Liquid-filled Ionization Chambers Research Articles

Calibration of a portal imaging device for high-precision dosimetry: a Monte Carlo study.

Today electronic portal imaging devices (EPID's) are used primarily to verify patient positioning. They have, however, also the potential as 2D-dosimeters and could be used as such for transit dosimetry or dose reconstruction. It has been proven that such devices, especially liquid filled ionization chambers, have a stable dose response relationship which can be described in terms of the physical properties of the EPID and the pulsed linac radiation. For absolute dosimetry however, an accurate method of calibration to an absolute dose is needed. In this work, we concentrate on calibration against dose in a homogeneous water phantom. Using a Monte Carlo model of the detector we calculated dose spread kernels in units of absolute dose per incident energy fluence and compared them to calculated dose spread kernels in water at different depths. The energy of the incident pencil beams varied between 0.5 and 18 MeV. At the depth of dose maximum in water for a 6 MV beam (1.5 cm) and for a 18 MV beam (3.0 cm) we observed large absolute differences between water and detector dose above an incident energy of 4 MeV but only small relative differences in the most frequent energy range of the beam energy spectra. It is shown that for a 6 MV beam the absolute reference dose measured at 1.5 cm water depth differs from the absolute detector dose by 3.8%. At depth 1.2 cm in water, however, the relative dose differences are almost constant between 2 and 6 MeV. The effects of changes in the energy spectrum of the beam on the dose responses in water and in the detector are also investigated. We show that differences larger than 2% can occur for different beam qualities of the incident photon beam behind water slabs of different thicknesses. It is therefore concluded that for high-precision dosimetry such effects have to be taken into account. Nevertheless, the precise information about the dose response of the detector provided in this Monte Carlo study forms the basis of extracting directly the basic radiometric quantities photon fluence and photon energy fluence from the detector's signal using a deconvolution algorithm. The results are therefore promising for future application in absolute transit dosimetry and absolute dose reconstruction.

The dose response relationship of a liquid-filled electronic portal imaging device.

To use a liquid-filled portal imaging device (EPID) for transmission dosimetry, it is necessary to understand its dosimetric properties. Therefore, the relation between the pixel values (i.e., ionization currents) of an electronic portal imaging device and the dose rate measured with an ionization chamber in a mini-phantom was investigated. First, a model was introduced to describe the ionization current of the matrix of liquid-filled ionization chambers for pulsed radiation. With this model the relation between ionization current and dose rate is explained qualitatively. Next, buildup measurements were performed at different photon beam energies to assess the amount of buildup material required to obtain electronic equilibrium in the detector. This additional buildup material significantly decreased the image quality, which can hamper patient setup verification, at only the 25 MV beam. Pixel values were then compared with measurements made with a Farmer-type ionization chamber in a mini-phantom at various dose rates. In addition, the influence of a number of accelerator and EPID settings (photon beam energy, pulse rate frequency, gantry rotation angle, and image acquisition modes) on the pixel value was investigated. Subsequently, the dose response relationships of three commercially obtained EPIDs of the same type were compared. For all types of measurements the relation between ionization current and dose rate is described within 1% (1 SD) by an equation with two terms: one term proportional to the square root of the dose rate and another term linear to the dose rate. For images obtained under a typical clinical situation (applying the "normal" acquisition mode at an 8 MV beam with a pulse rate frequency of 400 Hz at a transmission dose rate of 100 cGy/min) the contribution of the square root and linear term to the EPID signal is 94% and 6%, respectively. The weight factors of both terms depend on the photon beam energy, pulse rate frequency, and image acquisition mode. It is concluded that the EPID is useful for dosimetry purposes with 1% (1 SD) accuracy, but that the dose response relationship has to be determined for each EPID and accelerator setting.

Transmission dosimetry with a liquid-filled electronic portal imaging device

To assess the accuracy of transmission dose rate measurements for various phantom-detector geometries, performed with an electronic portal imaging device (EPID) and to compare these transmission dose rate values with exit dose rate data. Transmission dose rate values on the central beam axis and beam profiles were measured with an EPID consisting of a matrix of liquid-filled ionization chambers. These data were compared with transmission and exit dose rate values, obtained using air-filled ionization chambers for a number of field sizes, phantom thickness, and phantom-detector distances. Various homogeneous and inhomogeneous phantoms were applied. The increase in dose rate with field size is larger for the EPID than in air, due to the larger amount of side scatter in the EPID. The difference has been taken into account by a deconvolution of the EPID images. An additional build-up layer on top of the commercial device is needed to reach dose maximum at the liquid ionization chambers for photon beam energies higher than about 4 MV. The transmission off-axis ratios (OAR) determined with the EPID and in air agreed within 2% for all tested cases, after deconvolution of the EPID signal. The agreement between the EPID-and exit-OAR decreased with increasing phantom-detector distance and the presence of inhomogeneities. For a phantom-detector distance of about 10 cm, the EPID- and exit-OARs agree within 2.5%. The difference could be up to 8% for an air inhomogeneity and a phantom-detector distance of 30 cm. The difference between EPID measurements and measurements in air can be explained by side scatter effects in the EPID and lack of adequate buildup, and can easily be taken into account. The loss of scatter compared with the situation at the exit side of the phantom explains the difference between transmission and exit dose values. At short phantom-detector distances, good agreement exists between transmission and exit dose rate. This implies that at this distance, the EPID can be used for simple comparison with exit dose calculations during patient treatments. At larger distances, more sophisticated conversion methods are required.

Dosimetric characteristics of a liquid-filled electronic portal imaging device

To determine the characteristics of a commercial electronic portal imaging device (EPID), based on a two-dimensional matrix of liquid-filled ionization chambers, for transmission dose measurements during patient treatment. Electronic portal imaging device measurements were performed in a cobalt-60 beam and two accelerator x-ray beams, and compared with measurements performed with a Farmer-type ionization chamber in air in a miniphantom and in an extended water phantom. The warming up time of the EPID is about 1 h. The long-term stability of the detector is better than 1% under reference conditions for a period of about 3 months. The signal of the ionization chambers follows approximately the square root of the dose rate, although the relation becomes more linear for larger (> 1 Gy/min) dose rates. The signal can be transformed to dose rate with an accuracy of 0.6% (1 SD). The short-term influence of integrated dose on the sensitivity of the ionization chambers is small. The sensitivity increases about 0.5% for all ionization chambers after an absorbed dose of 8 Gy and returns to its original value in less than 5 min after stopping the irradiation. This small increase in sensitivity can be ascribed to the electrode distance of the ionization chambers in commercial EPIDs, which is 0.8 +/- 0.1 mm. The sensitivity increase depends on the electrode distance and is 4% for a 1.4 mm electrode distance. The scattering properties of the EPID ionization chambers were between those of an ionization chamber in a miniphantom and in a water phantom. The matrix ionization chamber EPID has characteristics that make it very suitable for dose rate measurements. It is therefore a very promising device for in vivo dosimetry purposes.

Photon beam quality specification by narrow-beam transmission measurements

Radiation quality specifications in megavoltage photon beams are usually based on depth-dose measurements performed under reference conditions. Stopping-power ratios and various correction factors are then related to parameters such as TPR1020 which are extracted from depth-dose measurements. Stopping-power ratio determinations based on this concept are shown to be in error by more than 2% at high energies. Furthermore, electrons generated in the treatment head can, at high energies, contribute to the dose at a depth of 10 cm and thus significantly affect the TPR1020 ratio. This method is further shown to be inadequate when the dose in other parts of the field than the reference point is to be measured with ionization chamber dosimetry. A new standardized device for determining photon beam quality based on half value layer (HVL) measurements in water has been developed and thoroughly investigated in both a low-energy, (4 MV) and a high-energy beam. A relation between HVL and stopping-power ratios water-to-air was determined by comparative measurements with air ionization chambers and liquid-filled ionization chambers together with Fricke dosimetry. Furthermore, different radiation quality gradients in the photon fields for different types of field-flattening systems, and field-compensating methods are discussed.

Sampling methods for a matrix ionization chamber system.

To achieve increased image acquisition speed or better image quality, several read-out methods for a matrix ionization chamber system have been investigated. In this device, which is applied for portal imaging in radiation therapy, 256 x 256 small liquid-filled ionization chambers are scanned by switching the polarizing voltage applied to rows of chambers. The ionization current of each column is measured by a separate amplifier. In this approach, instead of measuring row by row, more complex switching schemes can be applied for the polarizing voltage. These schemes are useful either for varying speed and spatial resolution of the imaging device or for coded sampling of the ionization signal. The former option allows for doubling or quadrupling the acquisition speed with a small loss in image quality, or for obtaining a large improvement in signal-to-noise ratio at the cost of image resolution. In the latter option, coded sampling, the image is reconstructed mathematically from the measured signals. It is shown that in this case the application of Hadamard or derived matrices for sampling leads, under certain circumstances, to a noise reduction in the reconstructed image.

Analysis of electronic conduction pulses produced in liquid ionization chambers by high-energy radiation

The electron pulse shape produced by the passage of an ionizing particle through a liquid-filled ionization chamber is analyzed theoretically. In addition, the authors discuss the effect of an electric field on the observed pulse height Q/sub e/(t/sub d/), where t/sub d/ is the electron drift time. The analysis of the pulse shape yields information on the type of particle and on the purity of the liquid. For reliable operation, a condition of the liquid characterized by tau >>t/sub d/ (where tau is the electron lifetime) has to be achieved. A plot of the pulse peak height as a function of the electric field strength for cosmic ray particles can be used to deduce the electron lifetime. >

The physics and chemistry of room-temperature liquid-filled ionization chambers

The properties of excess electrons in non-polar liquids, such as tetramethylsilane and 2,2,4,4-tetramethylpentane, which are suitable for room-temperature liquid-filled ionization chambers are reviewed. Such properties as mobility, ionization yield, conduction band energy, trapping, and the influence of the electric field are considered.

Leitfähigkeit von Hexan bei Einwirkung von Y-Strahlen und ihre Bedeutung für die Dosimetrie

The influence of primary and secondary recombination on the γ-ray-induced conductivity of liquid n-hexane is studied experimentally and theoretically. Conductivity measurements were made with a parallel-plate ionization chamber, varying the electrical field, the electrode distance and the dose rate. The primary recombination can be explained with a theory developed by ONSAGER for the recombination of single ion pairs in dense gases, whereas JAFFE’S theory of column ionization is not applicable. Independent of the ion mobilities, for hexane in absence of an electric field a G-value of 0,13 ± 0,01 for the ion pairs escaping primary recombination is obtained; this corresponds to a mean escape probability of 3% and a mean effective initial distance of the ions in an ion pair of 80 mµ. Finally, the applicability of liquid-filled ionization chambers for dosimetry is discussed.

Induced conduction in dielectric liquids

A brief review is given of the extensive published work which deals with the induced conduction in liquid dielectrics and, in particular, the results concerning the mobility, recombination and diffusion of ions which are produced by ionization in saturated hydrocarbons are discussed in some detail. It is shown that for the mobility of positive and negative ions, as well as for the recombination coefficients, relations are valid which, apart from constant parameters depending on the type of liquid, contain only the viscosity. Using the formula which the author has derived for the viscosity of saturated hydrocarbons it is thus possible to express all the above coefficients for these substances in terms of temperature and molecular structure. The activation energy involved in these processes may also be established. Also reviewed is the recent work on liquefied noble gases which is significant for the study of the mechanism of ion mobility in relation to the molecular structure of these liquids. Lastly the possibility of using liquid-filled ionization chambers for dosimetry is discussed. A formula for the ionization current in such a chamber filled with saturated hydrocarbons is given and the effectiveness of such a device for a wide spectrum of quantum energy is established.