planning CT on the fly and calculated the difference between the CT determined isocenter and BB marked isocenter. The small residual rotations were manually examined with the in-house software using boney landmark. A total of 68 fractions spread over 12 patients were used in this study. Short Orfit masks were used as the patient immobilization device. Both daily and weekly QA were performed to ensure the agreement between the laser and Exactrac image isocenter was within 1 mm.

The distance between ExacTrac and CT-on-rail determined isocenter were 1.66±0.9 mm with 95% less than 3 mm. For the small residual rotation angle detected by the ExacTrac, CT data also showed good agreement. ExacTrac snap data between beams of an IMRT delivery demonstrated the intra-fraction motion can be controlled within 2 mm

CT data confirmed that ExacTrac can accurately setup both intracranical and HN patients. PTV margin for patients with the Orfit masks can be reduced to 3 mm based on the image isocenter QA, image registration accuracy and motion tracking ability of ExacTrac system.

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