(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: The present study has the objective to suggest QA limits for iris aperture variations by determining their implications in the dosimetry calculations.
Methods: The iris collimator, with 5, 7.5, 10, 12.5, 15, 20, 25, 30, 35, 40, 50 and 60 mm apertures, has being manufactured and calibrated to exhibit an aperture size precision under 0.2 mm. Accuray provides appropriate software to check iris spot sizes based on Gafchromic film. Usually, film is shot at 800 SAD for 600 MU and later analyzed (Fig.1A and setup on Fig.1B). The mechanical precision of the iris collimator is close to the observed uncertainties on spot film analyses, making difficult to set a limit for periodic QA. Commissioning output measurements (Fig.2) were used (SADs; 800, 650, 1000 mm). Output variations as percentage difference were observed by fitting parabolic curves at each point locally. Plots for output variations as a function of aperture were obtained with 0.15 mm variations (Fig.3A). Moreover, variations of apertures were performed on the fitted curves to obtain aperture size changes that produce an output change of 0.5% as function of aperture size (Fig. 3B).
Results: Fig.3A shows that for collimator apertures of 10 mm and higher, the output variation is under 0.5%, except for 5, and 7.5 mm apertures. Fig.3B shows that for apertures 15 mm and higher the apertures must vary about 0.5 mm or more to produce a 0.5% output difference.
Conclusion: The 5, and 7.5 mm apertures should never be used for treatments, as recommended by Accuray. The recommended aperture uncertainties for 10mm is 0.2 mm, for 12.5 mm is 0.35 mm, and for 15 mm and higher is 0.5 mm, and more is not recommend since the dose volume spacing is typically 1.02 x slide size x 1.02 mm.
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