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Clair Baldock - One of the best experts on this subject based on the ideXlab platform.
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issues involved in the quantitative 3d imaging of proton doses using optical ct and chemical Dosimeters
Physics in Medicine and Biology, 2015Co-Authors: Simon J Doran, Tina Gorjiara, J Adamovics, Zdenka Kuncic, Andrzej Kacperek, Clair BaldockAbstract:Dosimetry of proton beams using 3D imaging of chemical Dosimeters is complicated by a variation with proton linear energy transfer (LET) of the dose‐response (the so-called ‘quenching effect’). Simple theoretical arguments lead to the conclusion that the total absorbed dose from multiple irradiations with different LETs cannot be uniquely determined from postirradiation imaging measurements on the Dosimeter. Thus, a direct inversion of the imaging data is not possible and the proposition is made to use a forward model based on appropriate output from a planning system to predict the 3D response of the Dosimeter. In addition to the quenching effect, it is well known that chemical Dosimeters have a non-linear response at high doses. To the best of our knowledge it has not yet been determined how this phenomenon is affected by LET. The implications for dosimetry of a number of potential scenarios are examined. Dosimeter response as a function of depth (and hence LET) was measured for four samples of the radiochromic plastic PRESAGE ® , using an optical computed tomography readout and entrance doses of 2.0 Gy, 4.0 Gy, 7.8 Gy and 14.7 Gy, respectively. The Dosimeter response was separated into two components, a
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high resolution gel dosimetry of a hdr brachytherapy source using normoxic polymer gel Dosimeters preliminary study
Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment, 2006Co-Authors: C Hurley, C Mclucas, Giovanni Pedrazzini, Clair BaldockAbstract:Polymer gel dosimetry has been shown to be an effective tool in the analysis of radiotherapy treatments in cancer therapy, being used to map the dose distribution around an irradiation pattern of a polymer gel Dosimeter. Combined with high-resolution magnetic resonance imaging (MRI), polymer gel dosimetry can be an effective dosimetry tool to map dose distributions with high spatial resolution (∼100 μm). Previously polyacrylamide polymer gel dosimetry required a strict hypoxic environment to protect the gel from oxygen infiltration as oxygen inhibits the polymerization reaction used to correlate to absorbed dose. However, with the advent of normoxic polymer gels, a strict hypoxic environment is not required. Normoxic polymer gel Dosimeters can be manufactured under normal atmospheric conditions. This study assessed the use of a MAGIC normoxic polymer gel Dosimeter to accurately map the dose distribution of a single-line irradiation and a point source irradiation from a brachytherapy radiation source administered through a nylon catheter inserted into the gel Dosimeter. The phantoms were irradiated to a dose of 10 Gy at 2 mm from the source center and imaged using high-resolution MRI with an in-plane pixel size of 0.1055 mm/pixel. Good agreement was found between the dose points predicted by the computer treatment-planning system and the measured normalized dose profiles in the gel Dosimeter. The use of normoxic polymer gel Dosimeters with high-resolution MRI evaluation shows promise as an effective tool in applications requiring accurate dose distributions in high resolution, such as intravascular brachytherapy.
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a preliminary study of the novel application of normoxic polymer gel Dosimeters for the measurement of ctdi on diagnostic x ray ct scanners
Medical Physics, 2005Co-Authors: Brendan Hill, A J Venning, Clair BaldockAbstract:Computer tomography dose index (CTDI) is a measurement undertaken during acceptance testing and subsequent quality assurance measurements of diagnostic x-ray CT scanners for the determination of patient dose. Normoxic polymer gel Dosimeters have been used for the first time to measure dose and subsequently CTDI during acceptance testing of a CT scanner and compared with the conventional ionization chamber measurement for a range of imaging protocols. The normoxic polymer gel Dosimeter was additionally used to simultaneously determine slice-width dose profiles and CTDI in the transaxial plane, the measurements of which are usually determined with thermoluminescent dosimetry or film. The resulting CTDI for all slice widths calculated from the normoxic polymer gel Dosimeter were within corresponding ionization chamber CTDI values. Slice-width dose-profiles full-width half-maximum values from the normoxic polymer gel Dosimeter were compared to the slice sensitivity profiles and were within the tolerances of the manufacturer. Normoxic polymer gel Dosimeters have been shown to be a useful device for determining CTDI and dose distributions for CT equipment, and provide additional information not possible with just the use of an ionization chamber.
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ultrasonic absorption in polymer gel Dosimeters
Ultrasonics, 2003Co-Authors: Melissa L Mather, A F Collings, Nick Bajenov, Andrew K. Whittaker, Clair BaldockAbstract:Ultrasonic absorption in polymer gel Dosimeters was investigated. An ultrasonic interferometer was used to study the frequency (f) dependence of the absorption coefficient (alpha) in a polyacrylamide gel Dosimeter (PAG) in the frequency range 5-20 MHz. The frequency dependence of ultrasonic absorption deviated from that of an ideal viscous fluid. The presence of relaxation mechanisms was evidenced by the frequency dependence of alpha/f(2) and the dispersion in ultrasonic velocity. It was concluded that absorption in polymer gel Dosimeters is due to a number of relaxation processes which may include polymer-solvent interactions as well as relaxation due to motion of polymer side groups. The dependence of ultrasonic absorption on absorbed dose and formulation was also investigated in polymer gel Dosimeters as a function of pH and chemical composition. Changes in Dosimeter pH and chemical composition resulted in a variation in ultrasonic dose response curves. The observed dependence on pH was considered to be due to pH induced modifications in the radiation yield while changes in chemical composition resulted in differences in polymerisation kinetics. (C) 2003 Elsevier B.V. All rights reserved.
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an experimental study of the dose response of polymer gel Dosimeters imaged with x ray computed tomography
Physics in Medicine and Biology, 2001Co-Authors: Jamie Trapp, Sven Back, Martin Lepage, Gregory Michael, Clair BaldockAbstract:Changes in the linear attenuation coefficient of polymer gel Dosimeters post-irradiation enable the imaging of dose distributions by x-ray computed tomography (CT). Various compositions of polymer gel Dosimeters manufactured from acrylamide (AA), and N,N'-methylene-bis-acrylamide (BIS) comonomers and gelatin or agarose gelling agents were investigated. This work shows that increasing the comonomer concentration increases the CT-dose sensitivity of the polymer gel Dosimeter. This can be further increased by replacing gelatin with agarose. Varying the gelatin concentration however does not significantly change the CT-dose sensitivity. Among the compositions studied, dose resolution (D(delta)95%) was found to be optimal for polymer gel Dosimeters comprising 5% gelatin, 3% AA, 3% BIS and 89% water. (Less)
Martin Lepage - One of the best experts on this subject based on the ideXlab platform.
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severe dose inaccuracies caused by an oxygen antioxidant imbalance in normoxic polymer gel Dosimeters
Physics in Medicine and Biology, 2011Co-Authors: Mahbod Sedaghat, Rachel Bujold, Martin LepageAbstract:Two oxygen scavengers have been successfully tested to produce normoxic polymer gel Dosimeters under normal atmospheric conditions. The first is ascorbic acid and the second is a chloride (also sulfate) salt of tetrakis (hydroxymethyl) phosphonium. These antioxidants, added to the Dosimeter during gel preparation, chemically remove dissolved oxygen that otherwise inhibits propagation of the polymerization reaction during irradiation of the Dosimeter. These gel Dosimeters are radiosensitive after manufacture under normoxic conditions. However, we show herein that the accuracy of the dosimetric measurement is compromised due to chemical reactions of the antioxidant with radicals. In addition, we provide evidence that both antioxidant and oxygen act as radical scavengers that affect the amount of polymer formed in the gel Dosimeter. This can result in important dose inaccuracies in both methacrylic acid-based and acrylamide-based normoxic Dosimeter gels.
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an experimental study of the dose response of polymer gel Dosimeters imaged with x ray computed tomography
Physics in Medicine and Biology, 2001Co-Authors: Jamie Trapp, Sven Back, Martin Lepage, Gregory Michael, Clair BaldockAbstract:Changes in the linear attenuation coefficient of polymer gel Dosimeters post-irradiation enable the imaging of dose distributions by x-ray computed tomography (CT). Various compositions of polymer gel Dosimeters manufactured from acrylamide (AA), and N,N'-methylene-bis-acrylamide (BIS) comonomers and gelatin or agarose gelling agents were investigated. This work shows that increasing the comonomer concentration increases the CT-dose sensitivity of the polymer gel Dosimeter. This can be further increased by replacing gelatin with agarose. Varying the gelatin concentration however does not significantly change the CT-dose sensitivity. Among the compositions studied, dose resolution (D(delta)95%) was found to be optimal for polymer gel Dosimeters comprising 5% gelatin, 3% AA, 3% BIS and 89% water. (Less)
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modelling of post irradiation events in polymer gel Dosimeters
Physics in Medicine and Biology, 2001Co-Authors: Martin Lepage, Sven Back, Llewellyn Rintoul, Andrew K. Whittaker, Clair BaldockAbstract:The nuclear magnetic resonance (NMR) spin-spin relaxation time (T2) is related to the radiation-dependent concentration of polymer formed in polymer gel Dosimeters manufactured from monomers in an aqueous gelatin matrix. Changes in T2 with time post-irradiation have been reported in the literature but their nature is not fully understood. We investigated those changes with time after irradiation using FT-Raman spectroscopy and the precise determination of T2 at high magnetic field in a polymer gel Dosimeter. A model of fast exchange of magnetization taking into account ongoing gelation and strengthening of the gelatin matrix as well as the polymerization of the monomers with time is presented. Published data on the changes of T2 in gelatin gels as a function of post-manufacture time are used and fitted closely by the model presented. The same set of parameters characterizing the variations of T2 in gelatin gels and the increasing concentration of polymer determined from FT-Raman spectroscopy are used successfully in the modelling of irradiated polymer gel Dosimeters. Minimal variations in T2 in an irradiated PAG Dosimeter are observed after 13 h. (Less)
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modelling of post irradiation events in polymer gel Dosimeters
Physics in Medicine and Biology, 2001Co-Authors: Martin Lepage, Sven Back, Llewellyn Rintoul, Andrew K. Whittaker, Clive BaldockAbstract:The nuclear magnetic resonance (NMR) spin-spin relaxation time (T2) is related to the radiation-dependent concentration of polymer formed in polymer gel Dosimeters manufactured from monomers in an aqueous gelatin matrix. Changes in T2 with time post-irradiation have been reported in the literature but their nature is not fully understood. We investigated those changes with time after irradiation using FT-Raman spectroscopy and the precise determination of T2 at high magnetic field in a polymer gel Dosimeter. A model of fast exchange of magnetization taking into account ongoing gelation and strengthening of the gelatin matrix as well as the polymerization of the monomers with time is presented. Published data on the changes of T2 in gelatin gels as a function of post-manufacture time are used and fitted closely by the model presented. The same set of parameters characterizing the variations of T2 in gelatin gels and the increasing concentration of polymer determined from FT-Raman spectroscopy are used successfully in the modelling of irradiated polymer gel Dosimeters. Minimal variations in T2 in an irradiated PAG Dosimeter are observed after 13 h.
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an experimental study of the dose response of polymer gel Dosimeters imaged with x ray computed tomography
Physics in Medicine and Biology, 2001Co-Authors: Jamie Trapp, Sven Back, Martin Lepage, Gregory Michael, Clive BaldockAbstract:Changes in the linear attenuation coefficient of polymer gel Dosimeters post-irradiation enable the imaging of dose distributions by x-ray computed tomography (CT). Various compositions of polymer gel Dosimeters manufactured from acrylamide (AA), and N,N'-methylene-bis-acrylamide (BIS) comonomers and gelatin or agarose gelling agents were investigated. This work shows that increasing the comonomer concentration increases the CT-dose sensitivity of the polymer gel Dosimeter. This can be further increased by replacing gelatin with agarose. Varying the gelatin concentration however does not significantly change the CT-dose sensitivity. Among the compositions studied, dose resolution (D(delta)95%) was found to be optimal for polymer gel Dosimeters comprising 5% gelatin, 3% AA, 3% BIS and 89% water.
Sven Back - One of the best experts on this subject based on the ideXlab platform.
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modelling of post irradiation events in polymer gel Dosimeters
Physics in Medicine and Biology, 2001Co-Authors: Martin Lepage, Sven Back, Llewellyn Rintoul, Andrew K. Whittaker, Clair BaldockAbstract:The nuclear magnetic resonance (NMR) spin-spin relaxation time (T2) is related to the radiation-dependent concentration of polymer formed in polymer gel Dosimeters manufactured from monomers in an aqueous gelatin matrix. Changes in T2 with time post-irradiation have been reported in the literature but their nature is not fully understood. We investigated those changes with time after irradiation using FT-Raman spectroscopy and the precise determination of T2 at high magnetic field in a polymer gel Dosimeter. A model of fast exchange of magnetization taking into account ongoing gelation and strengthening of the gelatin matrix as well as the polymerization of the monomers with time is presented. Published data on the changes of T2 in gelatin gels as a function of post-manufacture time are used and fitted closely by the model presented. The same set of parameters characterizing the variations of T2 in gelatin gels and the increasing concentration of polymer determined from FT-Raman spectroscopy are used successfully in the modelling of irradiated polymer gel Dosimeters. Minimal variations in T2 in an irradiated PAG Dosimeter are observed after 13 h. (Less)
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an experimental study of the dose response of polymer gel Dosimeters imaged with x ray computed tomography
Physics in Medicine and Biology, 2001Co-Authors: Jamie Trapp, Sven Back, Martin Lepage, Gregory Michael, Clair BaldockAbstract:Changes in the linear attenuation coefficient of polymer gel Dosimeters post-irradiation enable the imaging of dose distributions by x-ray computed tomography (CT). Various compositions of polymer gel Dosimeters manufactured from acrylamide (AA), and N,N'-methylene-bis-acrylamide (BIS) comonomers and gelatin or agarose gelling agents were investigated. This work shows that increasing the comonomer concentration increases the CT-dose sensitivity of the polymer gel Dosimeter. This can be further increased by replacing gelatin with agarose. Varying the gelatin concentration however does not significantly change the CT-dose sensitivity. Among the compositions studied, dose resolution (D(delta)95%) was found to be optimal for polymer gel Dosimeters comprising 5% gelatin, 3% AA, 3% BIS and 89% water. (Less)
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modelling of post irradiation events in polymer gel Dosimeters
Physics in Medicine and Biology, 2001Co-Authors: Martin Lepage, Sven Back, Llewellyn Rintoul, Andrew K. Whittaker, Clive BaldockAbstract:The nuclear magnetic resonance (NMR) spin-spin relaxation time (T2) is related to the radiation-dependent concentration of polymer formed in polymer gel Dosimeters manufactured from monomers in an aqueous gelatin matrix. Changes in T2 with time post-irradiation have been reported in the literature but their nature is not fully understood. We investigated those changes with time after irradiation using FT-Raman spectroscopy and the precise determination of T2 at high magnetic field in a polymer gel Dosimeter. A model of fast exchange of magnetization taking into account ongoing gelation and strengthening of the gelatin matrix as well as the polymerization of the monomers with time is presented. Published data on the changes of T2 in gelatin gels as a function of post-manufacture time are used and fitted closely by the model presented. The same set of parameters characterizing the variations of T2 in gelatin gels and the increasing concentration of polymer determined from FT-Raman spectroscopy are used successfully in the modelling of irradiated polymer gel Dosimeters. Minimal variations in T2 in an irradiated PAG Dosimeter are observed after 13 h.
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an experimental study of the dose response of polymer gel Dosimeters imaged with x ray computed tomography
Physics in Medicine and Biology, 2001Co-Authors: Jamie Trapp, Sven Back, Martin Lepage, Gregory Michael, Clive BaldockAbstract:Changes in the linear attenuation coefficient of polymer gel Dosimeters post-irradiation enable the imaging of dose distributions by x-ray computed tomography (CT). Various compositions of polymer gel Dosimeters manufactured from acrylamide (AA), and N,N'-methylene-bis-acrylamide (BIS) comonomers and gelatin or agarose gelling agents were investigated. This work shows that increasing the comonomer concentration increases the CT-dose sensitivity of the polymer gel Dosimeter. This can be further increased by replacing gelatin with agarose. Varying the gelatin concentration however does not significantly change the CT-dose sensitivity. Among the compositions studied, dose resolution (D(delta)95%) was found to be optimal for polymer gel Dosimeters comprising 5% gelatin, 3% AA, 3% BIS and 89% water.
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the relationship between radiation induced chemical processes and transverse relaxation times in polymer gel Dosimeters
Physics in Medicine and Biology, 2001Co-Authors: Martin Lepage, Sven Back, Llew Rintoul, Andrew K. Whittaker, Clair BaldockAbstract:The effects of ionizing radiation in different compositions of polymer gel Dosimeters are investigated using FT-Raman spectroscopy and NMR T2 relaxation times. The Dosimeters are manufactured from different concentrations of comonomers (acrylamide and N,N'-methylene-bis-acrylamide) dispersed in different concentrations of an aqueous gelatin matrix. Results are analysed using a model of fast exchange of magnetization between three proton pools. The fraction of protons in each pool is determined using the known chemical composition of the Dosimeter and FT-Raman spectroscopy. Based on these results, the physical and chemical processes in interplay in the Dosimeters are examined in view of their effect on the changes in T2. The precipitation of growing macroradicals and the scavenging of free radicals by gelatin are used to explain the rate of polymerization. The model describes the changes in T2 as a function of the absorbed dose up to 50 Gy for the different compositions. This is expected to aid the theoretical design of new, more efficient Dosimeters, since it was demonstrated that the optimum Dosimeter (i.e, with the lowest dose resolution) must have a range of relaxation times which match the range of T2 values which can be determined with the lowest uncertainty using an MRI scanner.
Y De Deene - One of the best experts on this subject based on the ideXlab platform.
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radio physical properties of micelle leucodye 3d integrating gel Dosimeters
Physics in Medicine and Biology, 2011Co-Authors: Jan Vandecasteele, S Ghysel, Steven Baete, Y De DeeneAbstract:Recently, novel radiochromic leucodye micelle hydrogel Dosimeters were introduced in the literature. In these studies, gel measured electron depth dose profiles were compared with ion chamber depth dose data, from which it was concluded that leucocrystal violet-type Dosimeters were independent of dose rate. Similar conclusions were drawn for leucomalachite green-type Dosimeters, only after pre-irradiating the samples to a homogeneous radiation dose. However, in our extensive study of the radio-physical properties of leucocrystal violet- and leucomalachite green-type Dosimeters, a significant dose rate dependence was found. For a dose rate variation between 50 and 400 , a maximum difference of 75% was found in optical dose sensitivity for the leucomalachite green-type Dosimeter. Furthermore, the measured optical dose sensitivity of the leucomalachite green-type Dosimeter was four times lower than the value previously reported in the literature. For the leucocrystal violet-type Dosimeter, a maximum difference in optical dose sensitivity of 55% was found between 50 and 400 . A modified composition of the leucomalachite green-type Dosimeter is proposed. This Dosimeter is composed of gelatin, sodium dodecyl sulfate, chloroform, trichloroacetic acid and leucomalachite green. The optical dose sensitivity amounted to (dose rate 400 ). No energy dependence for photon energies between 6 and 18 MV was found. No temperature dependence during readout was found notwithstanding a temperature dependence during irradiation of 1.90 cGy °C−1 increase on a total dose of 100 cGy. The novel gel Dosimeter formulation exhibits an improved spatial stability (= 0.088 )) and good water/soft tissue equivalence. Nevertheless, the novel formulation was also found to have a significant, albeit reduced, dose rate dependence, as a maximum difference of 33% was found in optical dose sensitivity when the dose rate varied between 50 and 400 . By pre-irradiating the novel leucomalachite green-type Dosimeter to 500 cGy, the apparent difference in dose response between 200 and 400 was eliminated, similar to earlier findings. However, a dose response difference of 38% between 50 and 200 was still measured. On the basis of these experimental results it is concluded that the leucodye micelle gel Dosimeter is not yet optimal for dose verifications of high precision radiation therapy treatments. This study, however, indicates that the dose rate dependence has a potential for improvement. Future research is necessary to further minimize the dose rate dependence through extensive chemical analysis and optimization of the gel formulation. Some insights into the physicochemical mechanisms were obtained and are discussed in this paper.
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the influence of cooling rate on the accuracy of normoxic polymer gel Dosimeters
Physics in Medicine and Biology, 2007Co-Authors: Y De Deene, G Pittomvils, S VisalatchiAbstract:Polymer gel Dosimeters offer a wide range of applications in the three-dimensional verification of complex radiation dose distributions such as in intensity-modulated radiotherapy (IMRT). With the release of polymer gel Dosimeters that can be fabricated in normal atmospheric ('normoxic') conditions, the gel manufacturing process has been significantly simplified. Gel Dosimeters are calibrated by use of a series of calibration vials irradiated with known doses or by use of a calibration phantom with a known dose distribution. The overall accuracy of the polymer gel Dosimeters is determined by different dosimetric properties. In this study, we show the influence of the temperature history during storage of the gel Dosimeter on the dose response curve for two gel Dosimeters using the monomers acrylamide/N,N'-methylene-bis-acrylamide (nPAG) and methacrylic acid (nMAG) respectively and bis[tetrakis(hydroxymethyl)phosphonium]sulphate (THP) as antioxidant in both gel Dosimeters. This study reveals that differences in temperature history after fabrication of normoxic polymer gel Dosimeters may compromise the dosimetric accuracy. It was found that the acrylamide based gel Dosimeter (nPAG) is less dependent on the post-manufacture temperature history than the methacrylic acid based gel Dosimeter (nMAG). The importance of an equal temperature history for the gel Dosimeter and calibration vials is emphasized by this study. A reproducibility study has also been performed on the nPAG gel Dosimeter when additional efforts are made to control the temperature changes upon cooling.
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the fundamental radiation properties of normoxic polymer gel Dosimeters a comparison between a methacrylic acid based gel and acrylamide based gels
Physics in Medicine and Biology, 2006Co-Authors: Y De Deene, Karel Vergote, Corneel Claeys, Carlos De WagterAbstract:Polymer gel Dosimeters offer a wide range of applications in the three-dimensional verification of complex dose distributions such as in intensity-modulated radiotherapy. One of the major difficulties with polymer gel Dosimeters is their sensitivity to oxygen, as oxygen inhibits the radiation-induced polymerization reaction. For several years, oxygen was removed from the gels by bubbling the sol with inert gases for several hours during the gel fabrication. Also, the gel had to be poured in containers with low oxygen permeability and solubility. Recently, it was found that these technical difficulties can easily be solved by adding an antioxidant to the gel. These gels are called 'normoxic' gels as they can be produced under normal atmospheric conditions. In this study several properties of polymer gel Dosimeters have been investigated: the dose sensitivity, the temporal and spatial stability of the gel, the sensitivity of the dose response to temperature during irradiation and during MR imaging, the energy dependence and the dose-rate dependence. This study reveals that the normoxic polymer gel Dosimeter based on methacrylic acid (nMAG) studied in this work has inferior radiation properties as compared to the polyacrylamide gelatine (PAG) gel Dosimeters. It is shown that from the three different gel Dosimeters investigated in this study, the nPAG gel Dosimeter results in a less sensitive gel Dosimeter but with superior radiation properties as compared to the nMAG gel Dosimeter. The importance of investigating relevant radiation properties of gel Dosimeters apart from the radiation sensitivity—prior to their use for dosimetric validation experiments—is illustrated and emphasized throughout this study. Other combinations of monomer and gelling agent may result in more reliable normoxic polymer gel Dosimeters.
Flor Herreramartinez - One of the best experts on this subject based on the ideXlab platform.
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dosimetry for small fields in stereotactic radiosurgery using gafchromic md v2 55 film tld 100 and alanine Dosimeters
PLOS ONE, 2013Co-Authors: G Massillonjl, Diego Cuevaprocel, Porfirio Diazaguirre, Miguel Rodriguezponce, Flor HerreramartinezAbstract:This work investigated the suitability of passive Dosimeters for reference dosimetry in small fields with acceptable accuracy. Absorbed dose to water rate was determined in nine small radiation fields with diameters between 4 and 35 mm in a Leksell Gamma Knife (LGK) and a modified linear accelerator (linac) for stereotactic radiosurgery treatments. Measurements were made using Gafchromic film (MD-V2-55), alanine and thermoluminescent (TLD-100) Dosimeters and compared with conventional dosimetry systems. Detectors were calibrated in terms of absorbed dose to water in 60Co gamma-ray and 6 MV x-ray reference (10×10 cm2) fields using an ionization chamber calibrated at a standards laboratory. Absorbed dose to water rate computed with MD-V2-55 was higher than that obtained with the others Dosimeters, possibly due to a smaller volume averaging effect. Ratio between the dose-rates determined with each Dosimeter and those obtained with the film was evaluated for both treatment modalities. For the LGK, the ratio decreased as the Dosimeter size increased and remained constant for collimator diameters larger than 8 mm. The same behaviour was observed for the linac and the ratio increased with field size, independent of the Dosimeter used. These behaviours could be explained as an averaging volume effect due to dose gradient and lack of electronic equilibrium. Evaluation of the output factors for the LGK collimators indicated that, even when agreement was observed between Monte Carlo simulation and measurements with different Dosimeters, this does not warrant that the absorbed dose to water rate in the field was properly known and thus, investigation of the reference dosimetry should be an important issue. These results indicated that alanine Dosimeter provides a high degree of accuracy but cannot be used in fields smaller than 20 mm diameter. Gafchromic film can be considered as a suitable methodology for reference dosimetry. TLD Dosimeters are not appropriate in fields smaller than 10 mm diameters.