An evaluation of ionization chambers for the relative dosimetry of kilovoltage x‐ray beams

Abstract

In this work, the authors have evaluated ten different ionization chambers for the relative dosimetry of kilovoltage x-ray beams in the energy range of 50-280 kVp. Percentage depth doses in water and relative detector response (in Solid Water and in air) were measured for each of the x-ray beams studied using a number of chambers. Measured depth dose data were compared with Monte Carlo calculated depth doses using the EGSnrc Monte Carlo package and the BEAMnrc user code. The accuracy of the phase space files generated by BEAMnrc was verified by calculating the half-value layer and comparing with the measured half-value layer of each x-ray beam. The results indicate that the Advanced Markus, Markus, NACP, and Roos parallel plate ionization chambers were suitable for the measurement of depth dose data in this beam quality range with an uncertainty of less than 3%, including in the regions close to the water surface. While the relative detector response of the Farmer and scanning thimble chambers exhibited a better energy response, they were not suitable for depth dose measurements in the first 5 mm below the water surface with differences of up to 12% in the surface dose measurement for the 50 kVp x-ray beam. These differences were due to dose artifacts generated by the chamber size and the dose gradient. However, at depths greater than 5 mm, the Farmer and thimble scanning chambers gave uncertainties of less than 3% for the depth dose measurements for all beam energies. The PTW PinPoint 31006 chamber was found to give varying dose differences of up to 8% depending on the x-ray beam energy; this was attributed to the steel central electrode. The authors recommend that one of the parallel plate ionization chambers investigated be used to determine depth dose data for kilovoltage x-ray beams in the energy range studied and give correct dose information close to the surface and at depth in the water phantom.

Computed tomography dosimetry with high‐resolution detectors commonly used in radiotherapy — an energy dependence study

“…This implies that the combination of lower Z and higher density in case of the PTW Semiflex chambers results in a similar photoelectric absorption for chambers of comparable volume. The results for the Semiflex chamber PTW 31013 are consistent with the results from the previous study by Hill et al (20) who reported a variation of 11% for an energy range up to 280 kVp.…”

“…All investigated detectors, except the parallel plate chambers, possess rotation‐symmetric designs, which is crucial for CT dosimetry. The parallel plate chambers were included in this study for comparison with previous studies 20 , 21 …”

“…The relative detector response (RDR) of an ionization chamber or dosimetry diode was determined according to the previously published method 20 , 21 . The RDR for a detector of interest RDR(E) x at a particular beam quality, E, was determined as follows: $RDR{(E)}_x=P_Q{(E)}_{\mathit{f}}\cdot \frac{M{(E)}_{\mathit{\text{Ref}}}}{M{(E)}_x}$ where $P_Q{false(Efalse)}_{Ref}$ is the air‐kerma beam quality correction factor (see Table 1) for the PTW 60004 reference diode provided by the manufacturer within the calibration sheet, $M{false(Efalse)}_{Ref}$ is the electrometer reading for the PTW 60004 reference diode, and $M{false(Efalse)}_x$ is the reading for the ionization chamber or diode under investigation.…”

“…The uncertainties associated with a comparable measurement setup as used in this study have been analyzed by Hill et al (20) Uncertainty components attributed to the used detector, phantom material, movement of phantom, dose rate variations and linearity in output of the X‐ray tube, temperature, and pressure stability were taken into account. The overall uncertainty amounts to 1.5%.…”

“…In previous studies, several ionization chambers designed primarily for dosimetry of megavoltage photon beams have been investigated in terms of their usability in X‐ray therapy 19 , 20 , 21 . These studies showed the general feasibility of these detectors for kV dosimetry, provided that detector‐specific correction factors for the spectral composition of the radiation fields are taken into consideration.…”