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Our centre began offering stereotactic body radiation therapy (SBRT) treatments for peripheral lung lesions in 2011. As a high-precision technique, SBRT requires precise positioning of the target, and precise quality assurance (QA) of the imaging systems; these may be dependent on local equipment and procedures. We aimed to maintain target position within 3 mm throughout each treatment, and imaging and mechanical systems to at least 2 mm accuracy. A retrospective analysis was done of patient cone-beam (CB) data, and of our imaging system QA, to assess our spatial objectives and look for opportunities for improvement. The data indicated that, using our immobilization and imaging procedures, target position was maintained within 3 mm 96% of the time, and 75% within 2 mm, similar to results from other centres. Imaging system QA using the standard ball-bearing test showed system accuracy was maintained well within 1 mm. These results were compared with a simpler daily QA procedure using a Pentaguide phantom. The mean and standard deviation of the radial difference in the kV-MV isocenter coincidence for the two techniques was 0.62mm +/- 0.23mm. With appropriate choice of tolerance and action level, the morning QA was sufficient for identifying outliers requiring further investigation. This analysis gives us confidence in understanding the performance of our SBRT lung treatments, and gives baselines for analyzing changes to patient immobilization or imaging procedures.

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