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Ion Recombination Corrections Factor in Flattening Filter-Free and Conventional Photon Radiation

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Z Wang

B Yang , Z Wang*, J Qiu , Peking union medical college hospital, Beijing, Beijing


SU-I-GPD-T-479 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To investigate the relationships between Pion and different parameters ₍calculation methods, bias voltage, beam energy, dose rate, depth, different type of chamber and electrometers₎.

Methods: 6 MV, 10 MV, 6 MV₋FFF and 10 MV₋FFF x₋rays were fully commissioned on a Elekta Versa HD linear accelerator. First part of this works is to investigate a method to decide the Pion value. The Pion values for beams were measured at source₋to₋surface distances(SSD) of 100cm in a water tank phantom at a depth of 5.2 cm for 6 MV and 6 MV₋FFF beams and 10.2 cm for 10 MV and 10 MV₋FFF beams in a 10 * 10 cm² field. The results are calculated by two₋voltage method and with 1/V versus 1/Q curves ₍Jaffe plots₎ in different energies and different bias voltages to find a suitable method and bias voltage for Pion calculation. Second part, we compare the relationships between Pion and factors of dose rate, energy, types of chamber and vendors. Third part, we discussed the relationships of Pion and depth and if we need to introduce ion recombination correction factor in percentage depth dose curve measurements.

Results: Two-voltage method and Jaffé-plot method shows small differences (<1%) for all energies with 300 V-100 V , 400 V-200 V, 400 V-100 V bias voltage. All results for different chambers and vendors for all energies were within 2% from the unity(1 ≤Pion<1.02). The ion recombination effect caused by different dose rate is not substantially different. The Pion factor changes more than 2% in different depth for 10 MV-FFF beams.

Conclusion: We recommended a thoroughly Pion measurement in commissioning and quality assurance procedure.

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