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%.

Milroy D ，Patrocinio H ，Seuntjens J 《-》

Sci-Thur AM: Planning - 08: Validation of a commercial Monte Carlo code used for stereotactic radiosurgery and stereotactic body radiation therapy.

Our project consisted of validating the BrainLab iPlan Monte Carlo algorithm, used in conjunction with the stereotactic radiosurgery (SRS) mode of the Varian Novalis TX linear accelerator, for clinical use. Our approach was to "benchmark" the iPlan algorithm by comparing dose distributions with those obtained using a BEAMnrc model of the Novalis SRS mode. The BEAMnrc model was obtained by modifying an existing accelerator model to include the SRS flattening filter and source characteristics of the Novalis TX, and by reprogramming a component module to model the high definition 120-leaf multi-leaf collimator. The free parameters of interleaf air gap and leaf density were adjusted by matching to interleaf leakage profiles measured with EBT2 film. The BEAMnrc model was used to perform comparisons of depth dose curves and planar distributions for fields in homogeneous and heterogeneous slab phantoms between both MC codes and film. The source parameters of electron beam energy, size and angular spread were determined to be 6.6 MeV, 0.7 mm and 0.8 mm (cross and in-plane), and 1.27°, respectively. Comparisons between iPlan and EGSnrc MC codes show agreement within 2% for PDD curves, and a high pass rate (>98%) on gamma analysis (3%/3mm) for planar distributions, when the scored quantity is dose to medium. Discrepancies between both MC codes and film measurements were seen near bone inhomogeneities, where the film trend agrees somewhat with iPlan MC reporting dose-to-water. Further work is being performed to understand these differences and how film is used to measure dose near bone.

Milroy D ，Santos GD ，Seuntjens J ，Patrocinio H ... -  《-》

Monte Carlo modeling and simulations of the High Definition (HD120) micro MLC and validation against measurements for a 6 MV beam.

Borges C ，Zarza-Moreno M ，Heath E ，Teixeira N ，Vaz P ... -  《MEDICAL PHYSICS》

Monte Carlo linear accelerator simulation of megavoltage photon beams: independent determination of initial beam parameters.

Almberg SS ，Frengen J ，Kylling A ，Lindmo T ... -  《MEDICAL PHYSICS》

Development of a Monte Carlo model for the Brainlab microMLC.

Belec J ，Patrocinio H ，Verhaegen F 《PHYSICS IN MEDICINE AND BIOLOGY》