Extending the EclipseTM AcurosXB output factor table for small field radiosurgery.

Abstract

To investigate the necessity of extending the output factor table (OF Table) of the Varian EclipseTM Treatment Planning System for small field stereotactic radiosurgery (SRS) and stereotactic body radiosurgery (SBRT) treatments. 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 3×3cm to 40×40cm OF Table, except the OF Table in the new model was extended to cover the range from 1×1cm to 40×40cm. 80 small square and rectangular output factors were measured on a Varian TrueBeam utilizing a Standard Imaging Exradin W2-1×1 scintillator detector, inside a PTW BeamScan water tank with 95cm SSD at 5cm depth. Cerenkov contamination was corrected using a rectangular field method (2cm×15cm field). Nine Radiosurgery plans with primary jaw setting ranging from 0.7cm to 2.0cm 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 0.5×0.5cm primary jaw setting. For the 6FFF beam, the difference between the two beam models was ∼ 6% for 1×1cm jaw settings and 4% at 1.5×1.5cm, 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 ∼7.5% for 1×1cm, 5% for 1.5×1.5cm, and 3% for 2×2cm jaw settings, with the extended OF Table requiring higher MUs. For jaw settings smaller than 1×1cm, 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 0.5×0.5cm, calculated via Eclipse external beam, was more than 30% greater than that measured for both 6FFF and 6MV beams. Eclipse does a satisfactory job for primary jaw sizes down to 2cm. For jaw settings smaller than 1.5cm, the OF Table in Eclipse should be extended to improve the dose calculation accuracy.