Monte Carlo dosimetry for 125I and 60Co in eye plaque therapy

Monte Carlo calculations of radiation dosimetry using MORSE code are performed for 125I and 60Co point sources in a cylindrical head phantom that simulates the geometry of eye plaque therapy for choroidal melanoma. We obtain the dose variation in the eye at submillimeter intervals over distances as close as 1 mm and up to 2.5 cm from the source. The calculations for 125I are performed for the phantom media of water, protein, and a homogenized protein-water mixture simulating the composition of the eye. Relative dose functions for 125I for these phantom media are fitted to second-degree polynomials. Agreement is found with published results. The relative dose function for 60Co at eye position in the water head phantom is fitted to a third-degree polynomial and compared with that for 60Co at the center of a large water sphere. A boundary effect due to the head phantom-air interface on the dose distribution for 60Co is demonstrated. The dose falloff with distance is faster for the eye geometry compared with the bulk geometry. We also show that the relative dose distributions within the tumor are comparable for 125I and 60Co by comparing their relative dose functions. This result is consistent with the success of clinical trials of large melanoma treatments with 125I plaques.     

A toolbox for intensity modulated radiation therapy optimization

We have designed a toolbox that provides an environment for testing radiotherapy optimization techniques, objective functions, and constraints. A set of three-dimensional (3D) pencil beam dose distributions have been computed for a cylindrical phantom. The 6 MV pencil beams were computed using a superposition-based dose engine commissioned for an Elekta SL20 linear accelerator. Due to the cylindrical symmetry of the phantom, the pencil beam dose distributions for any arbitrary beam angle can be determined by simply rotating the pencil beam data sets. Thus, the full accuracy is maintained without the need for additional dose calculations or large data storage requirements. In addition to the pencil beam data sets, tools are included for (1) rotating the pencil beams, (2) calculating the beam's eye view, (3) drawing structures, (4) writing the pencil beam dose data out to the optimizer, and (5) visualizing the optimized results. The pencil beam data sets and the corresponding tools are available for download at http://medschool.umaryland.edu/departments/radiationoncology/pencilbeam/. With this toolbox, researchers will have the ability to rapidly test new optimization techniques and formulations for intensity modulated radiation therapy and 3D conformal radiotherapy.     

Reduction of eye lens radiation dose by orbital bismuth shielding in pediatric patients undergoing CT of the head: a Monte Carlo study

Our aim in the study was to assess the eye lens dose reduction resulting from the use of radioprotective bismuth garments to shield the eyes of pediatric patients undergoing head CT. The Monte Carlo N-particle transport code and mathematical humanoid phantoms representing the average individual at different ages were used to determine eye lens dose reduction accomplished with bismuth shielding of the eye in the following simulated CT scans: (a) scanning of the orbits, (b) scanning of the whole head, and (c) 20 degrees angled scanning of the brain excluding the orbits. The effect of bismuth shielding on the eye lens dose was also investigated using an anthropomorphic phantom and thermoluminescence dosimetry (TLD). Eye lens dose reduction achieved by bismuth shielding was measured in 16 patients undergoing multiphase CT scanning of the head. The patient's scans were divided in the following: CT examinations where the eye globes were entirely included (n=5), partly included (n=6) and excluded (n=5) from the scanned region. The eye lens dose reduction depended mainly on the scan boundaries set by an operator. The average eye lens dose reduction determined by Monte Carlo simulation was 38.2%, 33.0% and <1% for CT scans of the orbits, whole head, and brain with an angled gantry, respectively. The difference between the Monte Carlo derived eye lens dose reduction factor values and corresponding values determined directly by using the anthropomorphic phantom head was found less than 5%. The mean eye lens dose reduction achieved by bismuth shielding in pediatric patients were 34%, 20% and <2% when eye globes were entirely included, partly included and excluded from the scanned region, respectively. A significant reduction in eye lens dose may be achieved by using superficial orbital bismuth shielding during pediatric head CT scans. However, bismuth garments should not be used in children when the eyes are excluded from the primarily exposed region.     

体元大小对基于MonteCarlo的剂量计算的影响

目的：在放射治疗计划系统中,剂量计算之前需要对人体密度数据体元化。对于蒙特卡罗方法的模拟过程,当一个自由程跨过体元界面时,会应用自由程近似。选取的体元越小,将导致越多的自由程近似。本文采用蒙特卡罗方法模拟一个虚拟射线源入射到水箱中的反应,计算水箱中的剂量分布,通过比较水箱分层和不分层两种情况下中心轴百分深度剂量分布和离轴比分布,来探讨选用不同大小的体元对剂量分布的影响。方法：本文以6MeV的方形电子射线源为外照射源、以三维水箱为介质模型。使用PENELOPE程序包模拟电子束垂直入射到水箱中引起的电子与物质的相互作用。比较水箱在分层和不分层情况下中心轴百分深度剂量和离轴比分布。结果：通过比较水箱在分层和不分层情况下中心轴百分深度剂量和离轴比分布,发现差异很小。结论：选用不同大小的体元,蒙特卡罗近似处理自由程对剂量计算精度的影响很小。研究结果对蒙特卡罗方法在放射治疗中的临床应用具有指导意义。     

Leakage and scatter radiation from a double scattering based proton beamline

Proton beams offer several advantages over conventional radiation techniques for treating cancer and other diseases. These advantages might be negated if the leakage and scatter radiation from the beamline and patient are too large. Although the leakage and scatter radiation for the double scattering proton beamlines at the Loma Linda University Proton Treatment Facility were measured during the acceptance testing that occurred in the early 1990s, recent discussions in the radiotherapy community have prompted a reinvestigation of this contribution to the dose equivalent a patient receives. The dose and dose equivalent delivered to a large phantom patient outside a primary proton field were determined using five methods: simulations using Monte Carlo calculations, measurements with silver halide film, measurements with ionization chambers, measurements with rem meters, and measurements with CR-39 plastic nuclear track detectors. The Monte Carlo dose distribution was calculated in a coronal plane through the simulated patient that coincided with the central axis of the beam. Measurements with the ionization chambers, rem meters, and plastic nuclear track detectors were made at multiple locations within the same coronal plane. Measurements with the film were done in a plane perpendicular to the central axis of the beam and coincident with the surface of the phantom patient. In general, agreement between the five methods was good, but there were some differences. Measurements and simulations also tended to be in agreement with the original acceptance testing measurements and results from similar facilities published in the literature. Simulations illustrated that most of the neutrons entering the patient are produced in the final patient-specific aperture and precollimator just upstream of the aperture, not in the scattering system. These new results confirm that the dose equivalents received by patients outside the primary proton field from primary particles that leak through the nozzle are below the accepted standards for x-ray and electron beams. The total dose equivalent outside of the field is similar to that received by patients undergoing treatments with intensity modulated x-ray therapy. At the center of a patient for a whole course of treatment, the dose equivalent is comparable to that delivered by a single whole-body XCT scan.     

Dosimetric evaluation in heterogeneous tissue of anterior electron beam irradiation for treatment of retinoblastoma

A dosimetric study of anterior electron beam irradiation for treatment of retinoblastoma was performed to evaluate the influence of tissue heterogeneities on the dose distribution within the eye and the accuracy of the dose calculated by a pencil beam algorithm. Film measurements were made in a variety of polystyrene phantoms and in a removable polystyrene eye incorporated into a tissue substitute phantom constructed from a human skull. Measurements in polystyrene phantoms were used to demonstrate the algorithm's ability to predict the effect of a lens block placed in the beam, as well as the eye's irregular surface shape. The eye phantom was used to measure dose distributions within the eye in both the sagittal and transverse planes in order to test the algorithm's ability to predict the dose distribution when bony heterogeneities are present. Results show (1) that previous treatment planning conclusions based on flat, uniform phantoms for central-axis depth dose are adequate; (2) that a three-dimensional heterogeneity correction is required for accurate dose calculations; and (3) that if only a two-dimensional heterogeneity correction is used in calculating the dose, it is more accurate for the sagittal than the transverse plane.     

Integrating a MRI scanner with a 6 MV radiotherapy accelerator: dose increase at tissue-air interfaces in a lateral magnetic field due to returning electrons

In the framework of the development of the integration of a MRI-scanner with a linear accelerator, the influence of a lateral, magnetic field on the dose distribution has to be determined. Dose increase is expected at tissue-air boundaries, due to the electron return effect (ERE): electrons entering air will describe a circular path and return into the phantom causing extra dose deposition. Using IMRT with many beam directions, this exit dose will not constitute a problem. Dose levels behind air cavities will decrease because of the absence of electrons crossing the cavity. The ERE has been demonstrated both by simulation and experiment. Monte Carlo simulations are performed with GEANT4, irradiating a water-air-water phantom in a lateral magnetic field. Also an air tube in water has been simulated, resulting in slightly twisted regions of dose increase and decrease. Experimental demonstration is achieved by film measurement in a perspex-air-perspex phantom in an electromagnet. Although the ERE causes dose increase before air cavities, relatively flat dose profiles can be obtained for the investigated cases using opposite beam configurations. More research will be necessary whether this holds for more realistic geometries with the use of IMRT and whether the ERE can be turned to our advantage when treating small tumour sites at air cavities.     

Mathematical solutions of the TG-43 geometry function for curved line, ring, disk, sphere, dome and annulus sources, and applications for quality assurance

Analytic solutions for the TG-43 geometry function for curved line, ring, disk, sphere, dome and annulus shapes containing uniform distributions of air-kerma are derived. These geometry functions describe how dose distributions vary strictly due to source geometry and not including attenuation or scatter effects. This work extends the use of geometry functions for individual sources to applicators containing multiple sources. Such geometry functions may be used to verify dose distributions computed using advanced techniques, including QA of model-based dose calculation algorithms. The impact of source curvature on linear and planar implants is considered along with the specific clinical case of brachytherapy eye plaques. For eye plaques, the geometry function for a domed distribution is used with published Monte Carlo dose distributions to determine a radial dose function and anisotropy function which includes all the scatter and attenuation effects due to the phantom, eye plaque and sources. This TG-43 model of brachytherapy eye plaques exactly reproduces azimuthally averaged Monte Carlo calculations, both inside and outside the eye.     

Tissue inhomogeneity correction for brachytherapy sources in a heterogeneous phantom with cylindrical symmetry.

In brachytherapy it is customary to perform dose calculations for an implant assuming that the tumor and surrounding tissues constitute a uniform, homogeneous medium equivalent to water. In this work, the validity of the above assumption is studied quantitatively for points along the transverse axis of 103Pd, 125I, and 241Am brachytherapy sources, using measured and Monte Carlo calculated dose rates in homogeneous and heterogeneous media with cylindrical symmetry. The irradiation geometry chosen was a single source implanted in a Solid Water phantom which had a 1- or 2-cm-thick cylindrical Solid Water shell replaced by a polystyrene shell. The Monte Carlo simulations were performed using the integrated tiger series CYLTRAN Code. Experimental data were obtained for the same geometry to test the validity of the Monte Carlo calculations for a heterogeneous phantom. Measured dose rates just beyond a 2-cm-thick polystyrene heterogeneity were observed to be greater than those in a homogeneous Solid Water phantom by about 130%, 55%, and 10% for 103Pd, 125I, and 241Am, respectively. Thus the effect of a relatively small polystyrene heterogeneity in Solid Water can be substantial for lower energy photons. This perturbation of dose was found to increase steeply with decreasing energy and increasing size (thickness) of inhomogeneity. A simple dose calculation formalism has been developed to predict dose rate in a heterogeneous phantom with cylindrical symmetry, which uses as input the radial dose functions of the uniform media comprising the heterogeneous phantom. Dose rate predictions using this formalism are in reasonable agreement with the experimental data and the Monte Carlo calculated values.(ABSTRACT TRUNCATED AT 250 WORDS)     

A new look at CT dose measurement: beyond CTDI

Equations are derived for generating accumulated dose distributions and the dose line integral in a cylindrical dosimetry phantom for a helical CT scan series from the single slice dose profiles using convolution methods. This exposition will better clarify the nature of the dose distribution in helical CT, as well as providing the medical physicist with a better understanding of the physics involved in dose delivery and the measurement process. Also addressed is the concern that as radiation beam widths for multi-slice scanners get wider, the current methodology based on the measurement of the integral of the single slice profile using a 10 cm long ion chamber (CTDI100) may no longer be adequate. It is shown that this measurement would underestimate the equilibrium dose and dose line integral by about 20% in the center of the body phantom, and by about 10% in the center of the head phantom for a 20 mm nominal beam width in a multi-slice scanner. Rather than making the ion chamber even longer to collect the broad scatter tails of the single slice profile, an alternative to the CTDI method is suggested which involves using a small volume ion chamber, and scanning a length of phantom long enough to establish dose equilibrium at the location of the chamber. With a modern CT scanner, such a scan length can be covered in 15 s or less with a helical or axial series, so this method is not significantly more time-consuming than the long chamber method. The method is demonstrated experimentally herein.     

A polygon-surface reference Korean male phantom (PSRK-Man) and its direct implementation in Geant4 Monte Carlo simulation

Even though the hybrid phantom embodies both the anatomic reality of voxel phantoms and the deformability of stylized phantoms, it must be voxelized to be used in a Monte Carlo code for dose calculation or some imaging simulation, which incurs the inherent limitations of voxel phantoms. In the present study, a voxel phantom named VKH-Man (Visible Korean Human-Man), was converted to a polygon-surface phantom (PSRK-Man, Polygon-Surface Reference Korean-Man), which was then adjusted to the Reference Korean data. Subsequently, the PSRK-Man polygon phantom was directly, without any voxelization process, implemented in the Geant4 Monte Carlo code for dose calculations. The calculated dose values and computation time were then compared with those of HDRK-Man (High Definition Reference Korean-Man), a corresponding voxel phantom adjusted to the same Reference Korean data from the same VKH-Man voxel phantom. Our results showed that the calculated dose values of the PSRK-Man surface phantom agreed well with those of the HDRK-Man voxel phantom. The calculation speed for the PSRK-Man polygon phantom though was 70-150 times slower than that of the HDRK-Man voxel phantom; that speed, however, could be acceptable in some applications, in that direct use of the surface phantom PSRK-Man in Geant4 does not require a separate voxelization process. Computing speed can be enhanced, in future, either by optimizing the Monte Carlo transport kernel for the polygon surfaces or by using modern computing technologies such as grid computing and general-purpose computing on graphics processing units programming.     

A Monte Carlo-based method to estimate radiation dose from spiral CT: from phantom testing to patient-specific models

The purpose of this work is to develop and test a method to estimate the relative and absolute absorbed radiation dose from axial and spiral CT scans using a Monte Carlo approach. Initial testing was done in phantoms and preliminary results were obtained from a standard mathematical anthropomorphic model (MIRD V) and voxelized patient data. To accomplish this we have modified a general purpose Monte Carlo transport code (MCNP4B) to simulate the CT x-ray source and movement, and then to calculate absorbed radiation dose in desired objects. The movement of the source in either axial or spiral modes was modelled explicitly while the CT system components were modelled using published information about x-ray spectra as well as information provided by the manufacturer. Simulations were performed for single axial scans using the head and body computed tomography dose index (CTDI) polymethylmethacrylate phantoms at both central and peripheral positions for all available beam energies and slice thicknesses. For comparison, corresponding physical measurements of CTDI in phantom were made with an ion chamber. To obtain absolute dose values, simulations and measurements were performed in air at the scanner isocentre for each beam energy. To extend the verification, the CT scanner model was applied to the MIRD V model and compared with published results using similar technical factors. After verification of the model, the generalized source was simulated and applied to voxelized models of patient anatomy. The simulated and measured absolute dose data in phantom agreed to within 2% for the head phantom and within 4% for the body phantom at 120 and 140 kVp; this extends to 8% for the head and 9% for the body phantom across all available beam energies and positions. For the head phantom, the simulated and measured absolute dose data agree to within 2% across all slice thicknesses at 120 kVp. Our results in the MIRD phantom agree within 11% of all the different organ dose values published by the UK's ImPACT group for a scan using an equivalent scanner, kVp, collimation, pitch and mAs. The CT source model was shown to calculate both a relative and absolute radiation dose distribution throughout the entire volume in a patient-specific matrix geometry. Results of initial testing are promising and application to patient models was shown to be feasible.     

不同算法所得IMRT计划的剂量学验证评估

目的:评估分别采用AAA(Analytic Anisotropic Algorithm)算法和PBC(Pencil Beam Convolution)算法所制订的IMRT计划在质量学验证方面的差异。方法:选取20例肺部肿瘤患者,对每个病例分别用AAA和PBC两种算法进行剂量计算得到2个IMRT计划,将病人的IMRT计划移植至模体生成QA(quality assurance)计划,使用Mapcheck工具对QA计划分别进行剂量学验证,并对结果进行比较和分析。结果:采用AAA算法进行肺部肿痛的剂量运算时,其所得到的治疗计划在剂量验证的结果方面要明显优于PBC算法所得到的治疗计划,值得注意的是,在其中进一步选取7例靶区位置和形态较为复杂的病例,出现以上结果的现象会更加明显。结论:从剂量学验证的角度来看,对于类似于肺部肿瘤这种靶区周围存在明显密度差异的肿瘤,在选取IMRT计划中剂量运算法则时,AAA算法剂量学验证的γ通过率更高,运算更加准确,尤其是如果靶区较为复杂时,这种表现更加显著。     

Proton spectroscopic imaging of polyacrylamide gel dosimeters for absolute radiation dosimetry

Proton spectroscopy has been evaluated as a method for quantifying radiation induced changes in polyacrylamide gel dosimeters. A calibration was first performed using BANG-type gel samples receiving uniform doses of 6 MV photons from 0 to 9 Gy in 1 Gy intervals. The peak integral of the acrylic protons belonging to acrylamide and methylenebisacrylamide normalized to the water signal was plotted against absorbed dose. Response was approximately linear within the range 0-7 Gy. A large gel phantom irradiated with three, coplanar 3 x 3 cm square fields to 5.74 Gy at isocentre was then imaged with an echo filter technique to map the distribution of monomers directly. The image, normalized to the water signal, was converted into an absolute dose map. At the isocentre the measured dose was 5.69 Gy (SD = 0.09) which was in good agreement with the planned dose. The measured dose distribution elsewhere in the sample shows greater errors. A T2 derived dose map demonstrated a better relative distribution but gave an overestimate of the dose at isocentre of 18%. The data indicate that MR measurements of monomer concentration can complement T2-based measurements and can be used to verify absolute dose. Compared with the more usual T2 measurements for assessing gel polymerization, monomer concentration analysis is less sensitive to parameters such as gel pH and temperature, which can cause ambiguous relaxation time measurements and erroneous absolute dose calculations.     

Microdosimetric study for secondary neutrons in phantom produced by a 290 MeV/nucleon carbon beam

Absorbed doses from main charged-particle beams and charged-particle fragments have been measured with high accuracy for particle therapy, but there are few reports for doses from neutron components produced as fragments. This study describes the measurements on neutron doses produced by carbon beams; microdosimetric distributions of secondary neutrons produced by 290 MeV/nucleon carbon beams have been measured by using a tissue equivalent proportional counter at the Heavy Ion Medical Accelerator in Chiba, Japan at the National Institute of Radiological Sciences. The microdosimetric distributions of the secondary neutron were measured on the distal and lateral faces of a body-simulated acrylic phantom (300 mm height x 300 mm width x 253 mm thickness). To confirm the dose measurements, the neutron energy spectra produced by incident carbon beams in the acrylic phantom were simulated by the particle and heavy ion transport code system. The absorbed doses obtained by multiplying the simulated neutron energy spectra with the kerma factor calculated by MCNPX agree with the corresponding experimental data fairly well. Downstream of the Bragg peak, the ratio of the neutron dose to the carbon dose at the Bragg peak was found to be a maximum of 1.4 x 10(-4) and the ratio of neutron dose was a maximum of 3.0 x 10(-7) at a lateral face of the acrylic phantom. The ratios of neutrons to charged particle fragments were 11% to 89% in the absorbed doses at the lateral and the distal faces of the acrylic phantom. We can conclude that the treatment dose will not induce serious secondary neutron effects at distances greater than 90 mm from the Bragg peak in carbon particle therapy.     

Output factors and dose calculations for blocked x-ray fields

Output factors for blocked fields have been measured in a polystyrene phantom for four collimator field sizes and two blocking schemes using 6-MV x rays. For all measurements the phantom surface was at the calibration source-surface distance (SSD) because, as is shown, the calculation of dose to any point in a phantom at an arbitrary SSD can be expressed in terms of the output factor for the field size at the calibration distance. It is found that output factors are a function of both the surface field size of the blocked field and the collimator field size. Specifically, the output factor for a blocked field is less than that for the collimator field size used but greater than that for an unblocked field of the same surface field size formed by collimator settings only. A method is proposed for utilizing these data to calculate the output factor for any collimator and blocked field size. The validity of the method is checked by using it to calculate dose to a point in a phantom and comparing this to the measured dose.     

The dosimetry of 125I seed eye plaques

The ability of a brachytherapy treatment-planning computer program to calculate accurately the dose from 125I seeds at distances relevant to eye plaque therapy was investigated. Thermoluminescent dosimetry measurements were made in a plastic phantom at depths of 0.5, 0.97, and 1.5 cm, and results were corrected for finite dosimeter size and phantom effects. Doses were calculated at the same depths with an 125I seed linear source model that accounted for dose anisotropy. Measurements and calculations were found to agree within their mutual uncertainties. The presence of a gold plaque was found to reduce the dose at all measured depths by 8%.     

A Monte Carlo based method to estimate radiation dose from multidetector CT (MDCT): cylindrical and anthropomorphic phantoms

The purpose of this work was to extend the verification of Monte Carlo based methods for estimating radiation dose in computed tomography (CT) exams beyond a single CT scanner to a multidetector CT (MDCT) scanner, and from cylindrical CTDI phantom measurements to both cylindrical and physical anthropomorphic phantoms. Both cylindrical and physical anthropomorphic phantoms were scanned on an MDCT under the specified conditions. A pencil ionization chamber was used to record exposure for the cylindrical phantom, while MOSFET (metal oxide semiconductor field effect transistor) detectors were used to record exposure at the surface of the anthropomorphic phantom. Reference measurements were made in air at isocentre using the pencil ionization chamber under the specified conditions. Detailed Monte Carlo models were developed for the MDCT scanner to describe the x-ray source (spectra, bowtie filter, etc) and geometry factors (distance from focal spot to isocentre, source movement due to axial or helical scanning, etc). Models for the cylindrical (CTDI) phantoms were available from the previous work. For the anthropomorphic phantom, CT image data were used to create a detailed voxelized model of the phantom's geometry. Anthropomorphic phantom material compositions were provided by the manufacturer. A simulation of the physical scan was performed using the mathematical models of the scanner, phantom and specified scan parameters. Tallies were recorded at specific voxel locations corresponding to the MOSFET physical measurements. Simulations of air scans were performed to obtain normalization factors to convert results to absolute dose values. For the CTDI body (32 cm) phantom, measurements and simulation results agreed to within 3.5% across all conditions. For the anthropomorphic phantom, measured surface dose values from a contiguous axial scan showed significant variation and ranged from 8 mGy/100 mAs to 16 mGy/100 mAs. Results from helical scans of overlapping pitch (0.9375) and extended pitch (1.375) were also obtained. Comparisons between the MOSFET measurements and the absolute dose value derived from the Monte Carlo simulations demonstrate agreement in terms of absolute dose values as well as the spatially varying characteristics. This work demonstrates the ability to extend models from a single detector scanner using cylindrical phantoms to an MDCT scanner using both cylindrical and anthropomorphic phantoms. Future work will be extended to voxelized patient models of different sizes and to other MDCT scanners.     

Proton dose calculation based on in-air fluence measurements

Proton dose calculation algorithms--as well as photon and electron algorithms--are usually based on configuration measurements taken in a water phantom. The exceptions to this are proton dose calculation algorithms for modulated scanning beams. There, it is usual to measure the spot profiles in air. We use the concept of in-air configuration measurements also for scattering and uniform scanning (wobbling) proton delivery techniques. The dose calculation includes a separate step for the calculation of the in-air fluence distribution per energy layer. The in-air fluence calculation is specific to the technique and-to a lesser extent-design of the treatment machine. The actual dose calculation uses the in-air fluence as input and is generic for all proton machine designs and techniques.     

Updated Solid Water to water conversion factors for 125I and l03Pd brachytherapy sources

Dosimetric characteristics of brachytherapy sources are normally determined in water using a Monte Carlo simulation technique and in water equivalent phantom material using both experimental and Monte Carlo simulation techniques. The consensuses of these results are then calculated for clinical applications by converting experimental data obtained in water equivalent material to water using a conversion factor. These conversion factors are normally determined as a ratio of the Monte Carlo-simulated dose rate constant in liquid water to the dose rate constant in a water-equivalent phantom material. However, it has been noted that conversion factors utilized by some investigators have been derived using incorrect phantom material composition and incorrect cross-sectional data information. The impact of errors associated with the cross-sectional data and chemical composition of the phantom material used in dosimetric evaluation of brachytherapy sources has been investigated in this project. Results of these investigations have shown that the use of Solid Water with 1.7% calcium content, as compared to the 2.3% value stated by the manufacturer, may lead to 5% and 9% differences in conversion factors for 125I and 103Pd, respectively.