Measurement of Prepl Pwall factors in electron beams and in a 60Co beam for plane-parallel chambers

This paper describes a method to measure the product of Prepl Pwall correction factors for ionization chambers and presents our measured values of Prepl Pwall for Markus plane-parallel chambers in electron beams. It is shown that the measured values of Prepl Pwall can be fitted to an equation, Prepl Pwall = c1 + c2 R50 + c3 (R50)2, for Markus chambers at the new reference depth for electron beams (6 MeV < or = nominal energy E < or = 20 MeV). We also present our measured values of Prepl Pwall for NACP and Markus chambers in a water phantom irradiated in a 60Co beam.     

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.     

Radiation dose perturbation at tissue-titanium dental interfaces in head and neck cancer patients

PURPOSE: To determine the dose perturbation effects at the tissue-metal implant interfaces in head and neck cancer patients treated with 6 MV and 10 MV photon beams. METHODS AND MATERIALS: Phantom measurements were performed to investigate the magnitude of dose perturbation to the tissue adjacent to the titanium alloy implants with (100 mu and 500 mu thick) and without hydroxylapatite (HA) coating. Radiographic and radiochromic films were placed at the upper (and lower) surface of circular metal discs (diameter x thickness: 15 x 3.2, 48 x 3.2, 48 x 3.8 mm2) in a solid water phantom and were exposed perpendicular to radiation beams. The dosimeters were scanned with automatic film scanners. Using a thin-window parallel-plate ion chamber, dose perturbation were measured for a 48 x 3.2 mm2 disc. RESULTS: At the upper surface of the tissue-dental implant interface, the radiographic data indicate that for 15 x 3.2 mm2 uncoated, as well as 100 mu coated discs, dose perturbation is about +22.5% and +20.0% using 6 MV and 10 MV photon beams, respectively. For 48 x 3.2 mm2 discs, these values basically remain the same. However, for 48 x 3.8 mm2 discs, these values increase slightly to about +23.0% and +20.5% for 6 MV and 10 MV beams, respectively. For 48 x 3.2 mm2 discs with 500 mu coating, dose enhancement is slightly lower than that obtained for uncoated and 100 mu coated discs for each beam energy studied. At the lower interface for 15 x 3.2 mm2 and 48 x 3.2 mm2 uncoated and 100 mu coated discs, dose reduction is similar and is about -13.5% and -9.5% for 6 MV and 10 MV beams, respectively. For 48 x 3.8 mm2 discs, dose reduction is about -14.5% and -10.0% for 6 MV and 10 MV beams, respectively. For 48 x 3.2 mm2 discs with 500 mu coating, the dose reduction were slightly higher than those for uncoated and 100 mu coated discs. CONCLUSIONS: For the beam energies studied, dose enhancement is slightly larger for the lower energy beam. The results of dose perturbation were similar for 100 mu coated and uncoated discs. These results were slightly lower for the 500 mu coated discs but are not clinically significant. The dosimetry results obtained from radiochromic films were similar to the ones obtained from radiographic film. The dose enhancement results obtained from ion chamber dosimetry are higher than those obtained from film dosimetry. The ion chamber data represent the data at "true" tissue-titanium interface, whereas the ones obtained from film dosimetry represent the data at film-titanium interface.     

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.     

Dosimetric evaluation of a widely used kilovoltage x-ray unit for endocavitary radiotherapy

In this paper we present the dosimetric data of a Therapax DTX300 kilovoltage x-ray unit for endocavitary rectal irradiation. The unit if operated at tube voltage of 40-60 kVp (30 mA) with an added filtration of 0.2-0.4 mm Al generates acceptable beam qualities comparable to those of the original Papillon technique. Relative dosimetric measurements were performed at the cone end (37.2 cm SSD) of a 3 cm diameter rectal cone using various detectors to ensure the accuracy. A Monte Carlo method was used to calculate correction factors for the diode used in the percentage depth-dose (PDD) measurement, and to study the effect of the detector size on the beam profile. The PDD data were determined using the diode measurement corrected for its energy and angular response. It was found that the PTW N23342 and Markus parallel-plate chamber can be used directly to measure the PDD for this beam quality with 2% uncertainty. Measurement and Monte Carlo results have shown that the detector size has a significant effect on the penumbral profile. Film and diode detectors have a better spatial resolution compared to ionization chambers, but they may give an incorrect profile tail due to either nonlinear response at low energy or angular dependence. This can be corrected using the ionization-chamber measurement, based on the Monte Carlo analysis. The isodose distributions for this x-ray unit are presented.     

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.     

Detection of trisomy 3 in primary gastric B-cell lymphoma by using chromosome in situ hybridization on paraffin sections

Recently, new backscatter factors for low-energy x rays derived from Monte Carlo calculations have been recommended in the UK code of practice for kilovoltage dosimetry published by (IPEMB). As these data, presented as a function of half-value layer, do not take account of the variation of the x-ray spectra for a given HVL, we have undertaken an experimental study in order to determine BSG for the beam qualities provided by a Darpac 2000 therapy unit. A RTL detector such as Li2B4O7:Cu and parallel-plate ion chambers specially designed for low-energy x-ray dosimetry have been used. The results obtained show very good agreement between the TLD and the Monte Carlo calculations, confirming values obtained by other authors with lithium borate TLD. On the contrary, the results obtained with plane-parallel ion chambers show discrepancies up to 9% that are discussed.     

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.     

Calibration of ion chambers for use in mammography

There is at present no UK calibration service for ion chambers for mammography, where X-ray beams are produced from tubes having molybdenum targets and filters. This paper reports calibrations against a radiotherapy secondary standard (calibrated for beams from tungsten targets with aluminium filters) using beams from both types of target and filter. Two examples of the Radcal mammography dosimeter were found to have calibration factors which varied by less than 1% in molybdenum target beams from 30 to 40 kV. Differences between calibrations using the two types of X-ray beam did not exceed about 2%. All calibration factors were within about +/- 2% of 1.0. Errors are thought to be within +/- 3%. The results of an independent calibration of one of these dosimeters against a similar chamber calibrated by CEC are also reported. Calibrations of this kind can only be temporary expedients until adequate calibration facilities for mammography beams become available, but are nevertheless useful.     

A filtration method for improving film dosimetry in photon radiation therapy

Successful radiotherapy requires accurate dosimetry for treatment verification. Existing dosimeters such as ion chambers, TLD, and diodes have drawbacks such as relatively long measurement time and poor spatial resolution. These disadvantages become serious problems for dynamic-wedged beams. Thus the clinical use of dynamic wedges requires an improved dosimetry method. X-ray film may serve this purpose. However, x-ray film is not clinically accepted as a dosimeter for photon beams, because it overresponds to photons with energies below about 400 keV. This paper presents and develops a method which was initially proposed by Burch to improve the dose response of x-ray film in a phantom. The method is based on placing high-atomic number foils next to the film. The foils are used as filters to preferentially remove low-energy photons. The optimal film and filter configuration in a phantom was determined using a mathematical scheme derived in this study and a Monte Carlo technique (ITS code). The optimal configuration thus determined is as follows: the filter-to-film distance of 6 mm and the filter thickness of 0.15 mm for percent depth-dose measurement; the distance of 1 cm and the thickness of 0.25 mm for off-axis (dose) ratio measurement. The configuration was then tested with photon beams from a 4 MV linac. The test result indicates that the in-phantom dose distribution based on the optimal configuration agrees well with those measured by ion chambers.     

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.     

Intensity modulation with electrons: calculations, measurements and clinical applications

Intensity modulation of electron beams is one step towards truly conformal therapy. This can be realized with the MM50 racetrack microtron that utilizes a scanning beam technique. By adjusting the scan pattern it is possible to obtain arbitrary fluence distributions. Since the monitor chambers in the treatment head are segmented in both x- and y-directions it is possible to verify the fluence distribution to the patient at any time during the treatment. Intensity modulated electron beams have been measured with film and a plane parallel chamber and compared with calculations. The calculations were based on a pencil beam method. An intensity distribution at the multileaf collimator (MLC) level was calculated by superposition of measured pencil beams over scan patterns. By convolving this distribution with a Gaussian pencil beam, which has propagated from the MLC to the isocentre, a fluence distribution at isocentre level was obtained. The agreement between calculations and measurements was within 2% in dose or 1 mm in distance in the penumbra zones. A standard set of intensity modulated electron beams has been developed. These beams have been implemented in a treatment planning system and are used for manual optimization. A clinical example (prostate) of such an application is presented and compared with a standard irradiation technique.     

Dosimetric characteristics of a liquid-filled electronic portal imaging device

PURPOSE: 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. METHODS AND MATERIALS: 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. RESULTS: 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. CONCLUSION: 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.     

Correction factors for water-proofing sleeves in kilovoltage x-ray beams

This paper investigates the effect of the waterproofing sleeve on the calibration of kilovoltage photon beams (50-300 kV). The sleeve effect correction factor, ps has been calculated using the Monte Carlo method as the ratios of the air kerma in an air cavity of a cylindrical chamber without the waterproofing sleeve to that with a sleeve. Three sleeve materials have been studied, PMMA, nylon and polystyrene. The calculations were carried out using the EGS4 (Electron Gamma Shower version 4) code system with the application of a correlated-sampling variance-reduction technique. The results show that the sleeve correction factor for 1-mm thick nylon and polystyrene sleeves, ps varies from 0.992 to 1.000 and from 0.981 to 1.000, respectively, for the same beam quality range. The ps factor varies with sleeve thickness, beam quality and phantom depth. No significant dependence of the ps factor on field size and source-surface distance has been found. Measurements for PMMA, nylon and polystyrene sleeves of various thicknesses have also been carried out and show excellent agreement with Monte Carlo calculations.     

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.     

Initial recombination in a parallel-plate ionization chamber exposed to heavy ions

For exact determination of absorbed dose in heavy-ion irradiation fields which are used in radiation therapy and biological experiments, ionization chambers have been characterized with defined heavy-ion beams and correction factors. The LET (linear energy transfer) dependence of columnar recombination in a parallel-plate ionization chamber has been examined. Using 135 MeV/u carbon and neon beams, the ion collection efficiency was measured for several gases (air, carbon dioxide, argon and tissue-equivalent gas). 95 MeV/u argon beams and 90 MeV/u iron beams were also used for measurements of columnar recombination in air. As expected by Jaffe theory, the inverse of the ratio of the ionization charge to the saturated ionization charge had a linear relationship with the inverse of the electric field strength in the region below 0.002 V(-1) cm. The gradient of the line increases as the LET of the heavy ions increases. A strong LET dependence of the gradient was observed in air and carbon dioxide. The LET dependence was not observed in tissue-equivalent gas, nitrogen or argon. The exact depth-dose distribution of the heavy-ion beam was obtained by this correction of the initial recombination effect for the collected ionization charge. The columnar recombination in air was analysed using Jaffe theory; the obtained parameter b (a track radius) should be in the range between 0.001 cm and 0.005 cm, whereas the value obtained by Jaffe is 0.00179 cm. The value of the parameter b should increase as the LET of the heavy-ion beam increases in order to reproduce the experimental values of the initial recombination.     

Dose variation at bone/titanium interfaces using titanium hollow screw osseointegrating reconstruction plates

PURPOSE: To evaluate dose variations at bone/titanium interfaces in an experimental model designed to simulate postoperative radiotherapy in patients with mandibular reconstructions using a titanium hollow-screw osseointegrating reconstruction plate (THORP) system. MATERIALS AND METHODS: The model consisted of a 25 x 25 x 10 mm3 block of fresh bovine femoral diaphysis, to the surface of which a segment of THORP system reconstruction plate was fixed by means of a solid titanium screw 4 mm in diameter and 10 mm in length. Using specially designed thermoluminescent dosimeters (TLD) 2 mm in diameter and 0.13 mm in thickness, dose measurements were carried out at four distances from the screw axis (0.1, 0.3, 0.6, and 1 mm). 60Co and 6-MV photon beams were used at incidences both perpendicular and parallel ("axial") to the screw axis. RESULTS: For 6-MV X-ray beams incident perpendicular to the screw axis, the maximum dose enhancement (due to backscatter) and the maximum dose reduction (due to attenuation) at the bone/titanium interface were 5% (+/- 2%) and 6% (+/- 2%), respectively. The corresponding values for 60Co beams were 6% (+/- 5%) and 10% (+/- 5%). For the axial incidences, a maximum dose enhancement of 5-7% was noted for both 6-MV X-rays and 60Co for beams incident on the surface containing the THORP plate segment, whereas beams incident on the opposite surface induced only a very small dose enhancement (2-3%). CONCLUSION: Using a new experimental model, TLD measurements showed only marginally significant dose variations at bone/titanium interfaces around THORP screws, all measured values being very close to the uncertainty limits (+/- 5%) associated with the method. For both 60Co and 6-MV beams, dose variations appeared smaller for axial than for perpendicular incidences. Because photon beams used in head and neck cancer treatment are most often directed parallel to the screw axes, these results suggest that failures of prosthetic osseointegration are unlikely to be explained by an overdosage at the bone/titanium interface.     

Lipowitz metal shielding thickness for dose reduction of 6-20 MeV electrons

The relative dose reduction by Lipowitz metal of 6 to 20 MeV electrons from a Varian Associates Clinac-20 linear accelerator has been measured using a parallel plate thin wall ionization chamber. Metal thickness required for a 5% attenuation level for a 10 X 10 cm2 field are as follows: 6 MeV-2.3 mm, 9 MeV-4.4 mm, 12 MeV-8.5 mm, 16 MeV--18.0 mm, 20 MeV-25.0 mm.     

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.