Monte Carlo and water calorimetric determination of kilovoltage beam radiotherapy ionization chamber correction factors

Abstract

The in-phantom calibration method for radiotherapy kilovoltage x-ray beams requires ionization chamber correction factors. The overall ionization chamber correction factor accounts for changes in the chamber response due to the displacement of water by the chamber cavity and wall, the presence of the stem and the change in incident photon energy and angular distribution in the phantom to that in air. A waterproof sheath, if required, is accounted for in a sheath correction factor. The aim of this study is to determine chamber correction factors through Monte Carlo (MC) simulations and water calorimetry measurements. Correction factors are determined for the PTW TM30013, NE2571, IBA FC65-G, IBA FC65-P and Exradin A12 ionization chambers. They are compared to experimental values obtained at the German national metrology institute Physikalisch-Technische Bundesanstalt (PTB) with their water calorimetry-based absorbed dose to water primary standard and at other national metrological institutes. An uncertainty analysis considers the contributions to the uncertainty on the chamber correction factors from the field size, photon cross sections, photon fluence spectra and chamber wall and central electrode dimensions. The MC calculated chamber correction factors are within 2.2% of unity with a standard uncertainty of 0.3%. For the 50 kV and 100 - 140 kV radiation beam qualities, the calculated correction factors deviate from the measured correction factors (with a standard uncertainty of 1%) by up to 2.6%. The calculated chamber correction factors for the PTW TM30013 and Exradin A12 are consistent with those derived from the BIPM kilovoltage primary standard. The inconsistencies between the calculated and experimental chamber correction factors indicate the need to further investigate the accuracy of kilovoltage absorbed dose to water primary standards and the use of MC simulations to determine kilovoltage beam chamber correction factors.

Monte Carlo calculation of quality correction factors based on air kerma and absorbed dose to water in medium energy x-ray beams

“…Equation ( 5) is also based on the assumption, that the chamber readings M w,Q and M a,Q can be replaced by the dose values D det,w,Q and D det,a,Q . In a recent study Bancheri et al (2020) have shown, that the ratios of the correction factors (k w k ii ) w,Q / (k w k ii ) a,Q are indeed unity for spectra in the same energy range as investigated here, i.e. the impact of the correction k w • k ii on the perturbation correction p Q is also negligible.…”

“…The agreement of both data sets is within 0.2%, whereat the data using the egs_kerma user code seem to be systematically higher. Additionally, the data from the TG-61 protocol and new data from the publication of Bancheri et al (2020) are included. Bancheri also used the EGSnrc system applying the FLURZnrc and g user code.…”

“…Chica et al (2014) did calculate the k Q,Q0 values for the PTW 30 010 ion chamber for a large number of x-ray spectra with the Monte Carlo code PENELOPE, Andreo (2019) recently published Monte Carlo calculated, detector-independent dosimetric parameters for low-and medium-energy x-ray beams required for N K -based dosimetry protocols. A recent study by Bancheri et al (2020) published Monte Carlo based and measured perturbation factors p Q for the conversion of kerma based calibration coefficients into absorbed dose to water calibration coefficients for medium energy x-rays.…”

Determination of the absorbed dose to water for medium‐energy x‐ray beams using Fricke dosimetry

“…The values for overall chamber correction factors and water‐to‐air mass energy absorption coefficient ratios are now two decades old, and the uncertainties associated with them are high, as shown in Table V. Bancheri et al 42,47 recently reported the calculation of these correction factors for different ionization chamber models, including the NE2571 model, using MC simulation. They reported inconsistencies between the measured and calculated absorbed dose to water‐to‐air kerma ratio, indicating the need for further investigation.…”

Kilovoltage therapy is well and truly alive and needed in a modern radiotherapy centre

“…In fact, kilovoltage dosimetry is so important, the IAEA provides two separate sections in the TRS398 Code of Practice for both low and medium energy X-rays while the megavoltage X-ray beam dosimetry recommendations are contained within one section [17]. In addition, primary standard dosimetry laboratories around the world continue to maintain and develop their standards for kilovoltage X-ray beams [18,19].…”