A Study of Varian Linac Beam Data Equivalency for IMRT
A Dimofte*, T Zhu, Univ Pennsylvania, Philadelphia, PASU-E-T-393 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: The purpose of this study was to compare the photon beam data for four Varian (2300iX) accelerators in six categories: beam quality, phantom scatter factor, output ratio in air, primary off axis ratio, electron disequilibrium factor and benchmark measurements including dose per MU ratio at selected points and a few IMRT plans.
Methods: We compared the beam quality of these accelerators, characterizing the attenuation and beam hardening coefficients and percent depth dose (PDD) at 10cm depth. The phantom scatter factor (Sp) and output ratio in air (Sc) were measured in a full phantom and a miniphantom, respectively, at depth of 10cm and SAD = 100cm as a function of field sizes.
Results: PDD agreed within 1% and the attenuation coefficients agreed within 3% for the same energy. The output ratio in air Sc is almost machine independent but was energy dependent and agreed within 0.3%. For the same nominal energy, the phantom scatter factor (Sp) measured at 10cm depth agreed within 1.4% regardless of the accelerator. We delivered 3 IMRT plans (prostate, H&N and lung) to study the difference in dose delivery among the four linacs. The dose delivered was compared for depths of 5, 10 and 20cm and to the calculated dose. All measurements were performed using a 2D diode array and analyzed using the distance to agreement (DTA) criteria of 3% and 3mm. The agreement among linear accelerators is much better than the agreement between measurement and calculation, and are better than 3.5% for all IMRT cases studied.
Conclusion: Accelerators from the Varian manufacturer, with the same nominal energy and model, can be treated as identical to within 1.8% for conventional treatment as well as for IMRT treatment, provided that caution is given to the dosimetrical characteristics for OAR and small field dosimetry.