Room: Exhibit Hall
Purpose: To demonstrate some of the issues involved with determining electron cutout factors and demonstrate that doing so incorrectly can lead to significant deviations from the intended dose.
Methods: The electron cutout factors for 6cm, 10cm, 15cm, and 20cm applicators were measured for five energies on a Varian Truebeam. Factors were measured for cutouts from 3cm diameter up to the maximum dimension of the applicator, with 1cm resolution/spacing. The results were compared with Truebeams at four other sites, which used lower resolution. Two clinical cases are examined which calculate the cutout factor based on each institutionsâ€™ factors.
Results: It was observed that for small field sizes the gradient in the cutout factor was relatively large. The cutouts from different institutions agreed to within Â±1%. The cutout factor resulting from using a lower-resolution table was as much as 4.6% different for the first case and 4.2% different for the second case.
Conclusion: This work demonstrates that using a cutout factor table of insufficient resolution can cause a significant error in the dose delivered. It is important to have several points in the high-gradient region, although fewer may be used in the low-gradient region.