Showing papers on "Dose profile published in 1995"

Portal dosimetry using a liquid ion chamber matrix: Dose response studies

Abstract: Current intensive investigations of electronic portal imaging devices(EPIDs) have prompted their potential application to portal dosimetry. In this paper, the progress made in using a commercial liquid ion chamber matrix EPID for portal dosimetry is discussed. The pixel value of the liquid ion chamber element was calibrated against dose by exposing the imager to 6‐MV x‐ray beams of various intensities obtained with various thicknesses of lead attenuators and a range of source to detector distances. Absolute dose values were determined using an ion chamber on the central axis at the depth of maximum dose in a solid water phantom. The pixel values of the matrix were determined for various field sizes in order to evaluate the dependence of pixel value on dose at those field sizes. It was confirmed that the pixel value was proportional to the square root of the dose rate and was nearly independent of the field size. The 2D pixel values were converted to 2D dose maps in the water phantom after applying a correction for the effect of horns in the flood calibration field. The flood calibration field was used to obtain the relative sensitivity of each pixel. Good agreement was observed (normally better than 1% in relative standard deviation) between the converted dose distribution obtained from the pixel matrix and the direct dose measurement using an ion chamber scanned in a water phantom in regions of shallow dose gradient. For application to on‐line portal dosimetry, both the short‐ and long‐term stability of this EPID system were found to be within 1% relative standard deviation. This system, together with an accurate portal dose calculation procedure, can be used for on‐line radiotherapy dose verification.

Dosimetric characteristics of a liquid-filled electronic portal imaging device

Abstract: 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.

Instrumentation and dosimeter-size artifacts in quantitative thermoluminescence dosimetry of low-dose fields

Abstract: Thermoluminescencedosimetry is extensively used for quantitative dose measurements in various irradiation fields such as dosimetry of brachytherapy sources. In this application, small doses on the order of 0.5 cGy must be accurately measured, which requires careful control of instrumentation, energy‐dependence, and nonlinearity of detector response. Several investigators have observed the presence of some undesirable signals when the thermoluminescent dosimeters(TLDs) were read without any nitrogen gas flow in the TLD reader. Others have indicated that the ‘‘prereadout’’ annealing technique is the same as the ‘‘preirradiation’’ technique for doses above 10 cGy, but they have not extended their study to lower doses. The goal of this study is to investigate dependence of sensitivity and linearity of the TLD response to the flow of nitrogen gas in the TLD reader at low dose level, annealing technique, and TLD size. The effect of nitrogen flow sensitivity and linearity of two different sizes of lithium fluoride TLD‐100 chips has been studied. Our data indicate a large standard deviation of TLD sensitivity, up to a factor of 2, when TLDs were read without nitrogen gas flow in the TLD reader. In addition, a large deviation from linearity was observed for doses below 5 cGy. When the reading‐chamber was purged with nitrogen gas, dispersion of the responses of the TLDs that were exposed to the same dose fell to within 5%. At precision levels of 2% and 5%, the low dose limits are 1 cGy and 0.5 cGy, respectively, for large chips and 15 cGy and 1 cGy for small chips, if TLDs are read with nitrogen gas flow in the TLD reader. The full results of our investigation are presented.

MOSFET dosimeters: the role of encapsulation on dosimetric characteristics in mixed gamma-neutron and megavoltage X-ray fields

Abstract: This paper is concerned with the role of the package of MOSFETs used in measurements of gamma dose in mixed gamma-neutron fields, in high energy bremsstrahlung and soft X-ray fields. It is shown that a kovar cap should be avoided for dosimetry applications in the presence of strong thermal neutron fluences. In regions of strong electronic disequilibrium the "bare" or unencapsulated MOSFET is a unique tool for surface dose measurements and Monte Carlo model validation. For depths where electronic equilibrium exists (i.e. x>depth of D/sub max/) the MOSFET package is not critical. For low energy X-ray fields the energy dependence of the dose enhancement factor (DEF) will differ depending upon whether the irradiation is performed in free air geometry or on the surface of a phantom.

Thermoluminescence dosimetry and its applications in medicine--Part 2: History and applications.

Abstract: Thermoluminescence dosimetry (TLD) has been available for dosimetry of ionising radiation for nearly 100 years. The variety of materials and their different physical forms allow the determination of different radiation qualities over a wide range of absorbed dose. This makes TL dosimeters useful in radiation protection where dose levels of microGy are monitored as well as in radiotherapy where doses up to several Gray are to be measured. The major advantages of TL detectors are their small physical size and that no cables or auxiliary equipment is required during the dose assessment. Therefore TLD is a good method for point dose measurements in phantoms as well as for in vivo dosimetry on patients during radiotherapy treatment. As an integrative dosimetric technique, it can be applied to personal dosimetry and it lends itself to the determination of dose distributions due to multiple or moving radiation sources (e.g. conformal and dynamic radiotherapy, computed tomography). In addition, TL dosimeters are easy to transport, and they can be mailed. This makes them well suited for intercomparison of doses delivered in different institutions. The present article aims at describing the various applications TLD has found in medicine by taking into consideration the physics and practice of TLD measurements which have been discussed in the first part of this review (Australas. Phys. Eng. Sci. Med. 17: 175-199, 1994).

Tomotherapy: Optimized planning and delivery of radiation therapy

Abstract: In contemporary radiotherapy dose optimization, radiation beams and beam modifiers are iteratively selected until the dose distribution is acceptable. Another approach, referred to as the “inverse problem,” is: Given the dose prescription, compute the optimal set of photon beams while preventing unphysical solutions such as negative beam weights, and iterate to achieve the prescription as closely as possible. This solution to this inverse problem, which uses image reconstruction mathematics, entails the delivery of large numbers of nonuniform beam intensities to produce uniform dose distributions. These dose distributions can be arranged to conform very closely to even complex target volumes, yet spare surrounding sensitive tissue. Alternatively, the dose distributions can be arranged to generously treat a regional field and “conformally avoid” overtreating sensitive volumes within the field. Multiple dose prescriptions can be delivered without additional effort. We propose that a practical way of delivering optimized dose distributions would be to intensity modulate a photon beam, using collimator leaves intersecting a slit field of radiation. Modulation is achieved by varying the time that the leaves are blocking the field. A practical geometry to deliver such a beam is a computed tomography-like gantry configuration, which also lends itself to tomographic setup verification of dose delivered to the patient. We refer to such a delivery method as “tomotherapy.” Several types of tomotherapy simulations have been conducted. A fully three dimensional optimized treatment planning system using iterative filtered back-projection have been developed. We will present examples of conformal plans for breast and prostate radiotherapy. We have constructed an experimental apparatus for simulating helical tomotherapy delivery by simultaneously rotating and longitudinally translating a phantom past an intensity-modulated fan beam. A comparison between a computation and and experimentally realized plan is presented. A Monte Carlo simulation of the angular distribution and energy fluence spectrum of 10-MV photons produced by a tungsten target have been used to estimate the optimized shape and mass of a primary shielding required to meet regulatory standards.

Testicular doses in definitive radiation therapy for localized prostate cancer

Abstract: Purpose : To measure the dose received by the unshielded testes during a conventional course of 18 MV photon radiotherapy for localized prostate cancer and to identify the factors influencing it. Methods and Materials : For each of four patients, a wax block containing thermoluminescent chips was attached to the posterior aspect of the scrotum in close proximity to the testes on each day of treatment during a full course of radical radiotherapy, and dose measurements were obtained. The distances between the thermoluminescent chips and the beam edge were verified by measurement from port films. The accuracy of the in vivo measurements and the factors influencing the testis dose were studied using a phantom arrangement. Six factors were considered : (a) the relative contributions from primary and scattered radiation, (b) the amount of buildup required for the thermoluminescent chips that monitored testis dose, (c) the position of the testes within the scrotum, (d) field size, (e) distance from the field lower border, and (f) the effect of port films. Results : Median daily doses to the testes in four patients ranged from 5 to 7 cGy. Daily doses for the four patients ranged from 4 to 14 cGy. The total dose to the testes over the full course of therapy ranged from 1.8 to 2.4 Gy. The daily dose depended primarily on the distance from the field lower border. This was increased by approximately 2.5 cGy when a 6 MV port film was taken. The relative contributions from primary and scattered radiation were found to be similar. Dose measurements at the posterior aspect of the scrotum overestimated the testis dose by approximately 15%. Conclusion : The most important factors influencing the dose received by the testis are the distance from the testes to the field lower border and the occasion of a port film. A knowledge of the number of port films and the average distance from the field lower border to the testes allows a reasonable prediction of testes dose without daily measurement.

Total body irradiation with an arc and a gravity-oriented compensator.

Abstract: Purpose To deliver uniform dose distributions for total-body irradiation (TBI) with an arc field and a gravity-oriented compensator. This technique allows the patient to be treated lying on the floor in a small treatment room. Methods and materials Through the sweeping motion of the gantry, a continuous arc field can deliver a large field to a patient lying on the floor. The dose profile, however, would not be uniform if no compensator were used, due to the effects of inverse square variation of beam intensity with distance as well as the slanted depth in patient. To solve this problem, a gravity-oriented compensator made of cerrobend alloy was designed. This compensator has a cross-section of an inverted isosceles triangle, with the apex always pointing downward, due to gravity. By properly selecting the thickness of the compensator, the width of the base, and the distance between the pivots to the base, the difference in the path length through the compensator can be made just right to compensate the effects of inverse-square and slanted depth, thus producing a uniform dose profile. Results Arc fields with a gravity-oriented compensator were used for 6, 10, 15, and 18 MV photon beams. The arc field can cover a patient with a height up to 180 cm. The field width was chosen from 32 to 40 cm at the machine isocenter. The optimal thickness of the compensator was found to be 2.5 cm, and its base was 25 cm wide. The distance from the pivot points to the flat surface of the compensator proximal to the beam ranges from 13 to 14 cm for different beam energies. The dose uniformity at a depth of 10 cm is within +/-5% for all beam energies used in this study. Conclusions Highly uniform dose profiles for TBI treatments can be delivered with an arc and a gravity-oriented compensator. The proposed technique is simple and versatile. A single compensator can be used for all energies, because the amount of compensation can be adjusted by changing the distance to the pivot and/or the field size.

Measurement of patient entrance surface dose rates for fluoroscopic x-ray units

Abstract: Measurements of patient entrance surface dose rate provide valuable data for interpreting results from dose-area product studies on fluoroscopic X-ray equipment. Methods for measurement of entrance surface dose rate with backscatter and incident dose rate without backscatter have been investigated. Entrance surface dose rate is measured with an ionization chamber in contact with a tissue-equivalent phantom. Backscattered radiation contributes 27-45% to the measurement and is affected by field size and chamber position. Incident dose rate measured using a copper phantom provides an alternative approach. Consistent relationships between thicknesses of Perspex and copper giving similar incident dose rates under automatic gain control have been established for different tube potentials with and without a grid. This allows measurements of incident dose rate made using copper to be linked to corresponding thicknesses of tissue-equivalent material. Since only a few millimetres of copper are required, contributions from backscatter can be minimized and transport of phantoms is simplified. Incident dose can be related to dose-area product and entrance surface dose derived using backscatter factors. Such measurements play a valuable role in interpreting patient dose data and recommending options to reduce patient dose.

Study of dosimetric penumbra due to multileaf collimation on a medical linear accelerator

Abstract: Purpose : Investigations of dosimetric penumbra produced by multileaf collimation on a medical linear accelerator are presented. Methods and Materials : Multileaf collimators (MLCs) can be designed with at least three different shaped leaf-end profiles : straight, divergent, and curved. Assessment of the dosimetric effects of the collimator edge profiles was implemented using a fast Fourier transform (FFT) convolution algorithm. Accelerator source intensity was considered to have a Gaussian distribution. The calculated dose profile, for a source-to-surface distance of 100 cm and at depth of 10 cm in a water phantom, was fitted to a penumbral-forming function from which the penumbral width between 80% and 20% of the central axis dose was obtained. Results : Calculation performed at various field sizes showed that curved collimator leaf-end produces a wider penumbra than the diverging collimator leaf-side. Film measurements agreed with the calculations within an uncertainty of less than 2 mm. The effect of backup jaws for the MLC and of the lower pair of diverging diaphragms on dosimetric penumbra was also investigated. Conclusions : This study is useful for characterizing collimator edge effects and for optimizing new collimator designs.

Clinical use of carbon-loaded thermoluminescent dosimeters for skin dose determination

Abstract: Carbon-loaded thermoluminescent dosimeters (TLDs) are designed for surface/skin dose measurements. Following 4 years in clinical use at the Mater Hospital, the accuracy and clinical usefulness of the carbon-loaded TLDs was assessed. Teflon-based carbon-loaded lithium fluoride (LiF) disks with a diameter of 13 mm were used in the present study. The TLDs were compared with ion chamber readings and TLD extrapolation to determine the effective depth of the TLD measurement. In vivo measurements were made on patients receiving open-field treatments to the chest, abdomen, and groin. Skin entry dose or entry and exit dose were assessed in comparison with doses estimated form phantom measurements. The effective depth of measurement in a 6 MV therapeutic x-ray beam was found to be about 0.10 mm using TLD extrapolation as a comparison. Entrance surface dose measurements made on a solid water phantom agreed well with ion chamber and TLD extrapolation measurements, and black TLDs have an accuracy of {plus_minus} 5% ({plus_minus}2 SD). The dose predicted from black TLD readings correlate with observed skin reactions as assessed with reflectance spectroscopy. In vivo dosimetry with carbon-loaded TLDs proved to be a useful tool in assessing the dose delivered to the basal cell layer in the skinmore » of patients undergoing radiotherapy. 23 refs., 4 figs., 3 tabs.« less

Estimation of Effective Dose from CT Examination

Abstract: Tissue or organ doses related to radiological risk were determined for four different types of CT scanners with a spiral scan function. Dose measurements were performed using a Rando phantom and two types of thermoluminescent dosimeters. The effective doses recommended by the International Commission on Radiological Protection in 1990 were evaluated using the tissue or organ doses determined with the phantom measurement. The resultant effective dose per CT examination ranged from 4.6 to 10.8 mSv for chest examination and from 6.7 to 13.3 mSv for upper abdominal examination. It should be noted that the effective dose from CT examination will be increased by increasing in the frequency of CT examinations and technical development of CT scanners.

The response behaviour of LiF:Mg,Cu,P thermoluminescence dosimeters to high-energy electron beams used in radiotherapy

Abstract: To ensure the effectiveness of radiation-therapy treatments, both in-phantom and in-patient reliable dose measurements are required. Thermoluminescence dosimeters are used commonly for both applications. Among the various available materials, the relatively new LiF:Mg,Cu,P phosphor is a suitable candidate for quality control of in vivo dosimetry in electron-beam therapy. The response behaviour of LiF:Mg,Cu,P chips to 6-21 MeV electron beams used in radiotherapy was studied. Batch homogeneity, dose linearity, sensitivity change after use, dose and dose-rate response, energy dependence and fading characteristics were investigated. The contribution from each factor to the overall uncertainty in dose measurement was evaluated. The results of this work show that the LiF:Mg,Cu,P chips are comparable to the commonly used LiF:Mg,Ti ones, and support their use for in vivo electron-beam dosimetry to an accuracy within +/- 10%.

Considerations for superficial photon dosimetry.

Abstract: Dose measurements at superficial energies required special considerations First, care must be taken in selecting appropriate phantom materials Materials that are adequate tissue substitutes at megavoltage energies might not be adequate at superficial energies The suitability of a material can be judged by comparing its mass attenuation and mass energy absorption coefficients at superficial energies to those of the tissue of interest Second, very low energy x‐ray and electron contaminants must be removed from the superficial beam before they reach the detector For detectors with a very thin window, this can be achieved by placing thin film on top of the detector Failure to properly eliminate contaminants can result in a large increase in dose measured directly at the surface

Verification of water equivalence of FeMRI gels using Monte Carlo simulation

Abstract: Dosimetric characteristics of the FeMRI gels for various energy photon and electron beams were investigated using the Integrated Tiger Series (ITS) Monte Carlo codes. The FeMRI gels have been reported as being tissue‐substitutes which can be shaped to any desired contour for three‐dimensional (3D) mapping of dose distribution using magnetic resonance(MR) scanning. This study indicated that the central axis depth dose, off‐axis depth dose, transverse profile at the depth of 50% of maximum dose, and energy deposition for photons and electrons in the gels are close to those in water. From these comparisons, it is concluded that FeMRI gels are suitable substitutes for water for the purposes of dose measurements of both photons and electrons used in radiotherapy.

The radiation exposure of the patient from stable-xenon computed tomography.

Abstract: For stable-xenon computed tomography (CT), an X-ray examination for measurement of cerebral blood circulation in the brain, the radiation exposure of the patient was determined in order to estimate the risk of inducing cancer. Organ doses of brain, eyelenses, thyroid and gonads have been calculated using the measured air kerma free-in-air on the axis of rotation and organ-specific conversion factors calculated with the Monte Carlo method. Dose measurements with TLD-100 rods using a humanoid Alderson phantom were carried out for verification of the calculated organ doses. In the case of brain partially located in the region of primary radiation a mean organ dose of 39 mSv was calculated. The dose measurements showed dose equivalents between 6 and 68 mSv in different regions of the brain and consequently an inhomogeneous dose distribution. From an estimation of the radiation-induced risk using the effective dose of 1.6 mSv it follows that one additional fatal cancer per 12,500 stable-xenon CT examinations has to be expected. The organ doses of eyelenses and thyroid located in the region of scattered radiation are so low that biological effects are hardly to be expected. The calculated dose equivalents of 6.5 mSv and 0.5 mSv, respectively, are in good agreement with measurements. The organ dose of gonads amounted to less than 0.07 mSv.

Computerised Analysis of LiF GR-200 TL Signals: Application to Dose Measurements in the µGy Range

Abstract: The features of a simple program for the computerised analysis of the LiF GR-200 glow curve are described. The adverse influence of the high residual signals characteristic of this material has been effectively eliminated for doses in the range 0.5-300 μGy. This program for TL evaluation, together with some improvements in the reader temperature measurement and control, permit one to obtain very good dose results in this dose range employing only reader anneal. The good dosimetric performance achieved under these simple Working conditions has been shown in a detailed experiment whose results are also presented here. Very reproducible ambient dose results have been obtained in real working conditions exposing GR-200 dosemeters for times ranging between five hours and three months.

A study of the effect of cone shielding in intraoperative radiotherapy

Abstract: The primary goal of intraoperative radiation therapy is to irradiate the intraoperatively determined tumor target volume with a single fraction of tumoroidal dose while minimizing the dose to all adjacent healthy tissues. To reduce dose outside the treatment volume, lead sheets are often used to cover the external surface of the cone tip thus providing a shielding for the tissues outside the field. In this paper, the effect of the shielding on the depth dose distributions and dose profiles at different depths is studied based on experimental data. The results were also compared against an EGS4 Monte Carlo code for the same geometry as the measurements. The cones varied in size having diameters of 5 cm, 7 cm, and 9 cm, and the electron energies ranged from 6 MeV to 22 MeV. The depth dose curves and dose profiles (at two different depths in the phantom) were measured and computed with and without the lead shielding for the various combinations of cone sizes and electron energies using a water phantom to simulate the patient. It was found that the presence of lead increases on average across the treatment area the dose to the tumor from 2% up to 5%, while the dose outside the cone was reduced by as much as 75%. Both measurements and calculations were found to be in agreement.

Measurements and calculations of the absorbed dose distribution around a 60Co source.

Abstract: The data from Meisberger et al. [Radiology 90, 953–957 (1968)] are often used as a basis for dose calculations in brachytherapy. In order to describe the absorbed dose in water around a brachytherapy point source, Meisberger provided a polynomial fit for different isotopes taking into account the effect of attenuation and scattering. The validity of the Meisberger coefficients is restricted to distances up to 10 cm from the source, which is regarded to be satisfactory for most brachytherapy applications. However, for more distant organs it may lead to errors in calculated absorbed dose. For this reason dose measurements have been performed in air and in water around a high activity 60Co source used in high dose rate brachytherapy. Measurements were carried out to distances of 20 cm, using ionization chambers. These data show that at a distance of about 15 cm the amount of scattered radiation virtually equals the amount of primary radiation. This emphasizes the contribution of scattered radiation to the dose in healthy tissue far from the target volume, even with relatively high energy photon radiation of 60Co. It is also shown that the Meisberger data as well as the approach of Van Kleffens and Star [Int. J. Radiat. Oncol. Phys. 5, 557–563 (1979)] lead to significant errors in absorbed dose between distances of 10 and 20 cm from the source. In addition to these measurements, the Monte Carlo code has been used to calculate separately primary dose and scattereddose from a cobalt point source. The calculated results agree with the experimental data within 1% for a most distant dose scoring region.

A new sigmoidal function describing the small field dose profile data from a linear accelerator

Abstract: Dose profile data from small circular fields have been used in treatment dose planning for stereotactic radiosurgery. Generally, a two‐dimensional interpolation of the measured beam profiles from circular collimators is used to calculate the dose at any axial depth and radial distance from the central axis. Instead, the dose profile data can be transformed into a sigmoidal form. A new three parameter sigmoidal function was developed to fit the transformed (sigmoidal) dose profile data. The values of the three estimated parameters were found to follow either linearly or exponentially as a function of axial depth. Thus, instead of linear interpolation, these formulas can be used to calculate dose at any axial depth and radial distance from the central axis for circular collimators of various diameters. This new sigmoidal function provides another formula to describe dose profile data from circular collimator of small fields.

Radiation damage by neutrons to plastic scintillators

Abstract: Polystyrene based scintillator SCSN38, wavelength shifter Y7 with polymethylmethacrylate matrix and pure PMMA light guide GS218 have been irradiated in the mixed radiation field of a pool reactor. About 77% of the dose released in SCSN38 was caused by the /spl gamma/-field, 23% by fast neutrons. The total dose ranged from 2 to 105 kGy. The dose measurements were made using alanine dosimeters. Transmission and fluorescence of the samples have been measured before and several times after irradiation. The radiation damage results show no differences to irradiations in pure /spl gamma/-fields with corresponding released doses. >

A comparison of two photon planning algorithms for 8 MV and 25 MV X-ray beams in lung.

Abstract: We report results of a comparison of two photon planning algorithms, the Clarkson Scatter Integration algorithm and the Equivalent Tissue-air Ratio algorithm, using a simple lung phantom for 8 MV and 25 MV X-ray beams of field sizes 5 cm x 5cm and 10 cm x 10 cm. Central axis depth-dose distributions were measured with a thimble chamber or a Markus parallel-plate chamber. Dose profile distributions were measured with TLD rods and films. Measured dose distributions were then compared to predicted dose distributions. Both agorithms overestimate the dose at mid-lung as they do not account for the effect of electronic disequilibrium. The Clarkson algorithm consistently shows less accurate results in comparison with the ETAR algorithm. There is additional error in the case of the Clarkson algorithm because of the assumption of a unit density medium in calculating scatter, which gives an overestimate in the effective scatter-air ratios in lung. For a 5 cm x 5 cm field, the error of dose prediction (Dpredicted-Dmeasured) for 25 MV x-ray beam at mid-lung is 15.8% and 12.8% for Clarkson and ETAR algorithm respectively. At 8 MV the error is 9.3% and 5.1% respectively. In addition, both algorithms underestimate the penumbral width at mid-lung as they do not account for the penumbral flaring effect in low density medium. It is very important for medical physicists, radiation therapists and clinicians to be aware of the limitation of their radiotherapy treatment planning systems.

The Measurement of CT Scanner Radiation Dose Profile Using TL Dosemeters

Abstract: Thermoluminescence detectors in the form of thin discs were used for dose profile measurement of CT scanners CT/T8800, CT9800 and CT MAX made by General Electric, CT 1200 SX made by Picker and SOMATOM 2 made by Siemens. Effective energy of a CT scanner X ray beam was evaluated prior to calibration and measurement. Dose profiles of all nominal slice thicknesses down to 1.5 mm and of a multiple scan were measured for each CT scanner. CT dose indexes (CTDI) were derived from the dose profiles and compared with values measured by a pencil type ionisation chamber.