Ionization chamber shift correction and surface dose measurements in electron beams

Cylindrical ionization chambers produce perturbations (gradient and fluence) in the medium, and hence the point of measurement is not accurately defined in electron beam dosimetry. The gradient perturbation is often corrected by a shift method depending on the type of ion chamber. The shift is in the range of 0.33-0.85 times the inner radius (r) of the ion chamber, upstream from the centre of the chamber, depending upon the dosimetry protocol. This variation in shift causes the surface dose to be uncertain due to the high dose gradient. An investigation was conducted to estimate the effective point of measurement of cylindrical ion chambers in electron beams. Ionization measurements were taken with the ion chamber in air and in a phantom at source to chamber distances of <100 cm and >100 cm respectively. The data in air and in the phantom were fitted with the inverse square and electron depth dose functions, respectively. The intersection of the two functions provides an accurate estimate of the ion chamber shift and the surface dose. Our results show that the shift correction for an ion chamber is energy dependent. The measured shifts vary from 0.9r to 0.5r between 6 MeV and 20 MeV beams respectively. The surface dose measured with the ion chambers and mathematically determined values are in agreement to within 3%. The method presented in this report is unambiguous, fast and reliable for the estimation of surface dose and the shift needed in electron beam dosimetry.     

Monte Carlo dosimetry study of a 6 MV stereotactic radiosurgery unit

Small-field and stereotactic radiosurgery (SRS) dosimetry with radiation detectors, used for clinical practice, have often been questioned due to the lack of lateral electron equilibrium and uncertainty in beam energy. A dosimetry study was performed for a dedicated 6 MV SRS unit, capable of generating circular radiation fields with diameters of 1.25-5 cm at isocentre using the BEAM/EGS4 Monte Carlo code. With this code the accelerator was modelled for radiation fields with a diameter as small as 0.5 cm. The radiation fields and dosimetric characteristics (photon spectra, depth doses, lateral dose profiles and cone factors) in a water phantom were evaluated. The cone factor (St) for a specific cone c at depth d is defined as St(d, c) = D(d, c)/D(d, c(ref)), where c(ref) is the reference cone. To verify the Monte Carlo calculations, measurements were performed with detectors commonly used in SRS such as small-volume ion chambers, a diamond detector, TLDs and films. Results show that beam energies vary with cone diameter. For a 6 MV beam, the mean energies in water at the point of maximum dose for a 0.5 cm cone and a 5 cm cone are 2.05 MeV and 1.65 MeV respectively. The values of St obtained by the simulations are in good agreement with the results of the measurements for most detectors. When the lateral resolution of the detectors is taken into account, the results agree within a few per cent for most fields and detectors. The calculations showed a variation of St with depth in the water. Based on calculated electron spectra in water, the validity of the assumption that measured dose ratios are equal to measured detector readings was verified.     

An experimental evaluation of recent electron dosimetry codes of practice

As from the 1 January 1997, the recent IPEMB code of practice for electron dosimetry is the recommended protocol for electron beam dosimetry in the UK, replacing the previous HPA code of practice and its IPSM addendum. New recommendations for electron beam dosimetry have also been formulated recently by the AAPM and the IAEA on the use of parallel-plate ionization chambers in high-energy electron beams. Against this background, the procedures recommended in each of these codes of practice have been followed from intercomparison of the field instrument ionization chamber with a secondary standard through to the determination of absorbed dose at the reference position in the electron beam. Absorbed doses have been determined for a number of electron beam energies ranging from nominal 5 MeV through to 17 MeV, and for four different types of field instrument ionization chamber: an NE2571 graphite walled cylindrical chamber; an NACP parallel-plate chamber; a Markus parallel-plate chamber; and a Roos parallel-plate chamber. The differences in the determination of absorbed dose between the IPEMB protocol and the HPA/IPSM protocol vary from +0.5% to +1.6% at the depth of maximum dose. In addition the IPEMB measured doses are 0.2% larger than those measured following the IAEA code of practice. It may also be stated that the IPEMB measured doses at the depth of maximum dose are up to 1.5%, but generally less than 1.0%, lower than those measured by the AAPM protocol.     

Correction factors for parallel-plate chambers used in plastic phantoms in electron dosimetry

In electron beam dosimetry using parallel-plate chambers solid phantoms are sometimes necessary. To obtain the dose to water from the ionization obtained in the solid phantom, fluence correction factors and perturbation factors have to be applied. In this study fluence factors in a perturbation free geometry have been determined experimentally for common phantom materials. Wall perturbation factors for simulated Attix, NACP, and Roos chambers have also been determined for the same materials. Comparative Monte Carlo calculations have been performed using the EGS4 Monte Carlo code. Comparison with data in newly published protocols such as IAEA and IPEMB shows an agreement with the results obtained in this paper to within 1%, demonstrating that the data published in these protocols may be used with reasonable accuracy if recommended phantoms are used. The results also show that if unsuitable phantom materials are used, the wall perturbation factors may differ for different chambers and for different phantom materials by more than 3% and perturbation factors have to be considered in order to obtain a high accuracy in the dose determination.     

Study of dosimetry consistency for kilovoltage x-ray beams

In this paper, the consistency of kilovoltage (tube potentials between 40 and 300 kV) x-ray beam dosimetry using the "in-air" method and the in-phantom measurement has been studied. The procedures for the measurement of the central-axis depth-dose curve, which serve as a link between the dose at the reference depth to the dose elsewhere in a phantom, were examined. The uncertainties on the measured dose distributions were analyzed with the emphasis on the surface dose measurement. The Monte Carlo method was used to calculate the perturbation correction factors for a photon diode and a NACP plane-parallel ionization chamber at different depths in a water phantom irradiated by 100-300 kV (2.43 mm Al-3.67 mm Cu half-value layer) x-ray beams. The depth-dose curves measured with these two detectors, after correcting for the perturbation effect (up to 15% corrections), agreed with each other to within 1.5%. Comparisons of the doses at the phantom surface and at 2 cm depth in water for photon beams of 100-300 kV tube potential obtained using the "backscatter" method and those using the "in-phantom" measurement have shown that the "in-air" method can be equally applied to this energy range if the depth-dose curve can be measured accurately. To this end, measured depth ionization curves require depth-dependent correction factors.     

Polarity and ion recombination correction factors for ionization chambers employed in electron beam dosimetry

Polarity and ion recombination correction factors for the NACP (type-02) design parallel-plate ionization chamber employed in a recent UK national electron beam dosimetry intercomparison are derived over the full range of energies and measurement conditions encountered. In addition, these effects have been studied for a further four NACP chambers, a Markus parallel-plate chamber, a Roos parallel-plate chamber and a NE2571 graphite walled cylindrical ionization chamber.     

Correction factors for Farmer-type chambers for absorbed dose determination in 60Co and 192Ir brachytherapy dosimetry

This paper presents two methods for absorbed dose determination with ionization chambers at short distance from 60Co and 192Ir brachytherapy sources. The methods are modifications of the Bragg-Gray and large cavity principles given in the IAEA code of practice for high- and medium-energy photon beams. A non-uniformity correction factor to account for the non-uniform electron fluence in the air cavity is introduced into the methods. The absorbed dose rates were determined from ionization chamber measurements at distances between 1.5 and 5.0 cm from the brachytherapy sources. The agreement between the two methods is excellent in 60Co brachytherapy dosimetry. For 192Ir dosimetry, the difference is less than 2.5% at all distances. In absorbed dose rate calculations with the 60Co brachytherapy source, the ratios between calculated and experimentally determined absorbed dose rates are 0.987 and 0.994 depending on the method used for absorption and scatter correction. In 192Ir dosimetry, the large cavity principle gives almost identical values to those which can be obtained with the AAPM recommendations. Using the chambers according to the Bragg-Gray principle in 192Ir dosimetry, the agreement with AAPM calculated absorbed dose rates is within 2.5% at all distances. The uncertainty, expressed as one standard deviation, in the experimentally determined absorbed dose is estimated to be between 3 and 4%.     

Quantification of mean energy and photon contamination for accurate dosimetry of high-energy electron beams

The scientific background of the standard procedure for determination of the mean electron energy at the phantom surface (E0) from the half-value depth (R50) has been studied. The influence of energy, angular spread and range straggling on the shape of the depth dose distribution and the R50 and Rp ranges is described using the simple Gaussian range straggling model. The relation between the R50 and Rp ranges is derived in terms of the variance of the range straggling distribution. By describing the mean energy imparted by the electrons both as a surface integral over the incident energy fluence and as a volume integral over the associated absorbed dose distribution, the relation between E0 and different range concepts, such as R50 and the maximum dose and the surface dose related mean energy deposition ranges, Rm and R0, is analysed. In particular the influence of multiple electron scatter and phantom generated bremsstrahlung on R50 is derived. A simple analytical expression is derived for the ratio of the incident electron energy to the half-value depth. Also, an analytical expression is derived for the maximum energy deposition in monoenergetic plane-parallel electron beams in water for energies between 2 and 50 MeV. Simple linear relations describing the relative absorbed dose and mass ionization at the depth of the practical range deposited by the bremsstrahlung photons generated in the phantom are derived as a function of the incident electron energy. With these relations and a measurement of the extrapolated photon background at Rp, the treatment head generated bremsstrahlung distribution can be determined. The identification of this photon contamination allows an accurate calculation of the absorbed dose in electron beams with a high bremsstrahlung contamination by accounting for the difference in stopping power ratios between a clean electron beam and the photon contamination. The absorbed dose determined using ionization chambers in heavily photon contaminated (10%) electron beams may be too low--by as much as 1.5%--without correction.     

Optimization of 3D conformal electron beam therapy in inhomogeneous media by concomitant fluence and energy modulation

The possibilities of using simultaneous fluence and energy modulation techniques in electron beam therapy to shape the dose distribution and almost eliminate the influences of tissue inhomogeneities have been investigated. By using a radiobiologically based optimization algorithm the radiobiological properties of the tissues can be taken into account when trying to find the best possible dose delivery. First water phantoms with differently shaped surfaces were used to study the effect of surface irregularities. We also studied water phantoms with internal inhomogeneities consisting of air or cortical bone. It was possible to improve substantially the dose distribution by fluence modulation in these cases. In addition to the fluence modulation the most suitable single electron energy in each case was also determined. Finally, the simultaneous use of several preselected electron beam energies was also tested, each with an individually optimized fluence profile. One to six electron energies were used, resulting in a slow improvement in complication-free cure with increasing number of beam energies. To apply these techniques to a more clinically relevant situation a post-operative breast cancer patient was studied. For simplicity this patient was treated with only one anterior beam portal to clearly illustrate the effect of inhomogeneities like bone and lung on the dose distribution. It is shown that by using fluence modulation the influence of dose inhomogeneities can be significantly reduced. When two or more electron beam energies with individually optimized fluence profiles are used the dose conformality to the internal target volume is further increased, particularly for targets with complex shapes.     

Investigation of the chamber correction factor (k(ch)) for the UK secondary standard ionization chamber (NE2561/NE2611) using medium-energy x-rays

This paper evaluates the characteristics of ionization chambers for the measurement of absorbed dose to water for medium-energy x-rays. The values of the chamber correction factor, k(ch), used in the IPEMB code of practice for the UK secondary standard (NE2561/NE2611) ionization chamber are derived and their constituent factors examined. The comparison of the chambers' responses in air revealed that of the chambers tested only the NE2561, NE2571 and NE2505 exhibit a flat (within 5%) energy response in air. Under no circumstances should the NACP, Sanders electron chamber, or any chamber that has a wall made of high atomic number material, be used for medium-energy x-ray dosimetry. The measurements in water reveal that a chamber that has a substantial housing, such as the PTW Grenz chamber, should not be used to measure absorbed dose to water in this energy range. The value of k(ch) for an NE2561 chamber was determined by measuring the absorbed dose to water and comparing it with that for an NE2571 chamber, for which k(ch) data have been published. The chamber correction factor varies from 1.023 +/- 0.03 to 1.018 +/- 0.001 for x-ray beams with HVL between 0.15 and 4 mm Cu. The values agree with that for an NE2571 chamber within the experimental uncertainty. The corrections due to the stem, waterproof sleeve and replacement of the phantom material by the chamber for an NE2561 chamber are described.     

The effect of delta rays on the ionometric dosimetry of proton beams

The interface effects arising in the measurement of absorbed dose by ionization chambers, owing to the inhomogeneity between the walls and the gas, have been evaluated by an analytical model. The geometrical situation considered here is appropriate for representing the behaviour of a plane-parallel ionization chamber exposed to a radiotherapeutic beam of protons. Two gases, dry air and tissue equivalent gas (methane based), as well as six materials commonly used in ionization chamber walls, i.e. graphite, A-150 tissue equivalent plastic, C-522 air equivalent plastic, nylon type 6, polymethyl methacrylate and polystyrene, have been examined. The analysis of the results shows that, within the limits of the detector dimensions and proton energies commonly used in the dosimetry of radiotherapeutic beams, these effects, if not taken into account in the measurement interpretation, can entail deviations of up to about 2% with respect to the correct absorbed dose in gas.     

Performance evaluation of a diode array for enhanced dynamic wedge dosimetry

The performance of a diode array (Profiler) was evaluated by comparing its enhanced dynamic wedge (EDW) profiles measured at various depths with point measurements using a 0.03 cm3 ionization chamber on a commercial linear accelerator. The Profiler, which covers a 22.5 cm width, was used to measure larger field widths by concatenating three data sets into a larger field. An innovative wide-field calibration technique developed by the manufacturer of the device was used to calibrate the individual diode sensitivity, which can vary by more than 10%. Profiles of EDW measured with this device at several depths were used to construct isodose curves using the percentage depth dose curve measured by the ionization chamber. These isodose curves were used to check those generated by a commercial treatment planning system. The profiles measured with the diode array for both 8 and 18 MV photon beams agreed with those of the ionization chamber within a standard deviation of 0.4% in the field (defined as 80% of the field width) and within a maximum shift of less than 2 mm in the penumbra region. The percentage depth dose generally agreed to within 2% except in the buildup region. The Profiler was extremely useful as a quality assurance tool for EDW and as a dosimetry measurement device with tremendous savings in data acquisition time.     

Field study on the distribution of Entamoeba histolytica and Entamoeba dispar in the northern Philippines as detected by the polymerase chain reaction

The influence of the electron contamination at in vivo dosimetry with diodes on the patient surface has been investigated by introducing different accessories in the beam path and by changing the field size and SSD. The results show a clear correlation between the electron contamination at an effective measuring depth of the diode and the signal from the patient diode. When the electron contamination is taken into account the agreement between the diode values and the absorbed dose is greatly improved. More accurate in vivo dosimetry with less error margins is therefore possible if better predictions of the electron contamination in high-energy photon beams can be performed.     

Calibration of a mosfet detection system for 6-MV in vivo dosimetry

PURPOSE: Metal oxide semiconductor field-effect transistor (MOSFET) detectors were calibrated to perform in vivo dosimetry during 6-MV treatments, both in normal setup and total body irradiation (TBI) conditions. METHODS AND MATERIALS: MOSFET water-equivalent depth, dependence of the calibration factors (CFs) on the field sizes, MOSFET orientation, bias supply, accumulated dose, incidence angle, temperature, and spoiler-skin distance in TBI setup were investigated. MOSFET reproducibility was verified. The correlation between the water-equivalent midplane depth and the ratio of the exit MOSFET readout divided by the entrance MOSFET readout was studied. MOSFET midplane dosimetry in TBI setup was compared with thermoluminescent dosimetry in an anthropomorphic phantom. By using ionization chamber measurements, the TBI midplane dosimetry was also verified in the presence of cork as a lung substitute. RESULTS: The water-equivalent depth of the MOSFET is about 0.8 mm or 1.8 mm, depending on which sensor side faces the beam. The field size also affects this quantity; Monte Carlo simulations allow driving this behavior by changes in the contaminating electron mean energy. The CFs vary linearly as a function of the square field side, for fields ranging from 5 x 5 to 30 x 30 cm2. In TBI setup, varying the spoiler-skin distance between 5 mm and 10 cm affects the CFs within 5%. The MOSFET reproducibility is about 3% (2 SD) for the doses normally delivered to the patients. The effect of the accumulated dose on the sensor response is negligible. For beam incidence ranging from 0 degrees to 90 degrees, the MOSFET response varies within 7%. No monotonic correlation between the sensor response and the temperature is apparent. Good correlation between the water-equivalent midplane depth and the ratio of the exit MOSFET readout divided by the entrance MOSFET readout was found (the correlation coefficient is about 1). The MOSFET midplane dosimetry relevant to the anthropomorphic phantom irradiation is in agreement with TLD dosimetry within 5%. Ionization chamber and MOSFET midplane dosimetry in inhomogeneous phantoms are in agreement within 2%. CONCLUSION: MOSFET characteristics are suitable for the in vivo dosimetry relevant to 6-MV treatments, both in normal and TBI setup. The TBI midplane dosimetry using MOSFETs is valid also in the presence of the lung, which is the most critical organ, and allows verifying that calculation of the lung attenuator thicknesses based only on the density is not correct. Our MOSFET dosimetry system can be used also to determine the surface dose by using the water-equivalent depth and extrapolation methods. This procedure depends on the field size used.     

Mean energy, energy-range relationships and depth-scaling factors for clinical electron beams

Using Monte Carlo simulations we have studied the electron mean energy, Eo, and the most probable energy, Eo,p, at the phantom surface and their relationships with half-value depth, R50, and the practical range, Rp, for a variety of beams from five commercial medical accelerators with an energy range of 5-50 MeV. It is difficult to obtain a relation between R50 and Eo for all electrons at the surface because the number of scattered lower-energy electrons varies with the machine design. However, using only direct electrons to calculate Eo, there is a relationship which is in close agreement with that calculated using monoenergetic beams by Rogers and Bielajew [Med. Phys. 13, 687-694 (1986)]. We show that the empirical formula Eo,p = 0.22 + 1.98Rp + 0.0025R2p describes accurately the relationship between Rp and Eo,p for clinical beams of energies from 5 to 50 MeV with an accuracy of 3%. The electron mean energy, Ed, is calculated as a function of depth in water as well as plastic phantoms and is compared both with the relation, Ed = Eo (1-d/Rp), employed in AAPM protocols and with values in the IAEA Code of Practice. The conventional relations generally overestimate Ed over the entire therapeutic depth, e.g., the AAPM and IAEA overestimate Ed at dmax by up to 20% for an 18 MeV beam from a Clinac 2100C. It is also found that at all depths mean energies are 1%-3% higher near the field edges than at the central axis. We calculated depth-scaling factors for plastic phantoms by scaling the depth in plastics to the water-equivalent depth where the mean energies are equal. The depth-scaling factor is constant with depth in a given beam but there is a small variation ( < 1.5%) depending on the incident beam energies. Depth-scaling factors as a function of R50 in plastic or water are presented for clear polystyrene, white polystyrene and PMMA phantom materials. The calculated depth-scaling factor is found to be equal to R50water/R50plastic. This is just the AAPM definition of effective density but there are up to 2% discrepancies between our calculated values and those recommended by the AAPM and the IAEA protocols. We find that the depth-scaling factors obtained by using the ratio of continuous-slowing-down ranges are inaccurate and overestimate our calculated values by 1%-2% in all cases. We also find that for accurate work, it is incorrect to use a simple 1/r2 correction to convert from parallel beam depth-dose curves to point source depth-dose curves, especially for high-energy beams.     

Use of CEA TVS film for measuring high energy photon beam dose distributions

CEA TVS film is a therapy verification film that has been recently introduced in the North American market. This film features linear characteristic curves for photon energies from 137Cs to 18 MV as reported by Cheng and Das [Med. Phys. 23, 1225 (1996)]. In Saskatoon, TVS film was investigated for its application in the measurement of dose distributions with 4 and 18 MV linacs and a 60Co unit. The TVS film jacket has a layer of conductive material that has a minimal effect on the film's response. Film sensitivity generally increases for exposures normal to the incident beam as compared with parallel exposures, but was highly dependent on beam energy and depth of measurement. Fractional depth doses obtained in the parallel orientation agreed well with ion chamber measurements for the linac beams at depths beyond Dmax; ion chamber measurements differed by a maximum of 1.6% and 2.6% for the 4 and 18 MV beams, respectively. In the buildup region, an increase in film response was found when compared to the ion chamber measurements for both linac beams. With the 60Co beam, the TVS film showed an increase in sensitivity with depth as the proportion of scattered soft x rays increases; the maximum difference between ion chamber and film fractional depth doses was 7.8%. The TVS film demonstrates a substantial improvement over Kodak X-Omat V film for measuring depth doses in the parallel orientation, for all beams considered. Generally, the results confirm TVS film as an accurate and practical dosimeter for the measurement of dose distributions in high energy photon beams.     

Monte Carlo study of fluence perturbation effects on cavity dose response in clinical proton beams

Current protocols for clinical proton beam dosimetry have not implemented any chamber-dependent correction factors for absorbed dose determination. The present work initiates a Monte Carlo study of these factors with emphasis on proton fluence perturbation effects and preliminary calculations of perturbation effects from secondary electrons. The proton Monte Carlo code PTRAN was modified to allow simulation of proton transport in non-homogeneous geometries of both unmodulated and modulated beams. The dose to water derived from the dose calculated in an air cavity agrees well with results from analytical calculations assuming a displacement of the point of measurement. For unmodulated beams small differences, limited to 0.8%, could be partially attributed to proton multiple scattering. Effects of replacing water around the cavity with wall material are explained by the introduction of a water-equivalent wall thickness. For modulated beams no significant perturbation effects arise. Secondary electron spectra are calculated analytically. Preliminary electron transport calculations with EGS4 show that wall perturbations of the order of 1% could result. Perturbation effects caused by the energy transport of secondary particles from inelastic nuclear interactions have not been studied here. Inclusion of inelastic nuclear energy transfers in the cavity dose, assuming total local absorption, indicate that separate scaling of this contribution with the ratio of total inelastic nuclear cross sections could be important.     

The secretory intestinal transport of some beta-lactam antibiotics and anionic compounds: a mechanism contributing to poor oral absorption

PURPOSE: To measure the effect of silicon diode detectors used for in vivo dosimetry on beam characteristics and determine whether this effect is clinically significant. METHODS AND MATERIALS: Commercially available photon and electron diodes were placed on the central axis of photon and electron beams. The beam characteristics were measured for 6- and 10-MV photon and 6-20-MeV electron energies from a Varian Clinac 1800 medical linear accelerator. Water was used for the medium, and measurements were made for various clinically common field sizes and depths. RESULTS: Beam attenuations along the central axis were 10 and 7.5% for 6- and 10-MV photons, respectively. Electron beam dose reductions were between 13 and 25% for 20-6-MeV electrons. Photon beam flatness varied up to 7% at different depths, but the symmetry was not affected much. Electron beam flatness and symmetry were significantly changed to as much as 18 and 6%, respectively. CONCLUSION: Use of diode detectors on central axis of photon and electron beams for in vivo dosimetry causes significant attenuation and alteration of the beam characteristics. The percentage of the volume affected is significant (e.g., 23% of the volume in a 4 x 4 field gets 10% less dose for a 6-MV photon beam), especially if these diodes are used for in vivo dosimetry on the central axis every day for every treatment, as is done in some clinics. Other beam parameters such as penumbra and skin dose are also affected. It is therefore recommended that the diodes be used only as needed.     

Carbon beam dosimetry intercomparison at HIMAC

To verify international uniformity in carbon beam dosimetry, an intercomparison programme was carried out at the heavy ion medical accelerator (HIMAC). Dose measurements with ionization chambers were performed for both unmodulated and 6 cm modulated 290 MeV/nucleon carbon beams. Although two different dosimetry procedures were employed, the evaluated values of absorbed dose were in good agreement. This comparison established a common framework for ionization chamber dosimetry between two different carbon beam therapy facilities.