Extending the Eclipse™ AcurosXB output factor table for small field radiosurgery

Purpose: To investigate the necessity of extending the output factor table (OF Table) of the Varian Eclipse (TM) Treatment Planning System for small field stereotactic radiosurgery (SRS) and stereotactic body radiosurgery (SBRT) treatments. Methods: A new AcurosXB 15.6 beam model was created in the Eclipse Beam Configuration, which is identical to the one that has been used in the clinic with a default 3 x 3 cm to 40 x 40 cm OF Table, except the OF Table in the new model was extended to cover the range from 1 x 1 cm to 40 x 40 cm. 80 small square and rectangular output factors were measured on a Varian TrueBeam utilizing a Standard Imaging Exradin W2-1x1 scintillator detector, inside a PTW BeamScan water tank with 95 cm SSD at 5 cm depth. Cerenkov contamination was corrected using a rectangular field method (2 cm x 15 cm field). Nine Radiosurgery plans with primary jaw setting ranging from 0.7 cm to 2.0 cm were evaluated by both beam models. The monitor unit (MU) differences between the two beam models were calculated for identical 3-dimensional (3D) absolute dose distributions. Output factors, measured versus Eclipse calculated, were compared down to 0.5 x 0.5 cm primary jaw setting. Results: For the 6FFF beam, the difference between the two beam models was similar to 6% for 1 x 1 cm jaw settings and 4% at 1.5 x 1.5 cm, with the extended OF Table requiring higher MUs for the same dose prescription and same 3-dimensional isodose distribution. For the 6MV beam, the corresponding difference is similar to 7.5% for 1 x 1 cm, 5% for 1.5 x 1.5 cm, and 3% for 2 x 2 cm jaw settings, with the extended OF Table requiring higher MUs. For jaw settings smaller than 1 x 1 cm, measured dose can be considerably smaller than Eclipse predicted dose, even with the OF Table extension. This is reflected by the fact that the output factor for 0.5 x 0.5 cm, calculated via Eclipse external beam, was more than 30% greater than that measured for both 6FFF and 6MV beams. Conclusions: Eclipse does a satisfactory job for primary jaw sizes down to 2 cm. For jaw settings smaller than 1.5 cm, the OF Table in Eclipse should be extended to improve the dose calculation accuracy.