Benchmark Comparison Between Varian TrueBeam and Other Linac Models
T Zhu1*, X Liang2, A Dimofte3, (1) Univ Pennsylvania, Philadelphia, PA, (2) University of Pennsylvania, Philadelphia, AA, (3) Univ Pennsylvania, Philadelphia, PASU-E-T-574 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: We have examined the photon beam data agreement between the newly released Varian TrueBeam with other Varian Linacs.
Methods: Existing benchmark points were established for dose per MU at various points in the field for various collimator settings and SSD. The benchmark points are chosen to be representative of clinical extreme cases with minimal number. They are designed to examine both photon transport (including output ratio in air, phantom scattering, primary off-axis ratio, off-axis beam softening, collimator exchange effects) as well as electron transport (including 1x1 cm² field size, buildup regions, etc.). All measurements were made with a 0.1cc thimble ionization chamber except for in the small field and in the buildup regions, where a Scanditronix stereotactic diode was used. An IBA blue radiation analyzer was used for all point measurements to ensure accuracy of positioning. For the purpose of comparison, the calibration geometry 1 cGy/MU for all linacs is made to be the same by normalizing the measured ratio to the same calibration geometry.
Results: The benchmark points within transient electron equilibrium region agree to within 1% between the Varian TrueBeam and a Varian 2300iX machine. Primary off-axis ratio agrees to within 0.3% at 15 cm off-axis at both 10 and 20 cm depths. In the buildup region, the agreement for a 3 x 3 and 40 x 40 cm² fields at 0.5 cm depth agrees to within 2.1% for 6 MV and 2.5% for 15 MV, respectively. This is possibly due to the positioning error because of the large dose gradient. The dose per MU for 1x1 cm² field at 10 cm depth agrees to within 0.3% for 6 MV and 0.8% for 15 MV, respectively.
Conclusion: The photon beam data for the Varian TrueBeam agrees with the existing Varian 2300ix for IMRT for the same nominal energies, provided they are calibrated the same way.