Absorbed dose beam quality correction factors kQ for the NE2571 chamber in a 5 MV and a 10 MV photon beam

Dose to water(D-W) determination in clinical high-energy photon beams with ionization chambers calibrated in terms of absorbed dose to water has been proposed as an alternative to ionization chamber dosimetry based on air kerma calibrations. D-W in the clinical beam is derived using a k(Q) factor that scales the absorbed dose calibration factor in the reference beam to the absorbed dose calibration factor in the user beam. In the present study k(Q) values were determined for the NE2571 chamber in a 5 MV and a 10 MV high-energy photon beam generated at the 15 MeV high-intensity electron linac of the University of Gent. A set of three NE2571 chambers was calibrated relative to the Gent sealed water calorimeter both in Co-60 and in the linac beam at a depth of 5 cm and a source to detector distance of 100 cm. Two high-purity chemical water systems were used in the detection vessel of the calorimeter, H-2-saturated and Ar-saturated pure water, which are both supposed to give a zero heat defect. TPR1020 and %dd(10) have been evaluated as beam quality specifiers. Simulations using the BEAM/DOSXYZ Monte Carlo system were performed to evaluate potential corrections on the measured beam qualities. The average k(Q) values measured for the three NE2571 chambers in the 5 MV and 10 MV photon beams are 0.995 +/- 0.005 and 0.979 +/- 0.005 respectively. For the three chambers used, the maximum deviation of individual k(Q) values is 0.2%. The measured beam quality specifiers %dd(10) and TPR1020 are 67.0 and 0.705 for the 5 MV beam and 75.0 and 0.759 for the 10 MV beam. Although our beam design is very different from those used by other investigators for the measurement of k(Q) values, the agreement with their results is satisfactory showing a slightly better agreement when %dd(10) is used as the beam quality specifier.