Determination of N(pp)(gas) and N(pp)(D) for a plane-parallel chamber.

The purpose of this work is to investigate the consistency of determining N(pp)(gas) and N(pp)(D) by 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 (60)Co beam; and 3) in-air calibration in a (60)Co 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 (60)Co in-phantom methods was a solid water phantom (RW3). For the electron beam method, the nominal energies were 18 and 21 MeV. An acrylic buildup of 0.5 g/cm(2) thickness was used for the (60)Co in-air method. For each method, N(pp)(gas) and 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 x 10 cm(2) field size at the depth of the maximum for each electron beam. The values of N(pp)(gas) by 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 for N(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)(gas) and N(pp)(D) for the three 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.