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Biomedical subjects

J F Williamson

Publications and source records attributed to J F Williamson.

At least 37 records · Page 2Linked to original sources
PubMed2000

PERB11 (MIC): a polymorphic MHC gene is expressed in skin and single nucleotide polymorphisms are associated with psoriasis.

The susceptibility genes for psoriasis remain to be identified. At least one of these must be in the major histocompatibility complex (MHC) to explain associations with alleles at human leucocyte antigen (HLA)-A, -B, -C, -DR, -DQ and C4. In fact, most of these alleles are components of just two ancestral haplotypes (AHs) designated 13.1 and 57.1. Although relevant MHC gene(s) could be within a region of at least 4 Mb, most studies have favoured the area near HLA-B and -C. This region contains a large number of non-HLA genes, many of which are duplicated and polymorphic. Members of one such gene family, PERB11.1 and PERB11.2, are expressed in the skin and are encoded in the region between tumour necrosis factor and HLA-B. To investigate the relationship of PERB11.1 alleles to psoriasis, sequence based typing was performed on 97 patients classified according to age of onset and family history. The frequency of the PERB11.1*06 allele is 44% in type I psoriasis but only 7% in controls (Pc = 0.003 by Fisher's exact test, two-tailed). The major determinant of this association is a single nucleotide polymorphism (SNP) within intron 4. In normal and affected skin, expression of PERB11 is mainly in the basal layer of the epidermis including ducts and follicles. PERB11 is also present in the upper keratin layers but there is relative deficiency in the intermediate layers. These findings suggest a possible role for PERB11 and other MHC genes in the pathogenesis of psoriasis.

Base Sequence↗
PubMed2000

Validation of a precision radiochromic film dosimetry system for quantitative two-dimensional imaging of acute exposure dose distributions.

We present an evaluation of the precision and accuracy of image-based radiochromic film (RCF) dosimetry performed using a commercial RCF product (Gafchromic MD-55-2, Nuclear Associates, Inc.) and a commercial high-spatial resolution (100 microm pixel size) He-Ne scanning-laser film-digitizer (Personal Densitometer, Molecular Dynamics, Inc.) as an optical density (OD) imaging system. The precision and accuracy of this dosimetry system are evaluated by performing RCF imaging dosimetry in well characterized conformal external beam and brachytherapy high dose-rate (HDR) radiation fields. Benchmarking of image-based RCF dosimetry is necessary due to many potential errors inherent to RCF dosimetry including: a temperature-dependent time evolution of RCF dose response; nonuniform response of RCF; and optical-polarization artifacts. In addition, laser-densitometer imaging artifacts can produce systematic OD measurement errors as large as 35% in the presence of high OD gradients. We present a RCF exposure and readout protocol that was developed for the accurate dosimetry of high dose rate (HDR) radiation sources. This protocol follows and expands upon the guidelines set forth by the American Association of Physicists in Medicine (AAPM) Task Group 55 report. Particular attention is focused on the OD imaging system, a scanning-laser film digitizer, modified to eliminate OD artifacts that were not addressed in the AAPM Task Group 55 report. RCF precision using this technique was evaluated with films given uniform 6 MV x-ray doses between 1 and 200 Gy. RCF absolute dose accuracy using this technique was evaluated by comparing RCF measurements to small volume ionization chamber measurements for conformal external-beam sources and an experimentally validated Monte Carlo photon-transport simulation code for a 192Ir brachytherapy source. Pixel-to-pixel standard deviations of uniformly irradiated films were less than 1% for doses between 10 and 150 Gy; between 1% and 5% for lower doses down to 1 Gy and 1% and 1.5% for higher doses up to 200 Gy. Pixel averaging to form 200-800 microm pixels reduces these standard deviations by a factor of 2 to 5. Comparisons of absolute dose show agreement within 1.5%-4% of dose benchmarks, consistent with a highly accurate dosimeter limited by its observed precision and the precision of the dose standards to which it is compared. These results provide a comprehensive benchmarking of RCF, enabling its use in the commissioning of novel HDR therapy sources.

Brachytherapy↗
PubMed2000

Dosimetric modeling of the microselectron high-dose rate 192Ir source by the multigroup discrete ordinates method.

The DANTSYS multigroup discrete ordinates computer code is applied to quantitatively estimate the absorbed dose rate distributions in the vicinity of a microSelectron 192Ir high-dose-rate (HDR) source in two-dimensional cylindrical R-Z geometry. The source is modeled in a cylindrical water phantom of diameter 20 cm and height 20 cm. The results are also used for evaluation of the Task Group 43 (TG-43) dosimetric quantities. The DANTSYS accuracy is estimated by direct comparisons with corresponding Monte Carlo results. Our 210-group photon cross section library developed previously, together with angular quadratures consisting of 36 (S16) to 210 (S40) directions and associated weights per octant, are used in the DANTSYS simulations. Strong ray effects are observed but are significantly mitigated through the use of DANTSYS's stochastic ray-tracing first collision source algorithm. The DANTSYS simulations closely approximate Monte Carlo estimates of both direct dose calculations and TG-43 dosimetric quantities. The discrepancies with S20 angular quadrature (55 directions and weights per octant) or higher are shown to be less than +/- 5% (about 2.5 standard deviations of Monte Carlo calculations) everywhere except for limited regions along the Z axis of rotational symmetry, where technical limitations in the DANTSYS first collision source implementation makes adequate suppression of ray effects difficult to achieve. The efficiency of DANTSYS simulations is compared with that of the EGS4 Monte Carlo code. It is demonstrated that even with the 210-group cross section library, DANTSYS achieves two-fold efficiency gains using the the S20 quadrature set. The potential of discrete ordinates method for further efficiency improvements is also discussed.

Algorithms↗
PubMed2000

A simple technique for alignment of perineal needle template to ultrasound image grid for permanent prostate implants.

Proper alignment of a needle template and ultrasound software grid is required to accurately deliver permanent prostate seed implants optimized using pretreatment volume studies. Correct alignment may also reduce tissue edema, morbidity, and the time and labor required to deliver permanent prostate seed implants. A technique has been developed to rapidly assess (and, if necessary, improve) the alignment using a custom-designed water phantom. Verification of needle positions can be conducted within 1 mm and requires less than ten minutes. We have instituted the technique as a part of our periodic quality assurance program.

Biophysical Phenomena↗
PubMed2000

Recommendations of the American Association of Physicists in Medicine on 103Pd interstitial source calibration and dosimetry: implications for dose specification and prescription.

The National Institute of Standards and Technology (NIST) introduced a national standard for air kerma strength of the ThreaSeed Model 200 103Pd source (the only 103Pd seed available until 1999) in early 1999. Correct implementation of the NIST-99 standard requires the use of dose rate constants normalized to this same standard. Prior to the availability of this standard, the vendor's calibration procedure consisted of intercomparing Model 200 seeds with a 109Cd source with a NIST-traceable activity calibration. The AAPM undertook a comprehensive review of 103Pd source dosimetry including (i) comparison of the vendor and NIST-99 calibration standards; (ii) comparison of original Task Group 43 dosimetry parameters with more recent studies; (iii) evaluation of the vendor's calibration history; and (iv) evaluation of administered-to-prescribed dose ratios from the introduction of 103Pd sources in 1987 to the present. This review indicates that for a prescribed dose of 115 Gy, the administered doses were (a) 124 Gy for the period 1988-1997 and (b) 135 Gy for the period 1997-1999. The AAPM recommends that the following three steps should be undertaken concurrently to implement correctly the 1999 dosimetry data and NIST-99 standard for 103Pd source: (1) the vendor should provide calibrations in terms of air kerma strength traceable to NIST-99 standard, (2) the medical physicist should update the treatment planning system with properly normalized (to NIST-99) dosimetry parameters for the selected 103Pd source model, and (3) the radiation oncologist in collaboration with the medical physicist should decide which clinical experience they wish to duplicate; the one prior to 1997 or the one from 1997 to 1999. If the intent is to duplicate the experience prior to 1997, which is backed by the long-term follow-up and published outcome studies, then the prior prescriptions of 115 Gy should be replaced by 124 Gy to duplicate that experience.

Brachytherapy↗
PubMed2000

Monte Carlo modeling of the transverse-axis dose distribution of the model 200 103Pd interstitial brachytherapy source.

This study presents the first theoretical analysis of the absolute dose-rate distribution about the Model 200 103Pd interstitial brachytherapy source. Monte Carlo photon-transport (MCPT) simulation techniques have been used to evaluate the transverse-axis dose-rate distribution of the Model 200 source as a function of thickness of the Pd metal coating (containing the 103Pd) plated onto the surfaces of right cylindrical graphite pellets contained within the seed. The dose-rate constant, A, was realistically estimated by simulating the wide-angle, free-air chamber (WAFAC) calibration geometry. The WAFAC is the experimental realization of NIST's (National Institute of Standards and Technology) recently implemented primary standard of air-kerma strength (S(K)). Our results show that polar angle- and distance-dependent oblique filtration and shielding effects induce significant and unexpected photon fluence anisotropy near the transverse-axis and inverse square law deviations at typical calibration distances. Any source consisting of radioactivity deposited on a highly attenuating surface with sharp edges may exhibit such effects. In the case of the Model 200 seed, the Pd metal thickness does not significantly influence the relative dose distribution in water at distances less than 5 cm, but does make A sensitive to the S(K) measurement geometry. Fortunately, the WAFAC averages fluence over a sufficiently large aperture that the resultant A, 0.68 +/- 0.02 cGy x h(-1) x U(-1), is almost independent of Pd metal layer thickness and in close agreement with recent measurements and calculations. This value is 20% higher than that of the renormalized Task Group 43 A value.

Brachytherapy↗
PubMed2000

Monte Carlo-aided dosimetry of the Symmetra model I25.S06 125I, interstitial brachytherapy seed.

A dosimetric study of a new 125I seed for permanent prostate implant, the Symmetra 1251 Seed model I25.S06, has been undertaken utilizing Monte Carlo photon transport calculations. All dosimetric quantities recommended by the AAPM Task Group 43 (TG-43) report have been calculated. Quantities determined are dose rate constant, radial dose function, anisotropy function, anisotropy factor, and anisotropy constant. The recently (January 1999) revised NIST (National Institute of Standards and Technology) 1251 standard for air kerma strength calibration was taken into account as well as updated interaction cross-section data. Calculations were done for the competing model 6702 source for the purpose of comparison. The calculated dose-rate constants for the two seeds are 1.010 and 1.016 cGyh(-1)U(-1) for the Symmetra and model 6702 seeds, respectively. The latter value deviates from the value, 1.039 cGyh(-1)U(-1), recommended in the TG-43 report. The calculated radial dose function for the Symmetra new seed is more penetrating than that of the model 6711 seed (by 20% at 5 cm distance) but agrees closely (within statistical errors) with that of the model 6702 seed up to distances of 10 cm. The anisotropy function for the seed is also close to that for the 6702 seed with a tendency of somewhat more pronounced anisotropy (lower values at small angles from the longitudinal axis). Compared to the model 6711 seed, the Symmetra new seed is more isotropic. The anisotropy constants (the anisotropy function averaged with respect to angle and distance) for the three seed models are within 2%.

Air↗
PubMed1999

Plastic scintillator response to low-energy photons.

The plastic scintillator (PS) is a promising dosimeter for brachytherapy and other low-energy photon applications because of its high sensitivity and approximate tissue equivalence. As part of our project to develop a new PS material which maximizes sensitivity and radiological equivalence to water, we have measured the response, epsilon (light output/unit air kerma), of PS to low-energy bremsstrahlung (20 to 57 keV average energies) x-rays as well as photons emitted by 99mTc, 192Ir, and 137Cs sources, all of which were calibrated in terms of air kerma. The PS systems studied were a standard commercial PS, BC400 (Bicron Corporation, Newbury, OH), and our new sensitive and quench-resistant scintillator (polyvinyltoluene base and binary dye system) with and without 4% Cl loading intended to match the effective atomic number of water. For low-energy x-rays, epsilon was 20-57% relative to epsilon for 192Ir photons. Chlorine loading clearly reduced the energy dependence of epsilon, which ranged from 46% to 85% relative to 192Ir. However, even after using Monte Carlo photon-transport simulation to correct for the non-air equivalence of the PS, inherent dosimetric sensitivity still varied by 30% over the 20-400 keV energy range. Our work, one of the few measurements of PS response to low-energy photons, appears to confirm Birks' 1955 finding that ionization quenching reduces sensitivity to electrons below 125 keV. However, our results cannot be explained by Birks' widely used unimolecular quenching model.

Monte Carlo Method↗
PubMed1999

Guidance to users of Nycomed Amersham and North American Scientific, Inc., I-125 interstitial sources: dosimetry and calibration changes: recommendations of the American Association of Physicists in Medicine Radiation Therapy Committee Ad Hoc Subcommittee on Low-Energy Seed Dosimetry.

Dose calculations to patients undergoing implantation of 125I interstitial brachytherapy sources are affected by two recent changes in low-energy seed dosimetry: (a) implantation of a new primary air-kerma strength standard at the National Institute of Standards and Technology (NIST) on 1 January 1999 and (b) publication of revised dose-rate distributions in AAPM's Task Group 43 Report. The guidance herein represents AAPM's recommendations for users of 125I interstitial seed products marketed prior to 1 January 1999 (Nycomed Amersham models 6711 and 6702 and North American Scientific, Inc. models 3631 A/S and 3631 A/M. Implementation of Task Group 43 (TG43) 125I dose calculations involves revising data stored in files of radiation treatment planning software and lowering the prescribed dose to be delivered to patients by as much as 15% to avoid modifying the dose actually delivered to patients. The magnitude of the dose prescription change depends on the dosimetry data used prior to TG43 and the implant geometry. Adapting to the revised NIST calibration standard requires the user to increase the dose-rate constant (or its equivalent by 11.5%) but does not require modification of the prescribed dose. Failure to correctly implement these modifications can result in 20% or even 30% errors.

Brachytherapy↗
PubMed1999

Towards two-dimensional brachytherapy dosimetry using plastic scintillator: new highly efficient water equivalent plastic scintillator materials.

Plastic scintillator (PS) has been proposed for both one- and two-dimensional (1D and 2D) dose measurements for radiation therapy applications. For low-energy photon modalities (e.g., brachytherapy), an efficient water equivalent scintillator is needed. To perform 2D measurements, a high localization of the scintillation process is required. Guided by comparison of the mass energy absorption coefficients as a function of energy and of the dose distribution as a function of distance from the radioactive source, as modeled by Monte Carlo photon transport simulation, a small quantity of medium atomic number (Z) atoms (4% Cl) was incorporated in a polyvinyl toluene (PVT) based PS to approximate closely (within 10%) the radiological properties of water in the 20-662 keV energy range. However, the scintillation efficiency of commercial PS mixtures drops as much as 70% when loaded with high atomic number additives. We developed experimental techniques to assess the scintillation efficiency and locality of 15 new PS mixtures. These mixtures differ by the type of the scintillation dyes and the type of compound containing the medium Z atoms (chlorine). To achieve higher material stability, 4-chlorostyrene was used as a loading compound to ensure polymerization with the PVT base. Two of the new PS materials exhibited scintillation efficiencies within 30% of one of the most efficient commercially available products (BC-400), which is not water equivalent at such low energies. These new scintillator materials are promising candidates for the development of an accurate and efficient radiation dosimetry method not only for brachytherapy, but also for superficial and diagnostic applications.

Biophysical Phenomena↗
PubMed1999

Quantitative optical densitometry with scanning-laser film digitizers.

A new process for eliminating two types of artifacts inherent in commercially available transmission scanning-laser film digitizers is presented. The first kind of artifact results in nonreproducible interference-pattern fluctuations as large as 7%. The second kind results in spreading of transmitted light from low-to-high optical density (OD) in regions with rapidly varying ODs, producing errors as large as 50%. These OD artifacts cause the loss of precision for films with low-OD regions (first type) and the loss of accuracy for films with regions of high-OD near high-OD gradients (second type). Test radiochromic films, produced by uniform exposure to a 6 MV photon beam and a high dose rate 192Ir brachytherapy source, along with test radiographic films were used to characterize the artifacts of a commercially available scanning-laser film digitizer. The interference-pattern artifact was eliminated by digitizing the films on a masked diffusing ground-glass scanning bed. The light-transmission artifact was eliminated through discrete-fast-Fourier-transform (DFFT) deconvolution of transmission profiles with measured digitizer line-spread functions. Obtaining precise OD distributions after the DFFT deconvolution required prior removal of the interference-pattern artifact and application of a low-pass Wiener noise filter. Light-transmission artifacts are particularly significant for applications requiring measurement of high-gradient OD distributions, such as brachytherapy or conformal photon-beam film dosimetry and quantitation of two-dimensional electrophoresis gels. Errors as large as 15%-35% occur in OD distributions representative of these applications. The data collection and correction process developed in this study successfully removes these artifacts.

Biophysical Phenomena↗
PubMed1998

Dosimetric properties of a novel brachytherapy balloon applicator for the treatment of malignant brain-tumor resection-cavity margins.

PURPOSE: This paper characterizes the dosimetric properties of a novel balloon brachytherapy applicator for the treatment of the tissue surrounding the resection cavity of a malignant brain tumor. METHODS AND MATERIALS: The applicator consists of an inflatable silicone balloon reservoir attached to a positionable catheter that is intraoperatively implanted into the resection cavity and postoperatively filled with a liquid radionuclide solution. A simple dosimetric model, valid in homogeneous media and based on results from Monte Carlo photon-transport simulations, was used to determine the dosimetric characteristics of spherical geometry balloons filled with photon-emitting radionuclide solutions. Fractional depth-dose (FDD) profiles, along with activity densities, and total activities needed to achieve specified dose rates were studied as a function of photon energy and source-containment geometry. Dose-volume histograms (DVHs) were calculated to compare idealized balloon-applicator treatments to conventional 125I seed volume implants. RESULTS: For achievable activity densities and total activities, classical low dose rate (LDR) treatments of residual disease at distances of up to 1 cm from the resection cavity wall are possible with balloon applicators having radii between 0.5 cm and 2.5 cm. The dose penetration of these applicators increases approximately linearly with balloon radius. The FDD profile can be made significantly more or less penetrating by combining selection of radionuclide with source-geometry manipulation. Comparisons with 125I seed-implant DVHs show that the applicator can provide a more conformal therapy with no target tissue underdosing, less target tissue overdosing, and no healthy tissue "hot spots;" however, more healthy tissue volume receives a dose of the prescribed dosage or less. CONCLUSIONS: This device, when filled with 125I solution, is suitable for classical LDR treatments and may be preferable to 125I interstitial-seed implants in several respects. Manipulation of the dosimetric properties of the device can improve its characteristics for brain tumor treatment and may make it suitable for boosting the lumpectomy margins in conservative breast cancer treatment.

Brachytherapy↗
PubMed1998

Monte Carlo-based dose-rate tables for the Amersham CDCS.J and 3M model 6500 137Cs tubes.

PURPOSE: (1) To present reference-quality dose-rate distributions for the Amersham CDCS.J-type 137Cs intracavitary source (hitherto unavailable in the literature) and updated tables for the 3M model 6500/6D6C source. (2) To assess the accuracy of the widely used 1D pathlength (Sievert integral) algorithm for lightly filtered 137Cs tube sources. METHODS AND MATERIALS: A Monte Carlo photon-transport code is used to calculate the dose-rate distributions about the 3M source and the CDCS.J source based on radiographic examination of the sources and the vendors' specifications. Dose-rate distributions are provided in the form of Cartesian "away-and-along" lookup tables. Using a general form of the Sievert integral, calculated dose-rate distributions were compared to the Monte Carlo benchmark calculations treating the filtration coefficients as best-fit parameters as well as approximating them by linear energy absorption coefficients. In addition, the errors introduced by approximating the active source core by uniform cylinders or line sources was evaluated. RESULTS: The Model CDCS.J dose distribution differs from that of the 3M model 6500 source by -5.9% to +14.4% (root-mean-square [RMS] average: 2.6%). The RMS accuracy of the Sievert algorithm is 2.4% to 2.8% (error range of -1.4% to 7.6%) when filtration coefficients for steel and ceramic media are approximated by linear energy absorption coefficients. If the filtration coefficients are treated as parameters of best fit, selected to minimize the discrepancies between 1D pathlength and Monte Carlo calculations, the RMS error is reduced to 0.8% (error range of -1.8% to 4.1%). The optimal values of stainless steel and low-density ceramic or glass filtration coefficients are approximately independent of the source geometry. CONCLUSIONS: The widely used Sievert integral algorithm accurately characterizes the dose distribution around stainless-steel clad low-density matrix 137Cs sources, particularly if design-independent best-fit values of the filtration coefficients are used. Although both families of source designs studied produce similar dose distributions, source-design specific dose distributions should be used for clinical treatment planning and dose-algorithm validation.

Brachytherapy↗
PubMed1998

Report of the ad hoc committee of the AAPM radiation therapy committee on 125I sealed source dosimetry.

PURPOSE: Two developments in 125I-sealed source dosimetry have necessitated swift and accurate implementation of TG43 dosimetry in clinic: (a) the dosimetry constants of 125I endorsed by the AAPM Task Group 43 Report result in calculated dose rate that deviates by as much as 15% from currently accepted dose-rate distributions, and (b) The National Institute of Standards and Technology (NIST) has proposed modifying the 125I air-kerma strength standard by approximately 10%. METHODS AND MATERIALS: The ad hoc committee of AAPM Radiation Therapy Committee describes specific procedures to implement these two developments without causing confusion and mistakes. CONCLUSIONS: Confusion and mistakes may be avoided when the following two general steps are taken: 1) STEP I, TG-43 implementation, and 2) STEP II, new air-kerma strength standard implementation when available from NIST.

Brachytherapy↗
PubMed1998

Uterosacral space involvement in locally advanced carcinoma of the uterine cervix.

PURPOSE: Radiation therapy is the standard management for locally advanced cervical cancer, but it has not yielded fully satisfactory results; a relatively high incidence of local failure remains. Standard radiation therapy techniques combine external beam radiation and brachytherapy generating a homogeneously composite dose distribution covering the lateral parametria but may not be adequate in the uterosacral and perirectal areas due to the spatial arrangement of intracavitary system and the constraints of rectal tolerance. We hypothesize that these dosimetric characteristics might lead to a higher incidence of central/marginal failures when the uterosacral space is involved by locally advanced carcinoma of uterine cervix. METHODS AND MATERIALS: Between January 1970 and December 1989, 343 patients with clinical Stage IIIB cervical cancer were treated at the Mallinckrodt Institute of Radiology with radiation therapy alone. We identified 83 patients with clinical evidence of tumor in the uterosacral region; the remaining 260 patients either did not have uterosacral involvement or were unspecified. The dose of external beam irradiation ranged from 18.02 to 33.20 Gy to the central pelvis and 48.22 to 59.40 Gy to the lateral parametrium. The average total dose, including brachytherapy contribution, to point A and the lateral pelvis was 80.30 to 86.46 Gy and 60.50 to 73.40 Gy, respectively. External beam dose to the lateral parametria was, on average, 10 Gy higher in patients with uterosacral involvement. RESULTS: We categorized the patterns of pelvic failure into central/marginal (including medial parametrium) and lateral parametria. The cumulative incidence of central/marginal failure at 5 years was significantly higher in the group of patients with uterosacral involvement (36% compared with 21% for patients without uterosacral involvement or unspecified) (p = 0.002). Lateral parametrial failure was similar for patients with and without uterosacral involvement (39% and 38% at 5 years, respectively) (p = 0.42). The actuarial incidence of distant metastasis was identical in the two groups: 46% at 5 years. Multivariate analysis confirmed that uterosacral space involvement increased the risk of pelvic recurrence (p = 0.044) and was the most significant factor that influenced the central/marginal pelvic failure (p = 0.002). CONCLUSIONS: Uterosacral involvement by locally advanced carcinoma of the uterine cervix significantly increased overall pelvic failure and was the most significant prognosticator of central/marginal pelvic failure. This is the result of the spatial constraints of the standard intracavitary geometry that deliver inadequate dose posteriorly to encompass the uterosacral space. Plausible ways to compensate the underdose in the uterosacral space include increasing whole pelvis dose without compromising the intracavitary brachytherapy dose, using a supplemental interstitial implant or adding a posterior oblique external beam boost.

Brachytherapy↗
PubMed1998

High dose-rate brachytherapy treatment delivery: report of the AAPM Radiation Therapy Committee Task Group No. 59.

The goals of this task group are to examine the current high dose-rate (HDR) treatment delivery practices and to prepare a document to assure safe delivery of HDR treatments. The document consists of detailed HDR procedures for design of an HDR brachytherapy program, staffing and training, treatment specific quality assurance, and emergency procedures. The document provides an extensive quality assurance (QA) check list. It reviews all aspects of HDR treatment delivery safety, including prescription, treatment plan, treatment delivery, and radiation safety.

Brachytherapy↗
PubMed1998

Analytical approach to heterogeneity correction factor calculation for brachytherapy.

In brachytherapy treatment planning, the effects of tissue and applicator heterogeneities are commonly neglected due to lack of accurate, general, and fast three-dimensional (3D) dose-computational algorithms. A novel approach, based on analytical calculation of scattered photon fluxes inside and around a disk-shaped heterogeneity, has been developed for use in the three-dimensional scatter-subtraction algorithm. Specifically, our model predicts the central-ray dose distribution for a collimated photon isotropic source or brachytherapy "minibeam" in the presence of a slab of heterogeneous material. The model accounts for the lateral dimensions, location, composition, density, and thickness of the heterogeneity using precalculated scatter-to-primary ratios (SPRs) for the corresponding homogeneous problem. The model is applicable to the entire brachytherapy energy range (25 to 662 keV) and to a broad range of materials having atomic numbers of 13 to 82, densities of 2.7 g.cm-3 (Al) to 21.45 g.cm-3 (Pt) and thicknesses up to 1 mean free path. For this range of heterogeneous materials, the heterogeneity correction factors (HCFs) vary from 0.09 to 0.75. The model underestimates HCF when multiple scattering prevails and overestimates HCF when absorption dominates. However, the analytic model agrees with Monte Carlo photon transport (MCPT) benchmark calculations within 1.8% to 10% for 125I, 169Yb, 192Ir, and 137Cs for a wide variety of materials, with the exception of Ag. For 125I shielded by Ag, where the mean discrepancy can exceed 25%, the error is due to K-edge characteristic x rays originating within the heterogeneity. The proposed approach provides reductions in CPU time required of 5 x 10(4)-10(5) and 100 in comparison with direct MCPT simulation and 1D numerical integration, respectively. The limitations of model applicability, as determined by the physical properties of heterogeneity material and accuracy required, are also discussed.

Algorithms↗