103Pd Sources Research Articles

Quantitative comparison between initial DNA-damage RBE of I-125 and Pd-103 brachytherapy sources through a hybrid simulation approach

Iodine-125 and palladium-103 are radioactive sources that are frequently used for interstitial brachytherapy purposes. The current study aims to quantitatively compare the initial DNA-damage relative biologic effectiveness (RBE) of these two brachytherapy sources through a hybrid Monte Carlo (MC) simulation approach. Two commercially available brachytherapy sources of SL-125/SH-125 and MED3633 were considered as 125I and 103Pd sources, respectively. At first, geometry of two selected brachytherapy sources was accurately simulated by MCNPX MC code. Then, energy spectrum of released secondary electrons at different distances of 0.5, 1, 3, and 5 cm was scored, and obtained spectra were imported to MCDS MC code. Finally, the RBE values, with respect to Co-60 reference source, were estimated through calculation of induced DNA damages, including single- and double-strand breaks (SSBs and DSBs), by MCDS code for both studied brachytherapy sources at different distances. No significant differences were observed between the calculated RBE values of both considered brachytherapy sources. Based on our findings, the RBE (DSB) for iodine-125 and palladium-103 is, respectively, equal to 1.14 and 1.15, while the RBE (SSB) is the same for both brachytherapy seeds and equal to 0.97.

Super-selective intra-arterial platinum-based chemotherapy concurrent with low-dose-rate plaque brachytherapy in the treatment of retinoblastoma: A simulation study.

Retinoblastoma is the most common cancer among children under 5 years of age. The common conventional methods for the treatment of retinoblastoma include chemotherapy and brachytherapy (BT). This study investigated the concurrent use of BT with 125I and 103Pd sources and chemotherapy with platinum-based chemotherapy drugs for retinoblastoma. The absorbed doses in different parts of the eye were measured with and without platinum. Platinum concentrations of 5, 7.5, 10, and 15 mg/g were evaluated, and the dose enhancement factors (DEFs) were calculated for different cases. For the 125I source, the DEFs at the tumor apex were 1.49, 1.67, 1.81, and 1.97 at concentrations of 5, 7.5, 10, and 15 mg/g, respectively. The DEF decreased dramatically beyond the apex at 0.85 cm from tumor base and was 0.87, 0.82, 0.76, and 0.63 for the abovementioned concentrations, respectively. For the 103Pd source, the DEFs were 1.15, 1.24, 1.21, and 1.07, respectively, at the apex and 0.76, 0.65, 0.56, and 0.39, respectively, beyond the apex. Our results showed that the concurrent use of low-dose-rate plaque BT and platinum-based chemotherapy significantly increased the tumor-absorbed dose and decreased the absorbed dose in areas outside the tumor and the treatment time.

APPLICABILITY OF PURE PROPANE GAS FOR MICRODOSIMETRY AT BRACHYTHERAPY ENERGIES: A FLUKA STUDY.

Applicability of pure propane gas for microdosimetric measurements at photon energies relevant in brachytherapy is studied using the Monte Carlo-based FLUKA code. Monoenergetic photons in the energy range of 20-1250 keV and brachytherapy sources such as 103Pd, 125I, 169Yb, 192Ir, 137Cs and 60Co are considered in the study. Using the calculated values of energy deposited in the sensitive region of LET-1/2 tissue-equivalent proportional counter filled with pure propane gas and tissue-equivalent propane gas, values of density scaling factor for the site sizes of 1 and 8 μm are obtained. The study shows that density of propane gas should be lowered by a factor of about 0.93 for 169Yb, 192Ir, 137Cs and 60Co sources for the site sizes of 1-8 μm. For 125I source, the density of propane gas requires a scaling of 0.93 for 1 μm site size, whereas for site sizes 2-8 μm, density need not be altered. 103Pd source does not require density scaling for site sizes 1-8 μm.

A Monte Carlo study on the radio-sensitization effect of gold nanoparticles in brachytherapy of prostate by 103Pd seeds

Abstract 103Pd seed is being used for prostate brachytherapy. Additionally, the dose enhancement effect of gold nanoparticles (GNP) has been reported in previous studies. The aim of this study was to characterize the dosimetric effect of gold nanoparticles in brachytherapy with a 103Pd source. Two brachytherapy seeds including 103 Pd source was simulated using MCNPX Monte Carlo code. The seeds’ models were validated by comparing the MC with reported results. Then, GNPs (10 nm in diameter) with a concentration of 7mg Au/g were simulated uniformly inside the prostate of a humanoid computational phantom. Additionally, the dose enhancement factor (DEF) of nanoparticles was calculated for both modeled brachytherapy seeds. A good agreement was found between the MC calculated and the reported dosimetric parameters. For both seeds, an average DEF of 23% was obtained in tumor volume for prostate brachytherapy. The application of GNPs in conjunction with 103Pd seed in brachytherapy can enhance the delivered dose to the tumor and consequently leads to better treatment outcome.

Presentation of a matrix-based method to calculate dose distribution in brachytherapy with photon-emitting sources.

The report of the task group number 43(TG-43) of AAPM has been known as the most common method to obtain the dose distribution around brachytherapy sources. The error caused by independent obtaining and rounding and combinational error caused by algebraic operations of each TG-43 dosimetry parameters increase the total error in the calculation of the dose distribution around the brachytherapy sources. The aim of the present study is to present and evaluate a matrix-based approach for simplifying and reducing calculation errors. In this study, the simulation method with MCNPX code was used to obtain the dose distribution. Four sources were simulated, and the dose matrix around these sources was obtained. Finally, the dose distribution obtained from the matrix-based method was compared with the dose distribution obtained from the method of TG-43 report. There is little difference between the values obtained from the two methods in some points. Absolute mean differences between the values obtained by these two methods were 1.4% for the 60Co, 3.52% for the 137Cs, 2.67% for the 192Ir, and 2.42% for the 103Pd sources. The advantage of the matrix-based method is its simple computing process and less computation time. Considering that the comparison of brachytherapy sources is not raised in calculations of treatment planning systems and also considering the more uncertainty in the calculation of the dose distribution in TG-43 method, it is recommended that dose distribution obtained from matrix-based method be used as input for treatment planning systems.

Influence of Implant Curvature for Improved Post-Surgical Dosimetry of Directional 103Pd Sources

Influence of Implant Curvature for Improved Post-Surgical Dosimetry of Directional 103Pd Sources

Performance of a palladium-103 line source for prostate brachytherapy implants: A Phase I trial

PurposeTo evaluate the use of a polymer-encapsulated palladium-103 (103 Pd) source with a unique linear radioactive distribution in prostate brachytherapy. This feasibility study assessed dosimetry, ease and efficiency of use, and side effects. The number of needles required for adequate coverage was the primary end point. Methods and MaterialsCivaString 103 Pd Model CS10 implants were preplanned for 25 patients. CivaStrings were custom manufactured according to plan. CivaStrings were implanted with 18 gauge needles. Post-implant dosimetry was performed at 3-6 weeks. ResultsMonotherapy (125 Gy) was prescribed for 11 implants. External beam radiation with CivaString boost (100 Gy) was prescribed for 14 implants. The mean time to implant the sources was 23.5 min. The number of planned needles and prostate sizes ranged from 14 to 25 and 21-101 cm3, respectively. 70% of implants in prostates less than 50 cm3 required ≤17 needles. Planned source strength ranged from 2.8 U/cm to 3.9 U/cm. Total source strength averaged 216 U (130-323 U) for monotherapy and 154 U (92.4-245 U) for boost. Nomograms were generated at both prescription dose levels. ConclusionsThe linear 103Pd source provides good dose coverage to the prostate. Prostate volume changes were minimal suggesting minimal swelling using the CivaString device.

Determination of TG-43 Dosimetric Parameters for Photon Emitting Brachytherapy Sources.

Objective:Brachytherapy sources are widely used for the treatment of cancer. The report of Task Group No. 43 (TG-43) of American Association of Physicists in Medicine is known as the most common method for the determination of dosimetric parameters for brachytherapy sources. The aim of this study is to obtain TG-43 dosimetric parameters for 60Co, 137Cs, 192Ir and 103Pd brachytherapy sources by Monte Carlo simulation. Methods:In this study, 60Co (model Co0.A86), 137Cs (model 6520-67), 192Ir (model BEBIG) and 103Pd (model OptiSeed) brachytherapy sources were simulated using MCNPX Monte Carlo code. To simulate the sources, the exact geometric characterization of each source was defined in Monte Carlo input programs. Dosimetric parameters including air kerma strength, dose rate constant, radial dose function and anisotropy function were calculated for each source. Each input program was run with sufficient number of particle histories. The maximum type A statistical uncertainty in the simulation of the 60Co, 137Cs, 192Ir and 103Pd sources, were equal to 4%, 4%, 3.19% and 6.50%, respectively. Results:The results for dosimetry parameters of dose rate constant, radial dose function and anisotropy function for the 60Co, 137Cs, 192Ir and 103Pd sources in this study demonstrated good agreement with other studies. Conclusion:Based on the good agreement between the results of this study and other studies, the TG-43 results for Co0.A86 60Co, 67-65200 137Cs, BEBIG 192Ir and OptiSeed 103Pd sources are validated and can be used as input data in treatment planning systems (TPSs) and to validate the TPS calculations.

Evaluation of dose enhancement in presence of gold nanoparticles in eye brachytherapy by 103Pd source.

In recent years, the use of gold nanoparticles in radiation therapy has been introduced as a new approach in radiotherapy. The aim of this study is to investigate the effect of gold nanoparticles (GNPs) in plaque brachytherapy for choroidal melanoma using Monte Carlo (MC) simulation. MCNPX code was used for simulation of human eye, 103Pd (model 200) brachytherapy source and the 20mm COMS eye plaque that was loaded with 24 103Pd seeds and standardized by Collaborative Ocular Melanoma Study (COMS). The tumour was defined from the inner surface of choroid with 0.55cm height and latticed with gold nanospheres and it was filled with different concentrations of 5, 10 and 15mg/g GNPs, separately. Dose rate and dose enhancement factor in tumour and normal tissues of the eye (without gold) was examined for this case and compared with gold-water mixture of the same concentrations distributed in the tumour. The results show that with increasing the concentration of GNPs, the dose in the tumour increases and the dose to the normal tissues decreases. Furthermore, the time that is required to deliver the prescribed dose to the tumour decreases. In the gold nanosphere case for 5, 10 and 15mg/g concentrations, the DEF in the apex of the tumour are 1.28, 1.46, 1.44 and at the distance of 6.5mm in the normal tissue (outside the tumour) this factor would be 0.82, 0.73 and 0.68. The comparison between two cases of gold nanospheres and gold-water mixture shows that when the gold concentrations are defined as mixed with water, the dose enhancement in the first depths are higher than when the gold-nanoparticles are distributed inside the tumour. Furthermore due to more reduced particle flux for water-mixture case, by an increase in the depth the dose enhancement in gold-nanosphere increases compared with gold-water mixture case.

Ocular brachytherapy dosimetry for 103Pd and 125I in the presence of gold nanoparticles: a Monte Carlo study.

The aim of the present Monte Carlo study is to evaluate the variation of energy deposition in healthy tissues in the human eye which is irradiated by brachytherapy sources in comparison with the resultant dose increase in the gold nanoparticle (GNP)‐loaded choroidal melanoma. The effects of these nanoparticles on normal tissues are compared between 103Pd and 125I as two ophthalmic brachytherapy sources. Dose distribution in the tumor and healthy tissues has been taken into account for both brachytherapy sources. Also, in certain points of the eye, the ratio of the absorbed dose by the normal tissue in the presence of GNPs to the absorbed dose by the same point in the absence of GNPs has been calculated. In addition, differences of the absorbed dose in the tumor observed in the comparison of simple water phantom and actual simulated human eye in presence of GNPs are also a matter of interest that have been considered in the present work. The difference between the eye globe and the water phantom is more obvious for 125I than that of the 103Pd when the ophthalmic dosimetry is done in the presence of GNPs. Whenever these nanoparticles are utilized in enhancing the absorbed dose by the tumor, the use of 125I brachytherapy source will greatly amplify the amount of dose enhancement factor (DEF) in the tumor site without inflicting much damage to healthy organs, when compared to the 103Pd source. For instance, in the concentration of 30 mg GNPs, the difference amongst the calculated DEF for 125I between these phantoms is 5.3%, while it is 2.45% for 103Pd. Furthermore, in Monte Carlo studies of eye brachytherapy, more precise definition of the eye phantom instead of a water phantom will become increasingly important when we use 125I as opposed to 103Pd.PACS number(s): 87.53.Jw, 87.85.Rs, 87.10.Rt

Effect of photon energy spectrum on dosimetric parameters of brachytherapy sources.

AimThe aim of this study is to quantify the influence of the photon energy spectrum of brachytherapy sources on task group No. 43 (TG-43) dosimetric parameters.BackgroundDifferent photon spectra are used for a specific radionuclide in Monte Carlo simulations of brachytherapy sources.Materials and methodsMCNPX code was used to simulate 125I, 103Pd, 169Yb, and 192Ir brachytherapy sources. Air kerma strength per activity, dose rate constant, radial dose function, and two dimensional (2D) anisotropy functions were calculated and isodose curves were plotted for three different photon energy spectra. The references for photon energy spectra were: published papers, Lawrence Berkeley National Laboratory (LBNL), and National Nuclear Data Center (NNDC). The data calculated by these photon energy spectra were compared.ResultsDose rate constant values showed a maximum difference of 24.07% for 103Pd source with different photon energy spectra. Radial dose function values based on different spectra were relatively the same. 2D anisotropy function values showed minor differences in most of distances and angles. There was not any detectable difference between the isodose contours.ConclusionsDosimetric parameters obtained with different photon spectra were relatively the same, however it is suggested that more accurate and updated photon energy spectra be used in Monte Carlo simulations. This would allow for calculation of reliable dosimetric data for source modeling and calculation in brachytherapy treatment planning systems.

A comparison study on various low energy sources in interstitial prostate brachytherapy.

PurposeLow energy sources are routinely used in prostate brachytherapy. 125I is one of the most commonly used sources. Low energy 131Cs source was introduced recently as a brachytherapy source. The aim of this study is to compare dose distributions of 125I, 103Pd, and 131Cs sources in interstitial brachytherapy of prostate.Material and methodsProstaSeed 125I brachytherapy source was simulated using MCNPX Monte Carlo code. Additionally, two hypothetical sources of 103Pd and 131Cs were simulated with the same geometry as the ProstaSeed 125I source, while having their specific emitted gamma spectra. These brachytherapy sources were simulated with distribution of forty-eight seeds in a phantom including prostate. The prostate was considered as a sphere with radius of 1.5 cm. Absolute and relative dose rates were obtained in various distances from the source along the transverse and longitudinal axes inside and outside the tumor. Furthermore, isodose curves were plotted around the sources.ResultsAnalyzing the initial dose profiles for various sources indicated that with the same time duration and air kerma strength, 131Cs delivers higher dose to tumor. However, relative dose rate inside the tumor is higher and outside the tumor is lower for the 103Pd source.ConclusionsThe higher initial absolute dose in cGy/(h.U) of 131Cs brachytherapy source is an advantage of this source over the others. The higher relative dose inside the tumor and lower relative dose outside the tumor for the 103Pd source are advantages of this later brachytherapy source. Based on the total dose the 125I source has advantage over the others due to its longer half-life.

Example of Monte Carlo uncertainty assessment in the field of radionuclide metrology

This chapter presents possible uses and examples of Monte Carlo methods for the evaluation of uncertainties in the field of radionuclide metrology. The method is already well documented in GUM supplement 1, but here we present a more restrictive approach, where the quantities of interest calculated by the Monte Carlo method are estimators of the expectation and standard deviation of the measurand, and the Monte Carlo method is used to propagate the uncertainties of the input parameters through the measurement model. This approach is illustrated by an example of the activity calibration of a 103Pd source by liquid scintillation counting and the calculation of a linear regression on experimental data points. An electronic supplement presents some algorithms which may be used to generate random numbers with various statistical distributions, for the implementation of this Monte Carlo calculation method.

SU‐F‐19A‐06: Experimental Investigation of the Energy Dependence of TLD Sensitivity in Low‐Energy Photon Beams

Purpose:To measure the energy dependence of TLD sensitivity in lowenergy photon beams with equivalent mono‐energetic energy matching those of 103Pd, 125I and 131Cs brachytherapy sources.Methods:A Pantek DXT 300 x‐ray unit (Precision X‐ray, Branford, CT), with stable digital voltage control down to 20 kV, was used to establish three lowenergy photon beams with narrow energy spread and equivalent monoenergetic energies matching those of 103Pd, 125I and 131Cs brachytherapy sources. The low‐energy x‐ray beams and a reference 6 MV photon beam were calibrated according to the AAPM TG‐61 and TG‐51 protocols, respectively, using a parallel‐plate low‐energy chamber and a Farmer cylindrical chamber with NIST traceable calibration factors. The dose response of model TLD‐100 micro‐cubes (1×1×1 mm3) in each beam was measured for five different batches of TLDs (each contained approximately 100 TLDs) that have different histories of irradiation and usage. Relative absorbed dose sensitivity was determined as the quotient of the slope of dose response for a beam‐of‐interest to that of the reference beam.Results:Equivalent mono‐energetic photon energies of the low‐energy beams established for 103Pd, 125I and 131Cs sources were 20.5, 27.5, and 30.1 keV, respectively. Each beam exhibited narrow spectral spread with energyhomogeneity index close to 90%. The relative absorbed‐dose sensitivity was found to vary between different batches of TLD with maximum differences of up to 8%. The mean and standard deviation determined from the five TLD batches was 1.453 ± 0.026, 1.541 ± 0.035 and 1.529 ± 0.051 for the simulated 103P, 125I and 131Cs beams, respectively.Conclusion:Our measured relative absorbed‐dose sensitivities are greater than the historically measured value of 1.41. We find that the relative absorbed‐dose sensitivity of TLD in the 103P beam is approximately 5% lower than that of 125I and 131Cs beams. Comparison of our results with other studies will be presented.

The use of TLDs for brachytherapy dosimetry

Thermoluminescent dosimeters (TLDs) are routinely used to measure the dose around brachytherapy sources due to their small size and high precision. This work presents a concise overview of the use of LiF:Mg,Ti TLDs for brachytherapy dosimetry including the experimental procedures required to achieve high-precision measurements as well as new results regarding the intrinsic energy dependence with some of the commonly used brachytherapy sources. Equations to correct TLD light output to air kerma are outlined and a description of the method to determine the intrinsic energy dependence is presented. For the intrinsic energy dependence investigation, a review of previously published results is presented as well as new experimental results using 125I, 103Pd, 192Ir, and miniature x-ray brachytherapy sources at the University of Wisconsin Medical Radiation Research Center (UWMRRC). The results of these experiments are consistent with previous work and give valuable insight to investigators using TLDs for brachytherapy measurements.

Photon activation therapy: a Monte Carlo study on dose enhancement by various sources and activation media

In the present study, a number of brachytherapy sources and activation media were simulated using MCNPX code and the results were analyzed based on the dose enhancement factor values. Furthermore, two new brachytherapy sources (¹³¹Cs and a hypothetical ¹⁷⁰Tm) were evaluated for their application in photon activation therapy (PAT). ¹²⁵I, ¹⁰³Pd, ¹³¹Cs and hypothetical ¹⁷⁰Tm brachytherapy sources were simulated in water and their dose rate constant and the radial dose functions were compared with previously published data. The sources were then simulated in a soft tissue phantom which was composed of Ag, I, Pt or Au as activation media uniformly distributed in the tumour volume. These simulations were performed using the MCNPX code, and dose enhancement factor (DEF) was obtained for 7, 18 and 30 mg/ml concentrations of the activation media. Each source, activation medium and concentration was evaluated separately in a separate simulation. The calculated dose rate constant and radial dose functions were in agreement with the published data for the aforementioned sources. The maximum DEF was found to be 5.58 for a combination of the ¹⁷⁰Tm source with 30 mg/ml concentration of I. The DEFs for ¹³¹Cs and ¹⁷⁰Tm sources for all the four activation media were higher than those for other sources and activation media. From this point of view, these two sources can be more useful in photon activation therapy with photon emitter sources. Furthermore, ¹³¹Cs and ¹⁷⁰Tm brachytherapy sources can be proposed as new options for use in the field of PAT.

SU‐E‐T‐32: Monte Carlo Determination of WAFAC Corrections for the Canadian LDR Primary Standard

Purpose: Determine correction factors for a wide angle free‐air chamber (WAFAC) using Monte Carlo techniques for the realization of the air‐kerma strength, Sk, for low dose rate (LDR) 125 I and 103Pd brachytherapy sources. This WAFAC will become the Canadian primary standard for LDR brachytherapy sources. Methods: The EGSnrc user‐code egsfac is used for estimating the correction factors required to determine the air‐kerma at the WAFACs point of measurement (POM) using MC simulation for a 125 I and a 103Pd source. The geometry library egs++ allows simulation of the sources and the WAFAC in detail. Spectra are taken from the most recently published data. Air and aluminum attenuation curves for the 125 I seed were also measured with the WAFAC. Results: Correction values compared with those reported by NIST for its LDR primary standard (of very similar design) are in excellent agreement when using the NIST approach of assuming an isotropic point source and using experimental main spectrum lines. Differences in both methodologies for determining the WAFAC corrections are discussed and their impact evaluated. The effect of the source and the holder on the scatter correction is quantified separately. The calculations resulted in a modification of the design to reduce collimator transmission. A 3.2 mm lead sheet was added behind the WAFAC aperture, attenuating the 1251 and the 103Pd photon beams to less than 0.02 %. MC attenuation curves for air and aluminum for the 125 I seed are in good agreement with experimental data. The largest correction is the aluminum filter attenuation which can be as large as 4 %. Conclusion: Despite an inconsistency in the scatter estimation, the NIST approximation has little impact on the determination of Sk. The scatter correction is sensitive to the collimation system dimensions. Using MC‐based air and aluminum attenuation corrections for the 125 I seed is experimentally justified.

SU‐E‐T‐455: A Treatment Planning Feasibility Study for Prostate LDR Brachytherapy Treatments Using the New 103‐Pd CivaString Source. Comparison with Clinical Cases Using the TheraSeed Model 200 103‐Pd Source

Purpose: This is a comparative dosimetric study using a new 103Pd CivaString source for prostate brachytherapy seed implants. Clinical plans using TheraSeed Model 200 103Pd seeds were used as the standard. The aims were to explore the feasibility and planning advantages with, and define guidelines for optimization and clinical implementation of, CivaString linear sources. Methods: Treatment plans from twelve patients implanted with Model 200 seeds were re‐planned using the CivaString source. Prostate volumes ranged from 23 to 61 cm3. CivaString dosimetric characterization was based on TG‐43 parameters derived from Monte Carlo simulations. Two CivaString plans were generated for each patient, first with a uniform source strength, then using higher‐strength sources peripherally and lower‐strength sources interiorly. The planning objective was matching the prostate D90 of the analogous Model 200‐based plan and was achieved by scaling CivaString source strength for a fixed but patient‐specific needle placement. Dosimetric quality was assessed in reference to prostate D90, V100, and V150, rectum V100, and urethra V150. Results: For virtually the same prostate D90, CivaString plans matched all dosimetric parameters yet provided a slight reduction in prostate V150. CivaString plans permitted reduction in the number of needles required to deliver implants, ranging from 55% to 68% compared with Model 200 plans. This allowed large prostate 103Pd source implants with 12–14 needles. In large‐volume prostate implants, use of two source strengths appeared beneficial to reduce V150 for prostate and urethra. Conclusion: Based on clinical scenarios, planning with the CivaString source significantly reduced the number of required needles while delivering similar dose distributions to the prostate, urethra, and rectum. Planning was dramatically simplified, and optimization was replaced by simple guidelines that allowed creation of high‐quality treatment plans within minutes.

COMS eye plaque brachytherapy dosimetric sensitivity to source photon energy and seed design

This study explores the influence of source photon energy on eye plaque brachytherapy dose distributions for a 16mm COMS plaque filled with 103Pd, 125I, or 131Cs sources or monoenergetic photon emissions ranging from 12keV to 100keV. Dose distributions were similarly created for all permutations of three common brachytherapy seed designs. Within this range, sources with average energy ≤22keV may reduce dose to the opposite eye wall by more than a factor of 2 while maintaining tolerable proximal sclera doses when prescribing to depths of 9mm or less. Current commercially-available brachytherapy sources can exhibit up to 15% relative dosimetric sensitivity to seed design at regions within the eye.

Evaluation of Permanent Lung Brachytherapy Implants Dose Distributions and Tissue Heterogeneities Using 103Pd, 125I, and 131Cs

Permanent brachytherapy implants are used to treat early-stage lung cancer. However, the current brachytherapy dosimetry formalism assumes a homogeneous water medium and does not account for tissue heterogeneities. These heterogeneities have been investigated for 20-400 keV point sources to glean dosimetric trends independent of specific radionuclide emissions. The purpose of this study was to determine the dosimetric effects of tissue heterogeneities for permanent lung brachytherapy implants using realistic 103Pd, 125I, and 131Cs source models.