(Tuesday, 7/31/2018) 3:45 PM - 4:15 PM
Room: Exhibit Hall | Forum 4
Purpose: To evaluate the accuracy of the RayStation electron Monte Carlo algorithm to commission the system for clinical use.
Methods: Tests recommended by AAPM’s Medical Physics Practice Guidelines 5.a (MPPG 5.a) were performed, including evaluation of output factors, custom cutout geometries, oblique incidence, and heterogeneities. For MPPG 5.a testing, RayStation calculations were compared to point dose measurements, as well as depth dose and profile measurements using 2D gamma analysis. Additionally, to test the algorithm in the presence of heterogeneities and irregular surface contours, four complex phantom geometries designed to mimic clinical scenarios were used (nose, bone, breast, and lung). EBT3 radiochromic film measurements for a 10x10 field size and 100cm SSD were compared to dose calculations using 2D gamma analysis for these complex phantoms.
Results: All MPPG 5.a testing showed excellent agreement, well within suggested criteria, with the exception of output for the small custom cutout. To investigate this further, output factors were evaluated for various square cutouts at 100-110cm SSD. Of the 312 output factors evaluated, 21 (6.7%) were outside of 3% and 4 (1.3%) were outside of 5%, with these larger errors generally being for the smallest cutout sizes within a given applicator. For complex phantom geometries, the electron Monte Carlo algorithm was able to accurately handle the presence of bone and cork heterogeneities as well as irregular surface contours (cylindrical breast and triangular nose geometry). Gamma analysis showed an average of 89.3% pixels passing for 3%/3mm criteria.
Conclusion: RayStation’s electron Monte Carlo algorithm can accurately handle complex clinical geometries. However, the algorithm can result in calculation errors for small cutouts, and this calculation error is sensitive to parameters in the electron beam models. Clinical processes should take this limitation into account, i.e., with the use of secondary MU calculations and patient-specific measurements as necessary.
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