Determinations of N^pp_gasand N^pp_Dfor a plane-parallel chamber

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The purpose of this work is to investigate the consistency of determining N^pp_gasand N^pp_Dby using three independent calibration methods from the AAPM TG 39 and IAEA TRS 381 protocols: 1) calibration with a high-energy electron beam in a phantom; 2)in-phantom calibration in a ⁶⁰Co beam; and 3) in-air calibration in a ⁶⁰Co beam. The plane-parallel chamber considered was the PTW-Markus and the comparisons were made against a calibrated PTW cylindrical Farmer-type chamber 30001. The phantom material used for the electron beam and ⁶⁰Co in-phantom methods was a solid water phantom (RW3). For the electron beam method, the nominal energies were 18 and 21MeV. An acrylic buildup of 0.5 g/cm2 thickness was used for the ⁶⁰Co in-air method. For each method, N^pp_gasand N^pp_D were obtained for the plane-parallel chamber as proposed by the AAPM TG 39 and IAEA TRS 381 protocols. The absorbed doses were measured along the central axis at a distance of 100 cm (SSD =100 cm) with 10×10 cm²field size at the depth of the maximum for each electron beam. The values of N^pp_gasby the three independent calibration methods agreed to within±0.6%. This meant that any of the methods would give a fairly good value. Similar results were obtained forN^pp_D In. comparing the results for the electron beam method at energies of 18 and 21 MeV, the latter gave better agreement. The ratios of N^pp_gasand N^pp_Dfor the 159three methods were in agreement within 0.7%. The results for the absorbed dose intercomparison in the AAPM TG 39 and IAEA TRS 381 protocols showed that they agreed to within±0.7% which meant that any of the calibration methods and two different protocols would give an accurate result.