[Cost-effectiveness analysis of rabies immunization strategy based on dynamic-decision tree model].

Objective: To evaluate the cost-utility of different immunization strategies for rabies in China, and to provide a reference for determining the optimal immunization strategy. Methods: The system dynamics model was used to simulate the epidemic of canine rabies and a decision tree model was conducted to analysis different immune strategies. Relevant probabilities were obtained through literature search and on-site investigation. Sensitivity analysis was used to explore the important influenced factors. Results: At baseline, from a social perspective, 70% vaccination of dogs was the optimal strategy compared to current vaccination strategy (43% vaccination in dogs, human category-Ⅱ exposure vaccination/category-Ⅲ exposure vaccination combined with RIG). The total cost was 14 084 354 CNY, and the total utility value was 22 078 616.23 QALYs, and the incremental cost-utility ratio was-62 148 147 CNY/QALY; if human vaccination was considered, 55% vaccination of dogs combined with strategy one was the optimal strategy, its incremental cost-utility ratio was-444 620 557 CNY/QALY. The probability that an injured dog carries rabies virus was the most sensitive parameter. When it was greater than 0.005 03, strategy four was the optimal strategy. When it was less than 82/100 000, strategy one was the optimal strategy; when it was between 82/100 000 and 120/100 000, strategy two was the optimal strategy; when it was between 120/100 000 and 503/100 000, strategy two was the optimal strategy. Conclusion: It was conducive to increase the vaccination coverage of canine for the prevention and control of rabies.

Selecting the optimum conditions for two-dimensional difference gel electrophoresis of proteins expressed in Populus euphratica.

We selected the optimum conditions for two-dimensional difference gel electrophoresis (2D-DIGE) of proteins expressed in the heteromorphic leaves of Populus euphratica Oliv. by adjusting the isoelectric focusing, the loading quantity, the concentration of the electrophoretic gel, and other parameters. The results of our study showed that protein separation was improved with many clear protein spots observed, and the differentiations were obvious. The findings of this study will be useful for future studies of protein expression in the heteromorphic leaves of P. euphratica.

Angular dependence of the MOSFET dosimeter and its impact on in vivo surface dose measurement in breast cancer treatment.

The focus of this study is the angular dependence of two types of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) dosimeters (MOSFET20 and OneDose/OneDosePlus) when used for surface dose measurements. External beam radiationat different gantry angles were delivered to a cubic solid water phantom with a MOSFET placed on the top surface at CAX. The long axis of the MOSFET was oriented along the gantry axis of rotation, with the dosimeter (bubble side) facing the radiation source. MOSFET-measured surface doses were compared against calibrated radiochromic film readings. It was found that both types of MOSFET dosimeters exhibited larger than previously reported angular dependence when measuring surface dose in beams at large oblique angles. For the MOSFET20 dosimeter the measured surface dose deviation against film readings was as high as 17% when the incident angle was 72 degrees to the norm of the phantom surface. It is concluded that some MOSFET dosimeters may have a strong angular dependence when placed on the surface of water-equivalent material, even though they may have an isotropic angular response when surrounded by uniform medium. Extra on-surface calibration maybe necessary before using MOSFET dosimeters for skin dose measurement in tangential fields.

Parameterization of brachytherapy source phase space file for Monte Carlo-based clinical brachytherapy dose calculation.

A common approach to implementing the Monte Carlo method for the calculation of brachytherapy radiation dose deposition is to use a phase space file containing information on particles emitted from a brachytherapy source. However, the loading of the phase space file during the dose calculation consumes a large amount of computer random access memory, imposing a higher requirement for computer hardware. In this study, we propose a method to parameterize the information (e.g., particle location, direction and energy) stored in the phase space file by using several probability distributions. This method was implemented for dose calculations of a commercial Ir-192 high dose rate source. Dose calculation accuracy of the parameterized source was compared to the results observed using the full phase space file in a simple water phantom and in a clinical breast cancer case. The results showed the parameterized source at a size of 200 kB was as accurate as the phase space file represented source of 1.1 GB. By using the parameterized source representation, a compact Monte Carlo job can be designed, which allows an easy setup for parallel computing in brachytherapy planning.

SU-E-T-10: Monte Carlo Study of the Dose Enhancement Factor (DEF) for Gold Nano-Particle (GNP) on the Cellular Level.

PURPOSE: In megavoltage external beam radiotherapy, in vivo cell experiments suggest GNP could be used as a radiosensitizer by having radiation dose enhancement factor (DEF) significantly larger than 1. However, Monte Carlo (MC) simulations published in the literature failed to give prove, in which most of them only simulated the interactions between the radiation beams and a single GNP. In this study, we built a multi-GNPs model considering possible spatial arrangements of GNPs relative to a cell to calculate the DEFs of GNPs. METHODS: Geant4 MC code with G4DNA physics model which can trace electrons down to eV level was used. Two types of geometry models representing different GNP-cell binding were created with each GNP modeled individually: (1) shell model with GNPs randomly and sparsely distributed in a shell in water mimicking when the GNPs were binding to the cell membrane, and (2) sphere model with GNPs randomly and sparsely distributed in a sphere in water mimicking when GNPs were floating inside the cytoplasm. Photon and electron spectrum at 5 cm in depth in water from a Varian 6MV beam was used as the radiation source. Dose to water inside the shell or the sphere representing cytoplasm were scored and compared to situations without GNPs to calculate the DEF. We also looked into the variation of DEFs due to different GNP sizes and concentrations. RESULTS: A 35 um water cubic were successfully built in Geant4 with spatial resolution of 100 nm. Preliminary results shown under 200 keV electron irradiation, 100 nm GNPs in the shell model shown increased dose to cell at the beam entrance (DEF = 1.08). CONCLUSIONS: The computation is undergoing for different GNP sizes and concentrations. Meaningful results are expected on the completion of this study.

SU-E-J-41: Fluoroscopy Based Adaptive Setup Approach for Thoracic Cancer IGRT.

PURPOSE: To develop a fluoroscopy imaging based approach that can determine the magnitude and phase variation in respiratory motion against the treatment planning images in order to quantify the online patient setup deviation in thoracic cancer IGRT for the real time adaptive patient positionadjustment. METHODS: A numerical phantom was generated to test the strength of the approach. The 2D phantom consist a static outside wall and an inner target moving in temporal pattern similar to the respiratory motion. Four motion parameters: frequency (0.5Hz∼2Hz), amplitude (40∼60mm), position offset (5∼10mm), and phase shift (starting phase 0, 45, and 90 degrees) vary to generate phantom image sets with different motions. White noise of level SNR=5 was added to all phantom images to simulate the impact of clinical image quality. A manifold based machine learning technique was used to construct the respiratory motion model under thestandard condition (frequency 1 Hz, amplitude 50mm, and no position offset and/or phase shift). Then the phase and position shift in other phantom image series were quantified by finding the MAP solution to fit theembedded motion to the standard motion model. RESULTS: The proposed approach can detect the variation in motion patterns between two image sets. The method is insensitive to frequency changes and image noise up to SNR=5, but is very effective at capturing and quantifying the change in motion amplitude, position shift, and also the phase shift. CONCLUSIONS: This work proposed an effective mathematical approach to quantify the difference in motion between the pre-treatment images and the treatment planning images by extraction of the motion model in fluoroscopy images using the machine learning technique. By applying the approach during online patient setup, the positioning deviation can be separated from the respiratory motion and adjusted to minimize the normal tissue toxicity in gated IGRT.

Experimental determination of depth-scaling factors and central axis depth dose for clinical electron beams.

Depth-scaling factors rho(eff) for clear polystyrene and polymethylmethacrylate (PMMA) phantoms have been determined experimentally as a function of nominal electron-beam energy in the range 6 to 22 MeV. Values of rho(eff) have been calculated from the ratio rho(eff) = R(wat)(50) / R(med)(50), where R(wat)(50) and R(med)(50) are the measured depths of 50% ionization in electron solid water and plastic (clear polystyrene and PMMA) phantoms, respectively. Measurements were made using an Attix chamber in an electron solid water phantom, a Holt chamber in a clear polystyrene phantom, and a Markus chamber in a PMMA phantom. The average value of measured rho(poly)(eff) was found to be 0.999 +/- 0.009. This is higher than the value of 0.975 recommended by Task Group 25 (TG-25) of the American Association of Physicists in Medicine (AAPM) by 2.5%. Depending on energy, the maximum differences between the AAPM TG-25-recommended and the measured values lie in the range 1% to 3.5%. Similarly, the average value of measured rho(PMMA)(eff) was found to be 1.168 +/- 0.023. This is higher than the AAPM TG-25-recommended value of 1.115, by 5%. Depending on energy, the maximum differences between the AAPM TG-25-recommended and the measured values lie in the range 3% to 8%. Central axis depth dose curves in water were generated for 6, 15, and 20 MeV electron beams from measured depth-ionization data in PMMA and clear polystyrene phantoms following the recommendations of the AAPM TG-25 report and using both TG-25-recommended and experimentally determined values of depth-scaling factors rho(eff). For both phantoms, either the TG-25-recommended value or the experimentally determined values of rho(eff) yielded agreement to within about 2 mm among all depth doses in water at the depths of clinical relevance.

Shielding effects of metallic encapsulations and radiographic contrast agents for catheter-based intravascular brachytherapy.

PURPOSE/OBJECTIVE: Both photon- and beta-emitting radionuclides for intravascular brachytherapy (IVB) are under active investigation for prevention of restenosis following conventional angioplasty with or without stents. High atomic number materials are usually present in the coronary vessels undergoing treatment in the form of metallic encapsulations, stents, calcified plaque, or radiographic contrast agent. The high atomic number materials are likely to interfere with the photons and betas and, thus, change the dosimetry in the treatment volume. The purpose of this study is to investigate the shielding effects caused by the presence of high atomic number materials in IVB. MATERIALS AND METHODS: Dose rates at various distances in water, with and without the presence of various high atomic number materials, were calculated using Monte Carlo simulation techniques for photon and electron transport in extended media. The high atomic number materials investigated included titanium, stainless steel, calcified plaque, Hypaque, and Omnipaque. A wide range of monoenergetic photon and electron sources and several photon- and beta-emitting radionuclides, which have been under consideration for IVB, were used. The energy of the monoenergetic photon sources was in the range from 10 keV to 1 MeV, and that of the monoenergetic electron sources in the range from 0.5 to 2 MeV. Photon-emitting radionuclides (192)Ir, (125)I, and (103)Pd and beta-emitting radionuclides (90)Y, (32)P, and (188)Re were also considered. RESULTS: It was found that the high atomic number materials interfere considerably with the transport of photons of relatively low energies (below 40 keV) and all electron sources. When the energy of photon exceeds 100 keV, the interference becomes minimum for the high atomic number materials that are likely to be present in clinical situations. The shielding correction factors (SCFs) for dose rate at 2 mm from center were essentially 1.00 for photon energies above 100 keV; as the energy decreased below 100 keV, the SCF became smaller reaching a value of almost 0 for the lowest energy studied, 10 keV. For the photon source of (192)Ir, the SCF was essentially 1.00; while for the photon sources of (125)I and (103)Pd, shielding corrections were considerably lower than 1.00 depending on the type and thickness of the high atomic number material. For the beta emitting sources, the shielding effect can be expressed as a loss in effective penetration depth. This loss depends both on the material and its thickness. For titanium and stainless steel, the loss of range was about two and four times the thickness of the metal. CONCLUSIONS: The effects of high atomic number materials, such as metallic stents, calcified plaque, and contrast agents are minimal for high energy photon emitters, such as (192)Ir. The effects are pronounced for beta emitters and low energy photon emitters, and must be included in dosimetry planning.

Multicenter phase II trial of temozolomide in patients with glioblastoma multiforme at first relapse.

BACKGROUND: Recurrent glioblastoma multiforme (GBM) is resistant to most therapeutic endeavors, with low response rates and survival rarely exceeding six months. There are no clearly established chemotherapeutic regimens and the aim of treatment is palliation with improvement in the quality of life. PATIENTS AND METHODS: We report an open-label, uncontrolled, multicenter phase II trial of temozolomide in 138 patients (intent-to-treat [ITT] population) with glioblastoma multiforme at first relapse and a Karnofsky performance status (KPS) > or = 70. One hundred twenty-eight patients were histologically confirmed with GBM or gliosarcoma (GS) by independent central review. Chemotherapy-naïve patients were treated with temozolomide 200 mg/m2/day orally for the first five days of a 28-day cycle. Patients previously treated with nitrosourea-containing adjuvant chemotherapy received 150 mg/m2/day for the first five days of a 28-day cycle. In the absence of grade 3 or 4 toxicity, patients on the 150 mg/m2 dose schedule were eligible for a 200 mg/m2 dose on the next cycle. RESULTS: The primary endpoint was six-month progression-free survival assessed with strict radiological and clinical criteria. Secondary endpoints included radiological response and Health-related Quality of Life (HQL). Progression-free survival at six months was 18% (95% confidence interval (CI): 11%-26%) for the eligible-histology population. Median progression-free survival and median overall survival were 2.1 months and 5.4 months, respectively. The six-month survival rate was 46%. The objective response rate (complete response and partial response) determined by independent central review of gadolinium-enhanced magnetic resonance imaging (MRI) scans was 8% for both the ITT and eligible-histology populations, with an additional 43% and 45% of patients, respectively, having stable disease (SD). Objectively assessed response and maintenance of a progression-free status were both associated with HQL benefits (characterized by improvements over baseline in HQL domains). Temozolomide had an acceptable safety profile, with only 9% of therapy cycles requiring a dose reduction due to thrombocytopenia. There was no evidence of cumulative hematologic toxicity. CONCLUSIONS: Temozolomide demonstrated modest clinical efficacy, with an acceptable safety profile and measurable improvement in quality of life in patients with recurrent GBM. The use of this drug should be explored further in an adjuvant setting and in combination with other agents.

Dosimetric penumbra effects in catheter-based intravascular brachytherapy using a centered photon or beta line source.

Purpose: In catheter-based intravascular brachytherapy, either photon or beta emitters are often used in a linear arrangement so that blood vessels of 10-30 mm lengths can be treated. With a line source, the dose gradient in the radial direction and longitudinal direction depend on the type of radionuclides used in the treatment. The purpose of this study was to investigate the dose fall-off at the edges of a linear source in a blood vessel for different types of photon and beta emitters.Materials/Methods: Dose distributions were calculated on cylindrical blood vessels of various radii. Radioactive sources of 192Ir, 125I, 103Pd, 188Re, 32P, and 90Y/Sr were studied. All the sources were assumed to be in the form of a line. The dose rate at a point in space produced by a radioactive source was computed by integrating the point dose rate kernel of the corresponding radionuclide over the radioactive line. The point dose rate kernel was computed with Monte Carlo simulation of radiation transport. The edge effects were characterized with three newly defined quantities: longitudinal dose uniformity (LDU), effective coverage length (ECL), and margin length (ML). LDU was defined as the ratio of dose at a distance along the long axis of the vessel to the dose at center. ECL was defined as the length over which the LDU was greater than 0.95. ML was defined as half of the length difference between source length L and ECL, which is essentially the length segment at each edge that is covered by the source physical length but is being underdosed.Results: All beta emitters provided more uniform dose distributions and covered a larger portion of blood vessels longitudinally than photon emitters. Typical MLs were 2-3 mm for beta emitters and 4-6 mm for gamma emitters. As the radial depth of the point of interest increased, both the LDU and ECL decreased and ML increased. The ML increased from 2 to 3 mm for beta emitters and from 4 to 6 mm for photon emitters when the radial depth of the point of interest increased from 1.5 to 2.5 mm (typical proximal and distal media points for a 3-mm diameter lumen). The ML increased with increasing source length for all radionuclides. For beta emitters the ML increased initially from 1.5 mm to more than 2.5 mm as source length increased from 5 to 10 mm. When the source length was longer than 15 mm, the ML remains nearly constant, about 3 mm. For photon emitters, ML increased continuously from 1.5 mm to more than 6.0 mm, as source length increased from 5 to 50 mm.Conclusions: A formalism to quantify the dose uniformity along the length of a blood vessel undergoing catheter-based intravascular brachytherapy has been developed. This formalism was used to study the edge effects at the ends of several beta and photon sources. The results indicated that for a centered source the ML at each end due to penumbra effects was about 2 to 3 mm for beta emitters; about 4-6 mm for photon emitters. The ML increases as the radial depth of point of interest in the vessel increases. The ML increases also with increasing source length, especially for photon sources.

On the need for massive additional shielding of a catheterization laboratory for the implementation of high dose rate 192Ir intravascular brachytherapy.

Purpose: There is a widespread belief in the cardiology and radiation oncology community that high dose rate 192Ir intravascular brachytherapy cannot be implemented without massive additional shielding of the conventional catheterization labs. The purpose of this work is to show that this is a myth, which is not based on sound radiation protection principles.Methods: Exposure rates in air were calculated for a variety of point and line sources of 192Ir. Exposures per treatment at different distances from the source were calculated for a typical intravascular brachytherapy treatment of a 15-Gy dose at a radial distance of 2 mm from the source and for source lengths in the range of 0 to 10 cm. Additionally, exposure rates outside the catheterization lab were calculated for various lead shielding thicknesses typical of conventional X-ray facilities. These rates were used along with the NCRP recommendations on radiation facility design to assess shielding requirements.Results: For a treatment dose of 15 Gy at 2 mm, the occupational exposure per treatment at 2 m in air without any tissue attenuation or shielding was 7.8 mR for a lesion length of 3.0 cm. This exposure/treatment is independent of the dose rate or the activity of the source. However, it increases as lesion length is increased, increasing from 5.4 to 24.9 mR as lesion length increased from 2 to 10 cm. Exposures in unrestricted areas outside the catheterization lab using the NCRP shielding rationale can be kept below 2 mR per treatment and using appropriate workload, use, and occupancy factors below 2 mR per week.Conclusions: The feasibility of implementing a high dose rate 192Ir intravascular brachytherapy program in a catheterization laboratory is totally independent of the dose rate or the activity of the source. If it is feasible to implement 192Ir brachytherapy in a conventional catheterization lab using low activity 192Ir seeds, then it is also feasible to do so with a high activity 192Ir afterloader.

Dosimetric characterization of a newly designed encapsulated interstitial brachytherapy source of iodine-125-model LS-1 BrachySeed.

A newly designed encapsulated 125I source has been introduced (Model LS-1 BrachySeed manufactured by DRAXIMAGE Inc.) for interstitial brachytherapy. In this source 125I radionuclide is contained in two ceramic beads positioned at each end of a titanium capsule. The source contains a rod of Pt-Ir, which serves as a radiographic marker for source localization in the patient. Principle photon emissions are 27.4 and 31.0 keV X-rays and a 35.5 keV gamma-ray. The 22.2 and 25.5 keV silver X-rays produced by fluorescence of the silver dopant in the ceramic bead radioisotope carriers, are also emitted. In this work, the dosimetric characteristics of the 125I source were measured with micro LiF TLD chips and dosimetry parameters were characterized based upon the American Association of Physicists in Medicine, Task Group, No. 43 formalism. The corrected 1999 National Institute of Standards and Technology standard for low energy interstitial brachytherapy sources was used to specify the air kerma strength of the sources used in this study. The dose rate constant of the sources was determined to be 1.02+/-0.07 cGy h(-1) U(-1). The radial dose function was measured and was found to be similar to that of the silver-based model 6711 125I source. However, the anisotropy function of the Model LS-1 BrachySeed source is considerably better than that of model 6711 125I source, especially on the points along and close to the longitudinal axis of the source. The BrachySeed model LS-1 provides more isotropic angular dose distribution in tissue than model 6711 125I source. The anisotropy constant for the model LS-1 source was determined to be 1.006, which is considerably better than the value of 0.93 for the model 6711 source.

Dose distribution along the transverse axis of a new 125I source for interstitial brachytherapy.

A new encapsulated source of 125I has been introduced for interstitial brachytherapy. This source, isoSTAR model 12501 (manufactured by Imagyn Corp.), consists of a welded titanium tube containing 125I as silver iodide uniformly coated on five silver beads. The dose rate constant and the radial dose function for this source were measured using lithium fluoride thermoluminescent dosimeters in a Solid Water phantom. The value of the dose rate constant is 0.95 cGy h(-1)U(-1) where the unit of air kerma strength is 1 U = 1 cGy h(-1)cm2. The air kerma strength was traceable to the year 2000 primary air kerma strength standard for the model 12501 source at the National Institute of Technology and Standards. The radial dose function for the source was very similar to that for the model 6711 source (manufactured by Nycomed Amersham) for radial distances up to 6 cm. However, the radial dose function is lower in value than that for the model 6702 source (manufactured by Nycomed Amersham) and the model MED3631-A/M Iogold source (manufactured by North American Scientific).

A novel balloon angioplasty catheter impregnated with beta-particle emitting radioisotopes for vascular brachytherapy to prevent restenosis; first in vivo results.

BACKGROUND: According to early clinical trials, vascular brachytherapy performed prior to or shortly after angioplasty is very effective in reducing restenosis rates. The purpose of this study was to investigate the effects of a novel radioactive catheter that allows simultaneous balloon angioplasty and beta-particle irradiation in the prevention of restenosis. MATERIAL AND METHODS: The balloon surface of an angioplasty catheter was impregnated with the radioisotope(32)P. Dosimetry calculations using a Monte Carlo method were performed at a radial distance of 0.2 mm from the balloon surface. Rabbit iliac arteries were dilated and simultaneously irradiated with a dose of 20 Gy delivered to the adventitia. Control arteries were only dilated and not irradiated. Neointimal areas, cell numbers and the perimeter of the arteries were measured by histomorphometry after 6 weeks. RESULTS: Neointima formation was reduced after balloon dilatation and simultaneous beta-particle irradiation using the(32)P impregnated angioplasty catheter as compared to balloon dilatation alone with a non-impregnated catheter (0.09+/-0.06 vs 0.27+/-0.09 mm(2)neointimal area and 168+/-45 vs 360+/-133 cells/0.05 mm(2)neointima, P<0.001 vs control, respectively). In addition, balloon dilatation with the(32)P impregnated angioplasty catheter increased the vessel perimeter as compared to balloon dilatation with a non-impregnated catheter (4. 7+/-0.2 vs 3.9+/-0.3 mm, P<0.001 vs control). CONCLUSIONS: Simultaneous balloon dilatation and vascular brachytherapy with a novel(32)P impregnated angioplasty catheter markedly reduces restenosis in vivo by preventing neointimal hyperplasia and constrictive vascular remodelling.

A phase II study of temozolomide vs. procarbazine in patients with glioblastoma multiforme at first relapse.

A randomized, multicentre, open-label, phase II study compared temozolomide (TMZ), an oral second-generation alkylating agent, and procarbazine (PCB) in 225 patients with glioblastoma multiforme at first relapse. Primary objectives were to determine progression-free survival (PFS) at 6 months and safety for TMZ and PCB in adult patients who failed conventional treatment. Secondary objectives were to assess overall survival and health-related quality of life (HRQL). TMZ was given orally at 200 mg/m(2)/day or 150 mg/m(2)/day (prior chemotherapy) for 5 days, repeated every 28 days. PCB was given orally at 150 mg/m(2)/day or 125 mg/m(2)/day (prior chemotherapy) for 28 days, repeated every 56 days. HRQL was assessed using the European Organization for Research and Treatment of Cancer Quality of Life Questionnaire (EORTC QLQ-C30 [+3]) and the Brain Cancer Module 20 (BCM20). The 6-month PFS rate for patients who received TMZ was 21%, which met the protocol objective. The 6-month PFS rate for those who received PCB was 8% (P = 0.008, for the comparison). Overall PFS significantly improved with TMZ, with a median PFS of 12.4 weeks in the TMZ group and 8.32 weeks in the PCB group (P = 0.0063). The 6-month overall survival rate for TMZ patients was 60% vs. 44% for PCB patients (P = 0.019). Freedom from disease progression was associated with maintenance of HRQL, regardless of treatment received. TMZ had an acceptable safety profile; most adverse events were mild or moderate in severity.

Dosimetric effects of edema in permanent prostate seed implants: a rigorous solution.

PURPOSE: To derive a rigorous analytic solution to the dosimetric effects of prostate edema so that its impact on the conventional pre-implant and post-implant dosimetry can be studied for any given radioactive isotope and edema characteristics. METHODS AND MATERIALS: The edema characteristics observed by Waterman et al (Int. J. Rad. Onc. Biol. Phys, 41:1069-1077; 1998) was used to model the time evolution of the prostate and the seed locations. The total dose to any part of prostate tissue from a seed implant was calculated analytically by parameterizing the dose fall-off from a radioactive seed as a single inverse power function of distance, with proper account of the edema-induced time evolution. The dosimetric impact of prostate edema was determined by comparing the dose calculated with full consideration of prostate edema to that calculated with the conventional dosimetry approach where the seed locations and the target volume are assumed to be stationary. RESULTS: A rigorous analytic solution on the relative dosimetric effects of prostate edema was obtained. This solution proved explicitly that the relative dosimetric effects of edema, as found in the previous numerical studies by Yue et. al. (Int. J. Radiat. Oncol. Biol. Phys. 43, 447-454, 1999), are independent of the size and the shape of the implant target volume and are independent of the number and the locations of the seeds implanted. It also showed that the magnitude of relative dosimetric effects is independent of the location of dose evaluation point within the edematous target volume. It implies that the relative dosimetric effects of prostate edema are universal with respect to a given isotope and edema characteristic. A set of master tables for the relative dosimetric effects of edema were obtained for a wide range of edema characteristics for both (125)I and (103)Pd prostate seed implants. CONCLUSIONS: A rigorous analytic solution of the relative dosimetric effects of prostate edema has been derived for a class of edema characterized by Waterman et al. The solution proved that the dosimetric effects caused by the edema are universal functions of edema characteristics for a given isotope. It provides an efficient tool to examine the relative dosimetric effects of edema for any given edema characteristics and for any isotopes that may be considered for prostate implants.

Dosimetric effects of needle divergence in prostate seed implant using 125I and 103Pd radioactive seeds.

In prostate seed implants, radioactive seeds are implanted into the prostate through a guiding needle with the help of a template and real-time imaging. The ideal locations of the guiding needles and the relative positions of the seeds in each needle are determined before the implantation under the assumption that the needles inserted at different locations will remain parallel. In actual implantation, the direction of the needle is subject variation. In this work, we studied how the dosimetry quality of an implant may be affected when the guiding needles deviate from its planned orientations. Needle divergence of varying degree was simulated on spherical models and actual patient implants. It was found that needle divergence degraded the dosimetric quality of an implant: The minimum target dose, the target dose coverage and therefore the tumor biological effective dose were quantitatively decreased as compared to the reference implant. The magnitude of degradation increased almost linearly with respect to the magnitude of needle divergence. For iodine-125 implants, the average reduction in minimum target dose was about 10% and 20% for needle divergence of standard deviation of 5(0) and 10(0), respectively. The dose coverage in the target was reduced by about 1% and 3% for needle divergence of standard deviation of 5(0) and 10(0), respectively. Implants designed with palladium-103 showed additional 5% reduction in minimum target dose while the effect on dose coverage was about the same as compared to the iodine-125 implants. The degree of dosimetry degradation was shown to be dependent on the size of target volume, the seed spacing used, the use of seeding margin, and on the actual configuration of needle orientations in a given implant. One needs to minimize the physical causes of needle divergence in order to minimize its impact on planned dosimetry. The study suggests that the displacement between a needle image and its planned grid point at the base of prostate should be kept less than 5 mm in order to minimize the reduction in D(min)(<5%) and the increase in cell-survival (< a factor of 10) from the planned dosimetry.

Measurement of dose-rate constant for 103Pd seeds with air kerma strength calibration based upon a primary national standard.

Recent developments in the past two years require a significant change in the dosimetry of 103Pd brachytherapy sources (Theraseed model 200, manufactured by Theragenics Corp., Atlanta, GA). Since their introduction in 1987, the air kerma strength of 103Pd sources for interstitial brachytherapy has been determined using a system of apparent activity measurement based upon the measurement of photon fluence at a reference distance along the transverse axis of the source free in air, using a NaI (T1) scintillation detector at the manufacturer's facilities. This detection system has been calibrated against a National Institute of Standards and Technology (NIST)-traceable activity standard of a 109Cd source. This system produced a highly consistent standard (within +/-2%) for over 12 years, with the exception of the last 109Cd source change in September 1997, which resulted in a change of 9% from the original 1987 standard. The second major development affecting 103Pd dosimetry is that on 13 January 1999 a primary national standard for the air kerma strength of 103Pd seeds was developed by NIST. This primary standard is based upon an absolute measurement of air kerma rate free in air at a reference distance from the source along its transverse axis using a wide angle free air chamber (WAFAC). In order to implement this new standard for the calibration of source strength in clinical dosimetry for interstitial implants, it is necessary to measure the dose-rate constant for the 103Pd seeds using a calibration of source strength based on the NIST 99 standard. In this work, a measurement of the dose-rate constant using lithium fluoride (LiF) thermoluminescent dosimeters (TLDs) in a water equivalent solid phantom is reported. The measured value of this constant is 0.65 +/- 0.05 cGy h(-1) U(-1), where the unit air kerma strength is 1 U = 1 cGy h(-1) cm2 = 1 microGy h(-1) m2, and is directly traceable to the NIST 99 standard. The implementation of the NIST 99 standard for 103Pd should be accompanied by a simultaneous adoption of the new dose-rate constant reported here. No changes in radial dose function, anisotropy function, anisotropy factor, and geometry function are needed. However, a change in prescribed dose may be necessary to deliver the same physical dose as before.