Abstract
Purpose: Beam modelling of 7MV unflat beam of Siemens ONCOR linear accelerator using Pinnacle3 planning system. Methods: Siemens ONCOR Impression plus linear accelerator equipped with 6MV (300MU/min, with FF) has recently approved beam of 7MV UF (2000MU/min, without FF). Beam quality index has been measured over last two years and has been consistent, varying within 1.0%. The output calibration was done with 0.65cc chamber following TRS398 protocol. KQ value was interpolated for the new beam. Output factors were measured at 10cm depth with 0.65cc chamber for field sizes above 5×5cm; and with 0.13cc chamber for smaller fields. The depth dose curves and beam profiles at different depths for various field sizes were collected using 0.13cc chamber in Blue phantom RFA (IBA dosimetry). The collected beam data (ASCII format) was imported into the Pinnacle3 V9.2 (Philips) treatment planning system. Results: Beam modelling was done in Pinnacle3 system. Measured beam data was compared against the available beam spectra of comparative beam energy. E_Tune All InSections sequence from the auto modelling option gave us a best fit for the measured data. Beam profiles were modelled as arbitrary beam profile. Few parameters obtained after completion of 7MV UF beam modelling were compared against 6MV flat beam data that was already modelled before. Beam spectrum for 7UF beam differs from 6MV spectrum. Maximum number of relative photons per energy is reached at 2MeV and 1.5MeV for 7UF and 6MV respectively. This slight increase in energy is also shown in beam quality index which is 0.681 for 7UF and 0.672 for 6MV. There is reduction in spectral off axis softening factor to 1.6878 (7UF) from 13.9628 (6MV) Conclusion: The modelled beam profiles at dmax were compared with the measured ones for three different field sizes and found be within 1% variation. This confirms the correctness of the modelling.
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