Experimental determination ofk_Qfactors for cylindrical ionization chambers in 10 cm × 10 cm and 3 cm × 3 cm photon beams from 4 MV to 25 MV

Abstract

For the ionometric determination of absorbed dose to water, Dw, in megavoltage photon beams from a linear accelerator, beam-quality-dependent correction factors, kQ, are used for the ionization chambers. By using a water calorimeter, these factors can be determined experimentally and with substantially lower standard uncertainties compared to calculated values of the kQ, which are published in various dosimetry protocols. In this investigation, kQ for different types of cylindrical ionization chambers (NE 2561, NE 2571, FC 65 G) were determined experimentally in 10 cm × 10 cm photon beams from 4 MV to 25 MV (corresponding beam quality index TPR20,10 from 0.64 to 0.80). The measurements were carried out at the linear accelerator facility of the Physikalisch-Technische Bundesanstalt. It is shown that the kQ factors for a single ionization chamber in 10 cm × 10 cm photon beams can be measured with a relative standard uncertainty of 0.31%. In addition to these measurements in 10 cm × 10 cm fields, kQ factors for the NE 2561 chamber were also determined in smaller 3 cm × 3 cm photon beams between 6 MV and 25 MV. In this case, relative standard uncertainties between 0.35 % and 0.38 % are achieved for the kQ factors. It is found for this ionization chamber, that the ratio of the kQ factors in 3 cm × 3 cm and in 10 cm × 10 cm beams increases with increasing TPR20,10 to reach a value of 1.0095 at TPR20,10 = 0.8 with a relative standard uncertainty of 0.4 %.

Monte Carlo-based investigations on the impact of removing the flattening filter on beam quality specifiers for photon beam dosimetry

“…In this work, the calculated k Q for the ionization chamber NE2561 in WFF beams is in good agreement with the data from Krauss and Kapsch for low energies. However, there is a 0.8% discrepancy between the k Q value from Krauss and Kapsch at TPR = 0.76 (Krauss and Kapsch: k Q = 0.982 ± 0.003; this work: k Q = 0.9743 ± 0.0008). On the other hand, our results are in good agreement with the calculations published by Lye et al .…”

“…and measured values published by Krauss and Kapsch and McEwen for WFF linear accelerators. Krauss and Kapsch determined k Q for two NE2561 chambers. Statistical uncertainties (one standard deviation) correspond to symbol sizes and are within 0.1%.…”

“…The calculated values are compared to the polynomial fits published by Muir et al . and measured values published by Krauss and Kapsch and McEwen for WFF linear accelerators. Krauss and Kapsch determined k Q for two NE2561 chambers.…”

“…The panels in the third and fourth column provide k Q as a function of mean photon energy below the accelerator head and at the point of measurement, respectively. Additionally, the k Q values for the NE2561 and the NE2571 ionization chambers determined by Krauss and Kapsch using a water calorimeter are shown in Fig. for reference.…”

First‐stage validation of a portable imageable MR‐compatible water calorimeter

“…To date, calorimetry has been used widely in conventional high energy photon beams, [6][7][8][9][10][11] and to a somewhat lesser extent in medium/high LET particle beams, 6,[11][12][13][14] high-energy electron beams, 11,[15][16][17][18] and brachytherapy. 11,[19][20][21] Calorimetry efforts in the area of MR-integrated linacs have been limited.…”