Room: AAPM ePoster Library
To investigate the necessity of extending Eclipse output factor tables for small field SRS/SBRT.
A beam model was created using Eclipse Beam Configuration for Acuros 15.6, which is identical to the one used for patient treatment, but with the output factor table extended to 1x1cm. The output factors entered came from measurements on a Varian TrueBeam® utilizing a W2 1x1 scintillator, inside a solid water setup with a 95cm SSD at 5cm of depth. Cerenkov contamination of W2 was corrected using method 3 in Max SD user manual. Nine Radiosurgery plans from four patients, with beam jaw settings ranging from 0.7cm to 1.5cm, were evaluated by both beam models. The 3-D isodose distribution was kept the same between the two beam models, and the difference between the required MU for the same 3D isodose distribution were calculated. Output factors, measured vs. Eclipse calculated, were compared down to a jaw setting of 0.5x0.5cm.
The difference between the two beam models was around 4% for 1x1cm jaw settings, and 2% at 1.5x1.5cm jaw settings, with the extended output factors table requiring higher MUs for the same dose prescription. For jaw settings smaller than 1x1cm, the point dose measurement results showed that it can be considerably lower than that of the Eclipse calculation, even with the output factor table extension. This is reflected in the fact that the output factor for 0.5x0.5cm, calculated via Eclipse external beam, was 42.6% higher than that measured for 6FFF, and 46.8% higher for 6MV, respectively.
Eclipse does a satisfactory job for field sizes down to 2cm. However, for field sizes smaller than 1.5cm, the output factor table in Eclipse should be extended to improve dose calculation accuracy. For jaw settings smaller than 1cm, further investigation on the accuracy of Eclipse is warranted.