The relative response of NE2561 and NE2611A ionization chambers in megavoltage x-ray beams.

The relative energy response of NE2561 and NE261 IA ionization chambers to megavoltage photon beams from the ARPANSA linac indicates significant differences between these two types of chamber. In 16 MV beams of TPR20(10) 0.779, differences of about 2% are observed. The results are expressed as ratios KQ of the beam quality correction factors kQ, where the kQ factor for each type of chamber is the ratio of the absorbed dose to water calibration factor ND, at the x-ray quality Q to that at 60Co. These results have implications for the use of generic kQ factors in dosimetry protocols and suggest that NE2561 and NE2611A ionization chambers cannot be assumed to be identical.     

The polarity effect for commercially available plane-parallel ionization chambers

The polarity effect was investigated for three different commercially available plane-parallel ionization chambers: the Memorial Pipe chamber, the Victoreen/Nuclear Associates model 30-329 chamber manufactured by PTW, Frieburg, and the Capintec PS-033 thin-window ionization chamber. The primary study was the polarity effect versus depth below the phantom surface for 6-, 10-, 18-, and 24-MV x-ray beams, and 9- and 22-MeV electron beams. The polarity effect in the region of nonelectronic equilibrium that exists at the interface of two dissimilar materials, polystyrene and aluminum, was investigated as well as the effects of field size. For the group of plane-parallel ionization chambers that we studied, we found a polarity effect of only 1%-2% for electron beams at the depth of dmax. At depths greater than dmax, the polarity effect for electrons increased and was as high as 4.5% for some chambers. When used in the buildup region of high-energy photon beams, these same chambers exhibited up to a 30% difference in collected charge between one polarity and the other. This effect and its relationship to physical chamber characteristics is discussed.     

Polarity effect of plane-parallel ionization chambers in electron radiation

Although plane-parallel ionization chambers have been in use for some time, there is still much to be learned about their performance characteristics. This work is concerned with the polarity effect in electron beams about which there is little published data. The investigations involved several popular ionization chambers; the PTW Markus chamber, the NACP chamber and its Calcam version, the Vinten-631 chamber, and the NE 2571 (Farmer-type) thimble chamber. The chambers were irradiated in electron beams of nominal energies between 4 MeV and 18 MeV. It was found in this study that the NACP, Markus and Vinten chambers require a correction of the order of 0.2% in the energy range between 4.5 MeV and 18 MeV. The overall behaviour of the Calcam chamber was similar with the exception of energies below 4 MeV. The depth dependence of the polarity effect seemed closely related to the mean beam energy at the depth of measurement. There is some evidence that the effect is also dependent on the angular spread of the electron beam and its spectrum. The authors considered how best to quantify the polarity effect practically, and propose that it should be expressed as a correction factor to be applied to readings with one particular chamber bias.     

Measurement of dose in the buildup region using fixed-separation plane-parallel ionization chambers

Accurate measurement of dose at the surface of a phantom and in the buildup region is a difficult task but one that is important for the proper treatment of patients. The instruments of choice for these measurements are extrapolation chambers but few institutions have these instruments at their disposal. As a result, fixed-separation plane-parallel ionization chambers are most commonly used for this purpose. Recent papers have re-emphasized the inaccuracies in the measurement of dose in the buildup region of normally incident photon beams when using fixed-separation plane-parallel ionization chambers. Data for Co-60, 6-, 10-, 18-, and 24-MV photon beams are presented that show the magnitude of this over response in the buildup region for several commercially available plane-parallel ionization chambers versus results obtained using both an extrapolation chamber and LiF thermoluminescent detectors. Differences in the percent depth dose at the surface of a phantom of greater than 19% were found for one of the chambers. All chambers over responded in the buildup region to some degree based upon their internal dimensions. The appropriateness of published corrections for these chambers is evaluated and guidelines for the accurate measurement of dose in the buildup region are presented.     

Dosimetry using plane-parallel ionization chambers in a 75 MeV clinical proton beam

Reference ionization chamber dosimetry in clinical proton beams is generally performed with cylindrical ionization chambers. However, when the measurement is performed in the presence of a large depth dose gradient or in a narrow spread out Bragg peak (SOBP), it could be advisable to use a plane-parallel chamber. Few recommendations and studies have been devoted to this subject. In this paper, experimental information on perturbation correction factors for four plane-parallel ionization chamber types in proton beams is presented. The experiments were performed in 75 MeV modulated and non-modulated proton beams. Monte Carlo calculations have been performed to support the conclusions of the experimental work. Overall, we were not able to find experimental evidence for significant differences between the secondary electron perturbation correction factors for plane-parallel chambers and those for a cylindrical NE2571. We found experimental ratios of perturbation correction factors that did not differ by more than 0.6% from unity for a Roos and two NACP02 chambers, and by not more than 1.2% for a Calcam-2 and two Markus chambers. Monte Carlo simulations result in corrections that are limited to 0.6% in absolute value, but given the overall uncertainties of the measurements, the deviations of the correction factors from unity could not be resolved from the experimental results. The results of the simulations thus support the experimental conclusion that perturbation correction factors for the set of plane-parallel chambers in both proton beams (relative to NE2571) do not deviate from unity by more than 1.2%. This confirms, within the experimental uncertainties, the assumption that the overall perturbation correction factor for a plane-parallel chamber in a low-energy proton beam is unity, made in IAEA TRS-398 and other dosimetry protocols. Given the large uncertainties of the gradient correction factors to be applied when using a cylindrical ionization chamber in a narrow SOBP or in the presence of a strong depth dose gradient, the level of agreement between plane-parallel and cylindrical ionization chambers observed in this study shows that plane-parallel chambers are a reliable alternative for reference dosimetry in low-energy proton beams.     

On the calibration of plane-parallel ionization chambers for electron beam dosimetry.

The procedure recommended by different dosimetry protocols for the determination of the absorbed dose to air chamber factor, ND,pp, of plane-parallel chambers, comparing absorbed dose determinations in a high-energy electron beam with a reference cylindrical chamber having a known ND,cyl factor, has been investigated. Attention has been focused on the case that the chamber serving as reference has a solid aluminium central electrode. It has been found that using a wide spread Farmer-type chamber (NE 2571), together with recommendations which specifically take into account central electrode corrections for electron beam dosimetry, kcelpcel = pcel-global(IAEA) = 1.008, yields inconsistent results compared with those obtained from a fully homogeneous ionization chamber; for the NE 2571 chamber, a value kcelpcel = pcel-global(IAEA) congruent to 1.0 has been obtained. Analytical calculations of kmkatt for Farmer-type cylindrical chambers and experimental determinations of the product kmkattkcelpcel in electron beams agree within experimental uncertainties, with no evidence of statistical significance for the commonly used assumption pcel = 1, which yields a 0.8% correction (due to kcel only) for the effect of the NE 2571 aluminium electrode in electron beam dosimetry. The use of a 'NACP-chamber' specific factor (kpp or kmkatt) to obtain ND,pp from NK,pp in NACP plane-parallel chambers has been found unsatisfactory, and direct experimental determinations of ND,pp are recommended instead. It is suggested that Standard Dosimetry Laboratories provide ND,pp calibration factors in 60Co beams.     

Polarity effect in plane-parallel ionization chambers using air or a dielectric liquid as ionization medium.

A plane-parallel ionization chamber having a sensitive volume of 2 mm3 and using the dielectric liquid tetramethylsilane as the sensitive medium instead of air is described. In the design of the chamber special attention was given to the factors that can cause unwanted currents in the cable, stem, or the chamber dielectric material. The chamber has been tested with respect to the polarity effect in regions of radiation fields where ordinary plane-parallel ionization chambers will often fail. These regions are the build-up region in photon fields, and the region close to the practical range for electrons where nonelectronic equilibrium is significant. Experimental results show that, despite the extremely small ionization volume in the liquid ionization chamber, the polarity effect never exceeds a few tenths of a percent in field positions where well-known commercially available chambers with much less spatial resolution designed for measurements in radiation therapy fields can show polarity effects of 5% to 30%. The origin of spurious currents and how they must be minimized in the design of either a liquid- or gas-filled ionization chamber is discussed.     

Monte Carlo calculations of correction factors for plane-parallel ionization chambers in clinical electron dosimetry

Recent standard dosimetry protocols recommend that plane-parallel ionization chambers be used in the measurements of depth-dose distributions or the calibration of low-energy electron beams with beam quality R50 <4 g/cm2. In electron dosimetry protocols with the plane-parallel chambers, the wall correction factor, Pwall, in water is assumed to be unity and the replacement correction factor, Prepl, is taken to be unity for well-guarded plane-parallel chambers, at all measurement depths. This study calculated Pwall and Prepl for NACP-02, Markus, and Roos plane-parallel chambers in clinical electron dosimetry using the EGSnrc Monte Carlo code system. The Pwall values for the plane-parallel chambers increased rapidly as a function of depth in water, especially at lower energy. The value around R50 for NACP-02 was about 10% greater than unity at 4 MeV. The effect was smaller for higher electron energies. Similarly, Prepl values with depth increased drastically at the region with the steep dose gradient for lower energy. For Markus Prepl departed more than 10% from unity close to R50 due to the narrow guard ring width. Prepl for NACP-02 and Roos was close to unity in the plateau region of depth-dose curves that includes a reference depth, dref. It was also found that the ratio of the dose to water and the dose to the sensitive volume in the air cavity for the plane-parallel chambers, Dw/[Dair]pp, at d(ref) differs significantly from that assumed by electron dosimetry protocols.     

Wall correction factors for calibration of plane-parallel ionization chambers with high-energy photon beams

Most dosimetry protocols recommend that calibration of plane-parallel ionization chambers be performed in an electron beam of sufficiently high energy by comparison with cylindrical chambers. For various plane-parallel chambers, the 1997 IAEA TRS-381 protocol includes an overall perturbation factor pQ for electron beams, a wall correction factor p(wall) for a 60Co beam and the product of two wall corrections k(att)k(m) for 60Co in-air calibration. The recommended values of p(wall) for plane-parallel chambers, however, are limited to certain phantom materials and a 60Co beam, and are not given for other phantom materials and x-ray beams. In this work, the p(wall) values of the commercially available NACP, PTW/Markus and PTW/Roos plane-parallel chambers in a solid water phantom have been determined with 60Co and 4 and 10 MV photon beams. The k(att)k(m) values for the NACP and PTW/Markus chambers have also been obtained. The wall correction factors p(wall) and k(att)k(m) have been determined by intercomparison with a calibrated Farmer chamber. The average value of p(wall) for these plane-parallel chambers was 1.005 +/- 0.1% (1 SD) for 60Co beams and 1.007 +/- 0.2% (1 SD) for both 4 MV and 10 MV photons. The k(att)k(m) values for the NACP and PTW/Markus chambers were about 1.5% lower than other published data.     

Monte Carlo-based treatment planning for a spoiler system with experimental validation using plane-parallel ionization chambers

A beam spoiler is often used to increase the build-up dose near the surface for treatment of superficial treatment areas. Photon-beam spoilers produce a large amount of contaminant electrons, conditions for which standard, commercial treatment-planning system dose-calculation algorithms are inadequate for producing accurate dose calculations. In this study, we implemented a Monte Carlo (MC) dose-calculation algorithm for this spoiler system. With and without a spoiler of 1 cm Lucite, depth doses and transverse profiles in the build-up region were measured for field sizes of 5 x 5 cm2 and 10 x 10 cm2 at the spoiler-to-surface distances (STSDs) of 6, 10 and 15 cm. An Attix chamber and a Markus chamber were used for depth doses, whereas a diode detector was used for transverse profiles. An MC simulation using BEAM/DOSXYZ was used to compare the calculated and the measured data. The MC calculations agreed with the Attix chamber measurements within 2% for all STSDs and field sizes, whereas the Markus data--even with corrections made-showed a discrepancy of about 3.5% with a maximum difference of 7.3% for a field size of 10 x 10 cm2 at an STSD of 6 cm. The MC treatment-planning system was successfully applied to a head-and-neck case using 6 MV photon beams with a beam spoiler.     

The behavior of several microionization chambers in small intensity modulated radiotherapy fields

The aim of this work is to compare different ion chambers available for dose measurements in small fields used in intensity modulated radiotherapy. Some dosimetric aspects, related to these small radiation fields, i.e., lack of electronic lateral equilibrium and steep dose region, must be evaluated, in order to obtain an accurate technique implementation. Furthermore, the size of the sensitive volume of the chambers compared with the mapping of the beams or segments needs consideration. If the size of the chamber is too large for the flatness of the field, the measurement can deviate from the expected absorbed dose at a point. We propose a comparison of various dosimetric values between different microionization chambers with respect to a smaller dosimeter, such as the diamond detector.     

The use of plane-parallel chambers in electron dosimetry without any cross-calibration

Current dosimetry protocols from AAPM, DIN and IAEA recommend a cross-calibration for plane-parallel chambers against a calibrated thimble chamber for electron dosimetry. The rationale for this is the assumed chamber-to-chamber variation of plane-parallel chambers and the large uncertainty in the wall perturbation factor (p(wall)60Co)pp at 60Co for plane-parallel chambers. We have confirmed the results of other authors that chamber-to-chamber variation of the investigated chambers of types Roos, Markus, Advanced Markus and Farmer is less than 0.3%. Starting with a calibration factor for absorbed dose to water and on the basis of the three dosimetry protocols AAPM TG-51, DIN 6800-2 (slightly modified) and IAEA TRS-398, values for (p(wall)60Co)Roos of 1.024 +/- 0.005, (p(wall)60Co)Markus of 1.016 +/- 0.005 and (p(wall)60Co)Advanced Markus of 1.014 +/- 0.005 have been determined. In future this will permit electron dosimetry with the above-listed plane-parallel chambers having a calibration factor N(D, w)60Co without the necessity for cross-calibration against a thimble chamber.     

The effective point of measurement of ionization chambers and the build-up anomaly in MV x-ray beams

A precision experimental investigation of the effective point of measurement (EPOM) of ion chambers in megavoltage beams has been carried out. A one-dimensional scanning phantom system was developed with an overall accuracy in the positioning of a chamber of better than 0.15 mm. Depth-dose data were acquired for a 25 MV beam from an Elekta Precise linac (field sizes of 10 x 10 cm and 25 x 25 cm) for measurement depths in the range 0.6-6 cm. The results confirmed the Monte Carlo calculations of an earlier theoretical investigation by Kawrakow [Med. Phys. 33, 1829-1839 (2006)] that the standard shift for cylindrical chambers, recommended in dosimetry protocols of -0.6r (where r is the internal radius of the cavity), is incorrect. A wide range of ion chambers were investigated and it was found that errors of up to 1.4 mm could occur for certain chamber designs (although typical errors for common chambers were around 0.5 mm). A comparison between measurements and Monte Carlo simulations showed that once the correct EPOM is used, the details of the linac geometry are correct, and the parameters of the electron beam striking the bremsstrahlung target have been adequately determined, the EGSnrc Monte Carlo package is capable of reproducing the experimental data to 0.2 mm or better. The investigation also confirmed that for the highest accuracy depth-dose curves in megavoltage photon beams one should use a well-guarded parallel-plate ion chamber. Three chamber designs were tested here and found to be satisfactory-the Scanditronix-Wellh?fer NACP-02, PTW Roos and Exradin All.     

Thermal and temporal response of ionization chambers in radiation dosimetry

The temporal and thermal response of various commonly used ion chambers (NEL, Exradin, PTW) with different wall and electrode materials is studied in a water phantom. Measurements were taken in heating water bath having an automatic temperature control mechanism. All chambers were submersed at 3 cm depth in water phantom and connected to separate electrometers for simultaneous readings for a given dose from a 6 MV beam. The temporal response was studied at approximately +/- 10 degrees C from room temperature, i.e., 10 degrees C and 30 degrees C. Temporal results show that all chambers reach quick equilibrium response within < 2 minutes of submersion in water. The steady-state thermal response was dependent upon the water temperature and wall and electrode compositions. The temperature and pressure corrected response of the NEL chamber was least affected by the changes in water temperature, where as the Exradin chamber has positive and PTW has negative slope with water temperature. The response varied within +/-1.5% between 10 degrees C-50 degrees C temperature and is mainly dependent on volume changes rather than the humidity. A correction factor based on thermal coefficients is derived for each chamber. It is concluded that ion chamber correction factors can be divided into first order; temperature and pressure connection, second order; volume correction for thermal expansion, and third order; humidity correction. To eliminate dosimetric error, the temporal and thermal response should be known for a chamber or water phantom temperature should be maintained close to room temperature.     

Experimental determination of pCo perturbation factors for plane-parallel chambers

For plane-parallel chambers used in electron dosimetry, modern dosimetry protocols recommend a cross-calibration against a calibrated cylindrical chamber. The rationale for this is the unacceptably large (up to 3-4%) chamber-to-chamber variations of the perturbation factors (pwall)Co, which have been reported for plane-parallel chambers of a given type. In some recent publications, it was shown that this is no longer the case for modern plane-parallel chambers. The aims of the present study are to obtain reliable information about the variation of the perturbation factors for modern types of plane-parallel chambers, and-if this variation is found to be acceptably small-to determine type-specific mean values for these perturbation factors which can be used for absorbed dose measurements in electron beams using plane-parallel chambers. In an extensive multi-center study, the individual perturbation factors pCo (which are usually assumed to be equal to (pwall)Co) for a total of 35 plane-parallel chambers of the Roos type, 15 chambers of the Markus type and 12 chambers of the Advanced Markus type were determined. From a total of 188 cross-calibration measurements, variations of the pCo values for different chambers of the same type of at most 1.0%, 0.9% and 0.6% were found for the chambers of the Roos, Markus and Advanced Markus types, respectively. The mean pCo values obtained from all measurements are [Formula: see text] and [Formula: see text]; the relative experimental standard deviation of the individual pCo values is less than 0.24% for all chamber types; the relative standard uncertainty of the mean pCo values is 1.1%.     

Influence of field size on a PTW type 23342 plane-parallel ionization chamber's response

The response of a PTW type 23342 plane-parallel ionization chamber, both in air and in phantom, was evaluated for x-ray tube potentials between 30 and 100 kV and radiation field diameters ranging from 30 to 70 mm. The experiments were performed with a calibrated Pantak x-ray machine and made use of the same set of x-ray qualities adopted by the PTB primary laboratory for the calibration of such chambers. A Plexiglas phantom (1.18 g cm(-3)) 110 mm long, 110 mm wide, and 80 mm deep was used for phantom measurements. X-ray qualities were characterized by using 99.99% pure aluminum filters. On the basis of the IAEA's TRS 398, the article discusses the dependence of the plane-parallel ionization chamber readings with field size in air and in phantom, its implication with regard to clinical dosimetry, cross-calibration, and dissemination of calibration factors.     

Assessment of small volume ionization chambers as reference dosimeters in high-energy photon beams

LNE-LNHB is involved in a European project aiming at establishing absorbed dose-to-water standards for photon-radiation fields down to 2 × 2 cm2. This requires the calibration of reference ionization chambers of small volume. Twenty-four ionization chambers of eight different types with volume ranging from 0.007 to 0.057 cm3 were tested in a ??Co beam. For each chamber, two major characteristics were investigated: (1) the stability of the measured current as a function of the irradiation time under continuous irradiation. At LNE-LNHB, the variation of the current should be less than ±0.1% in comparison with its first value (over a 16 h irradiation time); (2) the variation of the ionization current with the applied polarizing voltage and polarity. Leakage currents were also measured. Results show that (1) every tested PTW (31015, 31016 and 31014) and Exradin A1SL chambers demonstrate a satisfying stability under irradiation. Other types of chambers have a stability complying with the stability criterion for some or none of them. (2) IBA CC01, IBA CC04 and Exradin A1SL show a proper response as a function of applied voltage for both polarities. PTW, Exradin A14SL and Exradin A16 do not. Only three types of chambers were deemed suitable as reference chambers according to LNE-LNHB requirements and specifications from McEwen (2010 Med. Phys. 37 2179-93): Exradin A1SL chambers (3/3), IBA CC04 (2/3) and IBA CC01 (1/3). The Exradin A1SL type with an applied polarizing voltage of 150 V was chosen as an LNE-LNHB reference chamber type in 2 × 2 cm2 radiation fields.