Dose In Buildup Region Research Articles

Bare spherical gel dosimeter with optical computed tomography scanning

Radiochromic polyvinyl alcohol-iodide (PVA-I) hydrogel dosimeter with crosslinking by glutaraldehyde (GTA) has been reported. Because of the transparent gel’s mechanical strength, a study to cast samples in spherical shapes was initiated. The bare sample, 47 mm in diameter was mounted on a funnel and held by suction in a custom, 96 mm diameter, cylindrical vessel for 3D optical CT scanning. A solution of the same formulation without GTA provided refractive index matching inside the vessel for optic scanning and eliminated concentration gradients. The gel sphere remained in the vessel after draining the solution for the ‘in-air’ irradiation. A 20 Gy, maximum dose to the gel sphere was delivered by a single, 6 MV, x-ray beam. Comparison of central axis depth doses for the gel reconstruction and Monte Carlo calculation revealed similar results, indicating accurate 3D dosimetry within 1 mm of the gel surface will be possible. The gel recorded slightly greater dose in build-up region and slightly lower dose in the exit hemisphere relative to the calculation.

Effect of Diode-Based Transmission Detector Measurement on Dose Perturbation during Delivery of 6MV Photon

<h3>Purpose/Objective(s)</h3> Delivery dose quality assurance (QA) during treatment planning execution is the most direct and effective way to ensure that the prescribed dose is delivered accurately and the patient is irradiated safely. The purpose of this article is to evaluate the stability and sensitivity of the diode-based transmission detector (TD) installed on the beam path during radiotherapy, and analyze the effect of TD on the photon beam fluence and dose distribution. <h3>Materials/Methods</h3> The beam fluence with and without TD was measured for 6MV on a linear accelerator. The point doses at different depths (0.0cm∼15.0cm), different dose values (100MU∼500MU), different dose rates (100MU/min∼600MU/min) and different irradiation fields (3cm × 3cm∼20cm × 20cm) were measured by cylindrical three-dimensional water tank, and the transmission factors (TF) were calculated and analyzed. The percentage depth dose (PDD) and beam profile under different parameters were collected to analyze the effect of TD on ray quality and dose distribution. The effect of different irradiation fields on the γ passing rate was evaluated by a planar dosimeter. <h3>Results</h3> The diode-based TD has the high stability of 0.1% (1SD) and excellent position monitoring performance (≤0.2mm). The TF varies with the depth, dose, dose rate and irradiation fields. When the depth is 0cm, the TF has maximum value of 99.55%. For the depth of 0cm∼5cm, the TF increases with the increase of dose and dose rate (<i>R</i>=0.91, 0.75). For the depth of 5cm, the TF is least affected by dose and dose rate (1SD: 0.01%, 0.07%), and decreased with the increase of irradiation fields, 98.94% for 3cm × 3cm. When the depth is >5cm, it decreases with the dose increasing (<i>R</i>=−0.88∼−0.26), and increases with the dose rate increasing (<i>R</i>=0.78). The average TF under all the above conditions is 98.46%, that is, beam attenuation is 1.54%. The movement of the position of maximum dose depth is 0.124cm for 5cm × 5cm and 0.250cm for 3cm × 3cm to the surface of the water (0cm), otherwise ≤0.003cm. TD increases dose in build-up region, 1.49% for fields of 20cm × 20cm. TD effects ray quality (≤0.0028), beam symmetry and flatness (≤0.68%). The γ passing rate does not change under the commonly used indexes of 3%/3mm and 2%/2mm. <h3>Conclusion</h3> The diode-based TD has high stability and excellent monitoring performance. Although it has little effect on ray quality, beam symmetry, flatness and γ passing rate, it will cause beam attenuation and increase surface dose.

Development and dosimetric verification of 3D customized bolus in head and neck radiotherapy

The commercial flat bolus cannot form perfect contact with the irregular surface of the patient’s skin, resulting in an air gap. The purpose of this study was to evaluate the feasibility of using a 3D customized bolus from silicone rubber. The silicone rubber boluses were studied in basic characteristics. The 3D customized bolus was fabricated at the nose, cheek and neck regions. The point dose and planar dose differences were evaluated by comparing with virtual bolus. The hardness, thickness, density, Hounsfield unit (HU) and dose attenuation of the customized bolus were quite similar to a commercial bolus. When a 3D customized bolus was placed on the RANDO phantom, it can significantly increase buildup region doses and perfectly fit against the irregular surface shape. The average point dose differences of 3D customized bolus were −1.1%, while the commercial bolus plans showed −1.7%. The average gamma results for planar dose differences comparison of 3D customized bolus were 93.9%, while the commercial bolus plans were reduced to 91.9%. Overall, A silicone rubber bolus produced the feasible dosimetric properties and could save cost compared to a commercial bolus. The 3D printed customized bolus is a good buildup material and could potentially replace and improve treatment efficiency.

Dosimetry and radioprotection evaluations of very high energy electron beams

Very high energy electrons (VHEEs) represent a promising alternative for the treatment of deep-seated tumors over conventional radiotherapy (RT), owing to their favourable dosimetric characteristics. Given the high energy of the electrons, one of the concerns has been the production of photoneutrons. In this article we explore the consequence, in terms of neutron yield in a water phantom, of using a typical electron applicator in conjunction with a 2 GeV and 200 MeV VHEE beam. Additionally, we evaluate the resulting ambient neutron dose equivalent at various locations between the phantom and a concrete wall. Through Monte Carlo (MC) simulations it was found that an applicator acts to reduce the depth of the dose build-up region, giving rise to lower exit doses but higher entrance doses. Furthermore, neutrons are injected into the entrance region of the phantom. The highest dose equivalent found was approximately 1.7 mSv/Gy in the vicinity of the concrete wall. Nevertheless, we concluded that configurations of VHEEs studied in this article are similar to conventional proton therapy treatments in terms of their neutron yield and ambient dose equivalent. Therefore, a clinical implementation of VHEEs would likely not warrant additional radioprotection safeguards compared to conventional RT treatments.

Investigation of Buildup Region and Surface Dose: Comparison of Parallel Plane Ion Chamber, Treatment Planning System, and MC Simulation

In these days, Monte Carlo (MC) simulation is a method that can calculate the radiation dose that occurs in an environment in the most accurate way. The correct measurement of the dose occurring on the patient’s surface is of great importance to estimate the reactions that may occur on the patient’s skin. This importance encouraged us to do this study. The aim of this study is to determine buildup region and surface doses using MC simulation and to compare them with results of the parallel plane ion chamber and Treatment Planning System (TPS) measurements for 6-MV photon beams. Surface doses normalized to the maximum dose for the parallel plane ion chamber, MC simulation, fast photon (FP) algorithm, and collapsed cone convolution superposition (CC) algorithm are 13.6%, 30.28%, 0%, and 27.33%, respectively. The CC algorithm and parallel plane ion chamber measurements are compatible with MC simulation but the FP algorithm has calculated the dose less to a depth of 0.8 cm. Measuring the surface dose and the doses in the buildup region is of great importance in terms of accurately predicting the complications that may occur in the patient’s skin and taking precautions early. Using some methods and correction factors, the surface dose and the doses that may occur in the buildup region can be accurately calculated. It is recommended not to use the FP algorithm for stereotactic body radiation therapy and intensity-modulated radiation therapy treatments, as it cannot calculate doses correctly in the buildup region and surface.

Investigation of surface and buildup region doses for 6 MV high energy photon beams in the presence of a thermoplastic mask

Investigation of surface and buildup region doses for 6 MV high energy photon beams in the presence of a thermoplastic mask

Investigation of surface and buildup region doses for 6 MV high energy photon beams in the presence of a thermoplastic mask

Investigation of surface and buildup region doses for 6 MV high energy photon beams in the presence of a thermoplastic mask

Scattering kernels for fast neutron therapy treatment planning

The University of Washington (UW) Clinical Neutron Therapy System (CNTS) has been used to treat over 3300 patients. Treatment planning for these patients is currently performed using an MV x-ray model in Pinnacle® adapted to fit measurements of fast neutron output factors, wedge factors, depth-dose and lateral profiles. While this model has provided an adequate representation of the CNTS for 3D conformal treatment planning, later versions of Pinnacle did not allow for isocentric gantry rotation machines with a source-to-axis distance of 150 cm. This restriction limited the neutron model to version 9.0 while the photon and electron treatment planning at the UW had moved on to newer versions. Also, intensity modulated neutron therapy (IMNT) is underdevelopment at the UW, and the Pinnacle neutron model developed cannot be used for inverse treatment planning. We have used the MCNP6 Monte Carlo code system to develop Collapsed Cone (CC) and Singular Value Decomposition (SVD) neutron scattering kernels suitable for integration into the RayStation treatment planning system. Kernels were generated for monoenergetic neutrons with energies ranging from 1 keV to 51 MeV, i.e. the energy range most relevant to the CNTS. Percent depth dose (PDD) profiles computed in RayStation for the CC and SVD kernels are in excellent agreement with each other for depths beyond the beam’s dose build-up region (depths greater than about 1.7 cm) for open 2.8 × 2.8 cm2, 10.3 × 10.3 cm2, and 28.8 × 32.8 cm2 fields. Lateral profiles at several depths, as well as the PDD, calculated using the CC kernels in RayStation for the full CNTS energy spectrum pass a 3%/3 mm gamma test for field sizes of 2.8 × 2.8 cm2, 10.0 × 10.3 cm2, and 28.8 × 32.8 cm2.

A mathematical expression for depth-light curves of therapeutic proton beams in a quenching scintillator.

Recently, there has been increasing interest in the development of scintillator-based detectors for the measurement of depth-dose curves of therapeutic proton beams (Beaulieu and Beddar [2016], Phys Med Biol., 61:R305-R343). These detectors allow the measurement of single beam parameters such as the proton range or the reconstruction of the full three-dimensional dose distribution. Thus, scintillation detectors could play an important role in beam quality assurance, online beam monitoring, and proton imaging. However, the light output of the scintillator as a function of dose deposition is subject to quenching effects due to the high-specific energy loss of incident protons, particularly in the Bragg peak. The aim of this work is to develop a model that describes the percent depth-light curve in a quenching scintillator and allow the extraction of information about the beam range and the strength of the quenching. A mathematical expression of a depth-light curve, derived from a combination of Birks' law (Birks [1951], Proc Phys Soc A., 64:874) and Bortfeld's Bragg curve (Bortfeld [1997], Med Phys., 24:2024-2033) that is termed a "quenched Bragg" curve, is presented. The model is validated against simulation and measurement. A fit of the quenched Bragg model to simulated depth-light curves in a polystyrene-based scintillator shows good agreement between the two, with a maximum deviation of 2.5% at the Bragg peak. The differences are larger behind the Bragg peak and in the dose build-up region. In the same simulation, the difference between the reconstructed range and the reference proton range is found to be always smaller than 0.16mm. The comparison with measured data shows that the fitted beam range agrees with the reference range within their respective uncertainties. The quenched Bragg model is, therefore, an accurate tool for the range measurement from quenched depth-dose curves. Moreover, it allows the reconstruction of the beam energy spread, the particle fluence, and the magnitude of the quenching effect from a measured depth-light curve.

Measurement of build-up region dose with optical cone-beam computed tomography scanner

Measurement of the dose gradients from the entrance surface to depth is a standard task for characterizing an ionizing radiation beam. Most gel dosimeters provide spurious results near an air interface, limiting their value for this geometry. In this study, a 3D dosimeter system consisting of a low-diffusion, radiochromic hydrogel cast in a custom polyethylene terephthalate (PETE) vessel and imaged with a modified commercial optical cone-beam computed tomography (CBCT) scanner was employed. The cylindrical vessel wall and flat ends were constructed from a 0.025 cm thick PETE sheet. The optical CBCT scanner was modified to place the entire vessel in the centre of the field of view or to have the vessel base at the optical axis. Pre-irradiation and post-irradiation scans were acquired with the sample mounted in the standard and elevated positions. The sample was irradiated with a 2x2 cm square, 6 MV x-ray beam. Normalized attenuation coefficients from the central quarter of the reconstructed beam images were compared to a Monte Carlo depth dose calculation. Placing the vessel base at the optical axis allowed accurate dose measurements to within 0.2 cm of the entrance face and for the standard position to within 0.4 cm. These measurements validated the Monte Carlo calculation and provide an alternative to parallel plate ion chambers for dose measurement in the build-up region.

P213] Influence of beam spoiler and air gap on dose distribution in build-up region for X6 MV static field

Purpose Basing on CBCT images we noticed, that there are several different problems with reproducibility of bolus positioning, especially air gap occurrence. The aim of this study was to determine the influence of air gaps between bolus and phantom on dose distribution in build-up region for X6 MV. The novelty of the work is using low-density boluses fabricated with 3D printer. Methods Depth dose measurements were performed for X6 MV 10×10 cm2 open field, generated with Varian CL 2300 C/D. The measurements were carried out with the Markus plane parallel ionization chamber (PTW, 0.055 cm3 volume), connected to Unidos (PTW) electrometer. As 1 cm thick beam spoilers we used 3D-printed thermoplastic polyurethane (TPU) cuboids filled with 5%, 10%, 15% and 20% honeycomb structure (thickness of 0.5 g/cm2; 0.17 g/cm2; 0.22 g/cm2). To generate air gap we used homemade styrofoam frames. Measurements were performed at a physical depth of 1, 5, 10 and 15 mm for all beam spoilers without air gaps and with 10, 20, 30 and 40 mm air gaps, at constant 90 cm SSD. Results Our results showed that beam spoilers without air gap increased the dose in build-up region. The bigger spoiler filling is, the larger is surface dose. The same dependence can be seen on Percentage Depth-Dose curves, irrespectively of air gap size. Regardless of spoiler percentage filling with honeycomb structure, the larger air gap size is, the smaller dose on 1 mm depth. Conclusions Influence of beam spoiler filling and air gap on dose distribution in build-up region for X6 MV was investigated. Beam spoiler increased the dose in build-up region. The larger thickness of beam spoiler (larger filling) - the larger influence on build-up dose. Occurrence of air gaps lowers the surface dose. However, the influence of air gap in the range of 10–40 mm is not very visible. For bolus with density of 0.22 g/cm2; regardless of air gap size, the dose on 1 mm depth was always larger than 95% of maximum dose. The issue will be investigated for wedged and dynamic fields.

P153] A comparison of different dosimetry systems for surface and buildup region dose measurements

Purpose In this study, the surface doses obtained by optically stimulated luminescence (OSL), thermoluminescence (TLD) and markus parallel plane ion chamber were compared with TPS results and the availability of the systems in surface dosimetry was investigated. Methods The percentage depth dose (PDD) measurements of buildup region were made by TLD, OSL and markus pp ion chamber, at the surface, 1, 2, 5, 10 and 15 mm water equivalent depth at 100 cm source-detector distance for 5×5, 10×10, and 20×20 cm 2 field sizes.DD values were measured at the depth of 0.07 mm, which was suggested for surface dose measurements by ICRU Report 39. The effective measurement depth of each dosimeter was taken into account. The water equivalent thickness (WET) was considered to make an accurate comparison. Interpolations and extrapolations were performed for all systems to obtain the doses at the same depths. Under the same conditions, surface doses were calculated by TPS for different field sizes and were recorded. The buildup region doses measured by different dosimetry systems and calculated by TPS were compared for different field sizes. Results The overdoses occured in the buildup region were corrected for Markus chamber according to Gerbi’s method. The surface doses using 6 MV photon beams for 5×5, 10×10, and 20×20 cm 2 field sizes at 0.07 mm were found to be 14%, 24.75%, 13.17%, 7.74%; 19.69%, 31.78%, 21.63%, 11.04%; 30.87%, 43.80%, 28.71%, 13.61% for Markus chamber, TLD, OSL and TPS, respectively. Conclusions The lowest build-up region PDD values in all field sizes were obtained with the TPS. At 0.07 mm water equivalent depth, while the OSL and Markus pp. ion chamber results are close to each other, TLD results are higher in all field sizes. However, it is seen that the TLD gives closer results to Markus pp. ion chamber than OSL after 3 mm depth in all field sizes.

Absorption ratio of treatment couch and effect on surface and build-up region doses.

In this study, at different fields, energies and gantry angles, treatment couch and rails dose absorption ratio and treatment couch effect on surface and build-up region doses were examined. It is assumed that radiation attenuation is minimal because the carbon fiber couches have low density and it is not generally accounted for during treatment planning. Consequently, it leads to a major dosimetric mistake. Solid water phantom was used for relative dose measurement. The measurements were done using a Farmer ion chamber with 0.6cc volume and a parallel plane ion chamber starting from surface with 1mm depth intervals at 10×10cm2 field, SSD 100cm. Measurements were taken for situations where the beams intersect the couch and couch rails. Dose absorption ratio of carbon fiber couch obtained at gantry angle of 180° was 1.52%, 0.69%, 0.33% and 0.25% at different field sizes for 6MV. For 15MV, this ratio was 0.95%, 0.27%, 0.20% and 0.05%. The absorption ratio is between 3.4% and 1.22% when the beams intersect with couch rails. The couch effect increased surface dose from 14% to 70% for 6MV and from 11.34% to 53.03% for 15MV. The results showed that the carbon fiber couch increased surface dose during posterior irradiation. Therefore, the skin-sparing effect of the high energy beams was decreased. If the effect of couch is not considered, it may cause significant differences at dose which reaches the patient and may cause tissue problems such as erythema.

An Update of Couch Effect on the Attenuation of Megavoltage Radiotherapy Beam and the Variation of Absorbed Dose in the Build-up Region.

Fiber carbon is the most common material used in treating couch as it causes less beam attenuation than other materials. Beam attenuation replaces build-up region, reduces skin-sparing effect and causes target volume under dosage. In this study, we aimed to evaluate beam attenuation and variation of build-up region in 550 TxT radiotherapy couch. In this study, we utilized cylindrical PMMA Farmer chamber, DOSE-1 electrometer and set PMMA phantom in isocenter of gantry and the Farmer chamber on the phantom. Afterwards, the gantry rotated 10°, and attenuation was assessed. To measure build-up region, we used Markus chamber, Solid water phantom and DOSE-1 electrometer. Doing so, we set Solid water phantom on isocenter of gantry and placed Markus chamber in it, then we quantified the build-up region at 0° and 180° gantry angels and compared the obtained values. Notable attenuation and build-up region variation were observed in 550 TxT treatment table. The maximum rate of attenuation was 5.95% for 6 MV photon beam, at 5×5 cm2 field size and 130° gantry angle, while the maximum variation was 7 mm for 6 MV photon beam at 10×10 cm2 field size. Fiber carbon caused beam attenuation and variation in the build-up region. Therefore, the application of fiber carbon is recommended for planning radiotherapy to prevent skin side effects and to decrease the risk of cancer recurrence.

EP-1567: Investigation of dose buildup region from electron beam by of polymer films and ionization chamber

EP-1567: Investigation of dose buildup region from electron beam by of polymer films and ionization chamber

Comparison of build-up region doses in oblique tangential 6 MV photon beams calculated by AAA and CCC algorithms in breast Rando phantom

The purpose of this study is to compare the build-up region doses on breast Rando phantom surface with the bolus covered, the doses in breast Rando phantom and also the doses in a lung that is the heterogeneous region by two algorithms. The AAA in Eclipse TPS and the collapsed cone convolution algorithm in Pinnacle treatment planning system were used to plan in tangential field technique with 6 MV photon beam at 200 cGy total doses in Breast Rando phantom with bolus covered (5 mm and 10 mm). TLDs were calibrated with Cobalt-60 and used to measure the doses in irradiation process. The results in treatment planning show that the doses in build-up region and the doses in breast phantom were closely matched in both algorithms which are less than 2% differences. However, overestimate of doses in a lung (L2) were found in AAA with 13.78% and 6.06% differences at 5 mm and 10 mm bolus thickness, respectively when compared with CCC algorithm. The TLD measurements show the underestimate in buildup region and in breast phantom but the doses in a lung (L2) were overestimated when compared with the doses in the two plannings at both thicknesses of the bolus.

Surface and Buildup Region Dose Measurements with Markus Parallel-Plate Ionization Chamber, GafChromic EBT3 Film, and MOSFET Detector for High-Energy Photon Beams

The aim of the study was to investigate surface and buildup region doses for 6 MV and 15 MV photon beams using a Markus parallel-plate ionization chamber, GafChromic EBT3 film, and MOSFET detector for different field sizes and beam angles. The measurements were made in a water equivalent solid phantom at the surface and in the buildup region of the 6 MV and 15 MV photon beams at 100 cm source-detector distance for 5 × 5, 10 × 10, and 20 × 20 cm2field sizes and 0°, 30°, 60°, and 80° beam angles. The surface doses using 6 MV photon beams for 10 × 10 cm2field size were found to be 20.3%, 18.8%, and 25.5% for Markus chamber, EBT3 film, and MOSFET detector, respectively. The surface doses using 15 MV photon beams for 10 × 10 cm2field size were found to be 14.9%, 13.4%, and 16.4% for Markus chamber, EBT3 film, and MOSFET detector, respectively. The surface dose increased with field size for all dosimeters. As the angle of the incident radiation beam became more oblique, the surface dose increased. The effective measurement depths of dosimeters vary; thus, the results of the measurements could be different. This issue can lead to mistakes at surface and buildup dosimetry and must be taken into account.

Surface dose measurements and comparison of unflattened and flattened photon beams.

The purpose of this study was to evaluate the central axis dose in the build-up region and the surface dose of a 6 MV and 10 MV flattened photon beam (FB) and flattening filter free (FFF) therapeutic photon beam for different square field sizes (FSs) for a Varian Truebeam linear accelerator using parallel-plate ionization chamber and Gafchromic film. Knowledge of dosimetric characteristics in the build-up region and surface dose of the FFF is essential for clinical care. The dose measurements were also obtained empirically using two different commonly used dosimeters: a p-type photon semiconductor dosimeter and a cylindrical ionization chamber. Surface dose increased linearly with FS for both FB and FFF photon beams. The surface dose values of FFF were higher than the FB FSs. The measured surface dose clearly increases with increasing FS. The FFF beams have a modestly higher surface dose in the build-up region than the FB. The dependence of source to skin distance (SSD) is less significant in FFF beams when compared to the flattened beams at extended SSDs.

TH‐A‐19A‐07: The Effect of Particle Tracking Step Size Limit On Monte Carlo‐ Computed LET Spectrum of Therapeutic Proton Beams

Purpose:To investigate the effect of charged particle tracking step size limit in the determination of the LET spectrum of therapeutic proton beams using Monte Carlo simulations.Methods:The LET spectra at different depths in a water phantom from a 79.7 MeV spot‐scanning proton beam were calculated using Geant4. Five different tracking step limits 0.5 mm, 0.1 mm, 0.05 mm, 0.01 mm and 1 μm were adopted. The field size was set to 10×10 cm2 on the isocenter plane. A 40×40×6 cm3 water phantom was modelled as the irradiation target. The voxel size was set to 1×1×0.5 mm3 to obtain high resolution results. The LET spectra were scored ranging from 0.01 keV/μm to 104keV/μm in the logarithm scale. In addition, the proton energy spectra at different depths were also scored.Results:The LET spectra calculated using different step size limits were compared at four depths along the Bragg curve. At any depths, the spread of the LET spectra increases with the decrease of step size limit. In the dose buildup region (z = 1.9 cm) and in the region proximal to the Bragg peak (z = 3.95 cm), the frequency mean LET does not vary with decreasing step size limit. At Bragg peak (z = 4.75 cm) and in the distal edge (z = 4.85 cm), frequency mean LET decreases with decreasing step size limit. The energy spectrum at any specified depths does not vary with the step size limit.Conclusion:The calculated LET has a spectral distribution rather than a single value at any depths along the Bragg curve and the spread of the computed spectrum depends on the tracking step limit. Incorporating the LET spectrum distribution into the robust IMPT optimization plan may provide more accurate biological dose distribution than using the dose‐ or fluence‐averaged LET.NIH Program Project Grant P01CA021239.

SU-E-T-554: Comparison of Electron Disequilibrium Factor in External Photon Beams for Different Models of Linear Accelerators

Purpose: The dose in the buildup region of a photon beam is usually determined by the transport of the primary secondary electrons and the contaminating electrons from accelerator head. This can be quantified by the electron disequilibrium factor, E, defined as the ratio between total dose and equilibrium dose (proportional to total kerma), E = 1 in regions beyond buildup region. Ecan be different among accelerators of different models and/or manufactures of the same machine. This study compares E in photon beams from different machine models/ Methods: Photon beam data such as fractional depth dose curve (FDD) and phantom scatter factors as a function of field size and phantom depth were measured for different Linac machines. E was extrapolated from these fractional depth dose data while taking into account inverse-square law. The ranges of secondary electron were chosen as 3 and 6 cm for 6 and 15 MV photon beams, respectively. The field sizes range from 2x2 to 40x40 cm2. Results: The comparison indicates the standard deviations of electron contamination among different machines are about 2.4 - 3.3% at 5 mm depth for 6 MV and 1.2 - 3.9% at 1 cm depth for 15 MV for the same field size. The corresponding maximum deviations are 3.0 - 4.6% and 2 - 4% for 6 and 15 MV, respectively. Both standard and maximum deviations are independent of field sizes in the buildup region for 6 MV photons, and slightly decreasing with increasing field size at depths up to 1 cm for 15 MV photons. Conclusion: The deviations of electron disequilibrium factor for all studied Linacs are less than 3% beyond the depth of 0.5 cm for the photon beams for the full range of field sizes (2-40 cm) so long as they are from the same manufacturer.