Experimental determination of the beam quality dependence factors, k_Q, for ionization chambers used in photon and electron dosimetry

Abstract

Dosimetry in radiotherapy with ionization chambers calibrated in 60Co gamma beams in terms of absorbed dose to water, DW, can be performed if a factor conventionally denoted as kQ is known. The factor kQ depends on the beam quality and the chamber characteristics. Calculated values of the kQ factors for many types of ionization chamber have been recently published. In this work the experimental determination of the kQ factors for various ionization chambers was performed for 6 MV and 15 MV photon beams and for a 14 MeV electron beam. The kQ factors were determined by a procedure based on relative measurements performed with the ionization chamber and ferrous sulphate solution in 60Co gamma radiation and accelerator beams, respectively. The experimental kQ values are compared with the calculated values so far published. Theoretical and experimental kQ values are in fairly good agreement. The uncertainty in the experimental kQ factors determined in this work is less than about 1%, that is, appreciably smaller than the uncertainty of about 1.5% reported for the calculated values.

Measurement of ionization chamber absorbed dose factors in megavoltage photon beams

“…Included in this group are the so-called "Short Farmer" chambers with a volume around 0.23 cm 3 . Many of these chamber types ͑or previous versions͒ have been investigated previously [3][4][5][6][7][8][9][10][11][12]15 and the international community recommends such chambers for the output calibration of linear accelerators. ͑ii͒ Scanning chambers.…”

“…Stucki et al 5 also investigated these two chamber types at the Swiss metrology institute, METAS, while Pieksma et al 6 determined k Q factors for the NE2611 and Palmans et al 7,8 investigated the NE2571. Guerra et al 9 used Fricke dosimetry to determine factors for five types of chambers, including the NE2561, NE2571, and Capintec PR-06C. Seuntjens et al 10 calibrated six types of chamber against the NRC primary standard water calorimeter ͑NE2611, NE2571, NE2581, PR-06C, PTW30001, and Exradin A12͒.…”

Measured and Monte Carlo calculated kQ factors: Accuracy and comparison

“…Measured k Q factors have been determined in previous publications using water calorimetry [3][4][5][6][7][8] or Fricke dosimetry. [9][10][11][12] However, aside from the work of McEwen, 1 experimental determination of k Q has been limited to only a few chamber types. Additionally, many of the measured values suffer from large uncertainty (in most cases >0.5%) or use Fricke chemical dosimetry, which must be corrected for intrinsic energy dependence.…”

NCS Report 18: Code of practice for the absorbed dose determination in high energy photon and electron beams

“…Guerra et al [58] reported an experimental k Q value of 0.904 for a NE 2571 in a 14 MeV electron beam with an R 50 = 5.27 cm. The present NCS report predicts a value of 0.914 for these conditions.…”

“…Also, for at least three references the sleeve thickness was only 0.5 mm [53,54,58] and in one paper a nylon sleeve of 0.3 mm thickness was used [56]. These differences can make a small contribution to the spread in the data at the highest photon energies since it has been demonstrated that a 1 mm thick PMMA sleeve reduces the response of a Farmer type ionisation chamber by 0.2% ± 0.1% in a 20 MV beam [92].…”