MO-F-BRB-05: Monte Carlo Modeling of the Novalis TX Stereotactic Radiosurgery Mode

To model the stereotactic mode of the Varian-Brainlab Novalis TX linear accelerator using the BEAMnrc Monte Carlo user code Methods: The EGSnrc Monte Carlo user codes BEAMnrc and DOSXYZnrc were used for photon simulations and dose calculations, respectively. A Monte Carlo model of a Varian Clinac 21 EX was modified to model the stereotactic radiosurgery (SRS) mode of the Novalis, taking into account the smaller dimensions of the SRS flattening filter and limited field sizes. The parameters of source such as energy, size and angular spread, were readjusted following a new procedure outlined by Almberg et al, 2012. A component module, DYNVMLC, previously used to model the Varian Millennium 120 multi-leaf collimator (MLC), was reprogrammed to include the four leaf types of the Varian high definition 120 leaf MLC. Interleaf air-gap and leaf density were adjusted to match interleaf leakage profiles measured with EBT2 film. Subsequent validation included profiles, percent depth dose curves and output factors measured with ion chambers, and other film measurements.

From PDD measurements, the energy of the incident electron beam was determined to be 6.6 MeV. From penumbra measurements, the electron radial intensity distribution, given as the full width at half maximum of a Gaussian distribution, was found to be 0.7 mm (cross-plane) and 0.8 mm (in-plane). From profiles in water, the mean angular spread had to be adjusted to 1.27° to achieve an acceptable match. The interleaf air-gap and the density of the leaves of the HDMLC were determined to be 0.0047 cm and 18.5 g/cm³ , respectively.

The Almberg procedure was successfully implemented in determining the electron beam parameters to model the Novalis Tx's SRS mode. Dose profiles simulated with the new HDMLC component module agreed with measurements within 2%.