Absorbed Dose Distribution Research Articles

Retrospective dosimetry with the MAX/EGS4 exposure model for the radiological accident in Nesvizh-Belarus

On October 26, 1991 a fatal radiological accident occurred in a 60Co irradiation facility in the town of Nesvizh in Belarus. Following a jam in the product transport system, the operator entered the facility to clear the fault. On entering the irradiation room the operator bypassed a number of safety features, which prevented him from perceiving that the source rack was in the irradiation position. After the accident average whole body absorbed doses between 8 and 16 Gy have been determined by TLD measurements, by isodose rate distributions, by biological dosimetry and by ESR measurements of clothes and teeth. In an earlier investigation the MAX/EGS4 exposure model had been used to calculate absorbed dose distributions for the radiological accident in Yanango/Peru, which actually represented the simulation of exposure from a point source on the surface of the body. After updating the phantom as well as the Monte Carlo code, the MAX/EGS4 exposure model was used to calculate the absorbed dose distribution for the worker involved in the radiological accident in Nesvizh/Belarus. For this purpose, the arms of the MAX phantom had to be raised above the head, and a rectangular 60Co source was designed to represent the source rack used in the irradiation facility. Average organ absorbed doses, depth-absorbed doses, maximum absorbed dose and average whole body absorbed dose have been calculated and compared with the corresponding data given in the IAEA report of the accident.

Application of Macro Response Monte Carlo method for electron spectrum simulation

During the past years several variance reduction techniques for Monte Carlo electron transport have been developed in order to reduce the electron computation time transport for absorbed dose distribution. We have implemented the Macro Response Monte Carlo (MRMC) method to evaluate the electron spectrum which can be used as a phase space input for others simulation programs. Such technique uses probability distributions for electron histories previously simulated in spheres (called kugels). These probabilities are used to sample the primary electron final state, as well as the creation secondary electrons and photons. We have compared the MRMC electron spectra simulated in homogeneous phantom against the Geant4 spectra. The results showed an agreement better than 6% in the spectra peak energies and that MRMC code is up to 12 time faster than Geant4 simulations.

Three-Dimensional Radiobiologic Dosimetry: Application of Radiobiologic Modeling to Patient-Specific 3-Dimensional Imaging-Based Internal Dosimetry

Phantom-based and patient-specific imaging-based dosimetry methodologies have traditionally yielded mean organ-absorbed doses or spatial dose distributions over tumors and normal organs. In this work, radiobiologic modeling is introduced to convert the spatial distribution of absorbed dose into biologically effective dose and equivalent uniform dose parameters. The methodology is illustrated using data from a thyroid cancer patient treated with radioiodine. Three registered SPECT/CT scans were used to generate 3-dimensional images of radionuclide kinetics (clearance rate) and cumulated activity. The cumulated activity image and corresponding CT scan were provided as input into an EGSnrc-based Monte Carlo calculation: The cumulated activity image was used to define the distribution of decays, and an attenuation image derived from CT was used to define the corresponding spatial tissue density and composition distribution. The rate images were used to convert the spatial absorbed dose distribution to a biologically effective dose distribution, which was then used to estimate a single equivalent uniform dose for segmented volumes of interest. Equivalent uniform dose was also calculated from the absorbed dose distribution directly. We validate the method using simple models; compare the dose-volume histogram with a previously analyzed clinical case; and give the mean absorbed dose, mean biologically effective dose, and equivalent uniform dose for an illustrative case of a pediatric thyroid cancer patient with diffuse lung metastases. The mean absorbed dose, mean biologically effective dose, and equivalent uniform dose for the tumor were 57.7, 58.5, and 25.0 Gy, respectively. Corresponding values for normal lung tissue were 9.5, 9.8, and 8.3 Gy, respectively. The analysis demonstrates the impact of radiobiologic modeling on response prediction. The 57% reduction in the equivalent dose value for the tumor reflects a high level of dose nonuniformity in the tumor and a corresponding reduced likelihood of achieving a tumor response. Such analyses are expected to be useful in treatment planning for radionuclide therapy.

Dose distribution measurements by means of gel-layer dosimeters. Evaluation of algorithms for artifacts amendment

One of the major necessities that appears with the continuum development of conformal radiotherapies is a corresponding improvement of dosimetric techniques. Fricke gel-layer dosimetry, coupled with a suitably developed software, has proven to be a reliable technique for 3D absorbed dose distribution verification. The method is based on the measurement of optical transmittance of the gel-layers. The experimental assessment consists basically in a planar illumination source and a computer-controlled CCD camera for the acquisition of optical transmission images. Two possible sources of errors might alter the right dose distribution evaluation, namely defects in the gel structures and problems related to the non-uniform illumination source. Various algorithms for the detection and amendment of both kinds of artifacts were evaluated on a series of properly chosen gel-layers images. Dose profiles along the central axis of a phantom irradiated with different X-ray therapy fields and evaluated with the proposed algorithms were then compared with ionization chamber measurements. The obtained results indicate a good reliability of the proposed method.

Monte Carlo simulations of absorbed dose in a mouse phantom from 18-fluorine compounds.

The purpose of this study was to calculate internal absorbed dose distribution in mice from pre-clinical small animal PET imaging procedures with fluorine-18 labeled compounds (18FDG, 18FLT, and fluoride ion). The GATE Monte Carlo software and a realistic, voxel-based mouse phantom that included a subcutaneous tumor were used to perform simulations. Discretized time-activity curves obtained from dynamic in vivo studies with each of the compounds were used to set the activity concentration in the simulations. For 18FDG, a realistic range of uptake ratios was considered for the heart and tumor. For each simulated time frame, the biodistribution of the radionuclide in the phantom was considered constant, and a sufficient number of decays were simulated to achieve low statistical uncertainty. Absorbed dose, which was scaled to take into account radioactive decay, integration with time, and changes in biological distribution was reported in mGy per MBq of administered activity for several organs and uptake scenarios. The mean absorbed dose ranged from a few mGy/MBq to hundreds of mGy/MBq. Major organs receive an absorbed dose in a range for which biological effects have been reported. The effects on a given investigation are hard to predict; however, investigators should be aware of potential perturbations especially when the studied organ receives high absorbed dose and when longitudinal imaging protocols are considered.

A Quality-Control Method for SPECT-Based Dosimetry in Targeted Radionuclide Therapy

Dosimetry for targeted radionuclide therapy is necessary for treatment planning and radiation protection. Currently, there are no standard methods either for performing dosimetry or to evaluate the uncertainties inherent in the dosimetric calculations. In this paper, we present an experimental method using polymer gel dosimeters, whereby absorbed-dose distributions resulting from nonuniform distributions of activity may be determined directly from T(2) magnetic resonance imaging (MRI) as well as from scintigraphic images. A phantom containing a nonuniform distribution of I-131 was prepared by mixing 58 MBq of activity within the gel as it was solidifying. The resulting absorbed-dose distribution was determined directly from the MRI and from sequential single-photon emission computed tomography (SPECT) images using the Medical Internal Radiation Dose (MIRD) schema. The MRI data were quantified using 12 calibration vials uniformly irradiated by 0-12 MBq of I-131. The agreement between the two absorbed-dose maps was verified by convolving the MRI-based absorbed-dose map with the SPECT system point spread function, which gave a correlation coefficient of 0.96. It was seen that the absorbed-dose distribution, as imaged by the MRI, was misrepresented by the SPECT owing to its relatively poor spatial resolution, which included a shift of the voxel containing the maximum absorbed dose. This technique could provide an independent benchmark for assessing patient-specific dosimetry and, therefore, could be used as a basis for quality control for dosimetry.

The Use of BED And EUD Concepts in Heterogeneous Radioactivity Distributions on a Multicellular Scale for Targeted Radionuclide Therapy

There is evidence that nonuniform activity distributions within tumors might cause targeted radionuclide therapy (TRT) to fail. The aim of this study was to investigate the effects of the temporal and spatial behavior of the radioactivity in TRT, focusing on heterogeneous radiopharmaceutical distributions at a multicellular scale. Various activity distributions at the multicellular level from three radionuclides ((32)P, (90)Y, and (131)I) were simulated in cubic matrices (1- and 3-mm side). The in-house software package DOVE was used to calculate dose-rate maps, and survival fractions were calculated taking into account an up-take and a clearance phase. The effect from nonuniform activity distributions was analyzed in terms of dose volume histograms (DVHs), biologically effective dose (BED), and the effective uniform dose (EUD). The fraction of the absorbed dose that is "wasted," without producing a biological effect to the treatment, reaches 60% in the highly nonuniform distributions. For (32)P and (90)Y, the loss of therapeutic effectiveness was shown to be less than for (131)I. However, (90)Y, owing to its shorter physical half-life, presented lower mean BED values in almost every geometry, compared to (32)P and (131)I, and thus was less effective. (131)I, among all geometries, appeared to be more effective in more homogeneous activity distributions and in the 1-mm volume of interest, whereas it was the least effective radionuclide in the more heterogeneous activity distributions. (32)P presented the highest values of EUD, compared to (90)Y and (131)I. The EUD is a unique value that facilitates comparisons between different activity distributions in terms of treatment outcome. This study showed that as the degree of the heterogeneity in the dose distributions increases, the therapy effectiveness worsens. Nonuniform absorbed dose distributions can create a situation in which a fraction of cells are underirradiated, while another fraction of cells is "over-killed."

A 3-D Dosimeter for Complex-Shaped Foods Using Electron-Beam Irradiation

A chemical solution composed of paraffin wax, chloroform, and methyl yellow was shaped into a solid 3-D apple-phantom to determine absorbed dose from e-beams. The purpose of this study was to develop a 3-D dosimeter to visualize 3-D dose distribution in complex-shaped food products. Intensity data of an apple-phantom irradiated at 45° with a Van de Graaff accelerator was generated by slicing the phantom into 16 slices, each with 3.175 mm thickness. Three-dimensional representations were produced using the individual data for each phantom slice. The main advantage of using the apple-phantom is that the absorbed dose distribution can be visually seen in 3-D. It clearly helps to demonstrate that the dose distribution and dose depth profiles may be different depending on the reference plane chosen for the dosimetry analysis.

Challenges and progress in predicting biological responses to incorporated radioactivity.

Prediction of risks and therapeutic outcome in nuclear medicine largely rely on calculation of the absorbed dose. Absorbed dose specification is complex due to the wide variety of radiations emitted, non-uniform activity distribution, biokinetics, etc. Conventional organ absorbed dose estimates assumed that radioactivity is distributed uniformly throughout the organ. However, there have been dramatic improvements in dosimetry models that reflect the substructure of organs as well as tissue elements within them. These models rely on improved nuclear medicine imaging capabilities that facilitate determination of activity within voxels that represent tissue elements of approximately 0.2-1 cm(3). However, even these improved approaches assume that all cells within the tissue element receive the same dose. The tissue element may be comprised of a variety of cells having different radiosensitivities and different incorporated radioactivity. Furthermore, the extent to which non-uniform distributions of radioactivity within a small tissue element impact the absorbed dose distribution is strongly dependent on the number, type, and energy of the radiations emitted by the radionuclide. It is also necessary to know whether the dose to a given cell arises from radioactive decays within itself (self-dose) or decays in surrounding cells (cross-dose). Cellular response to self-dose can be considerably different than its response to cross-dose from the same radiopharmaceutical. Bystander effects can also play a role in the response. Evidence shows that even under conditions of 'uniform' distribution of radioactivity, a combination of organ dosimetry, voxel dosimetry and dosimetry at the cellular and multicellular levels can be required to predict response.

Investigation of an in-house laser CT scanner for reading normoxic gel

New radiation delivery techniques that aim at precise delivery of radiation dose such as Stereotactic Radiotherapy, 3D Conformal Radiotherapy (3DCRT) and Intensity Modulated Radiotherapy (IMRT) are more commonly used at present. These advanced techniques based on increased complexity of radiation delivery methods necessitate verification of computer calculated dose distribution by an accurate dosimetric method. Gel dosimeters are capable of recording dose distributions in 3D. To date, magnetic resonance imaging (MRI) has been most extensively used for the evaluation of absorbed dose distributions in gel dosimeters. However, due to the practical problems of availability, cost and artifacts, optical scanning of gel has been used by several researchers as an accurate and practical alternative to MRI. In this paper we discuss the construction of an in-house laser CT scanner and its performance for evaluating normoxic gel dosimeters.

High density dental materials and radiotherapy planning: Comparison of the dose predictions using superposition algorithm and fluence map Monte Carlo method with radiochromic film measurements

Background and purpose During radiotherapy planning high density dental materials create a major challenge in determining correct dose distribution inside patients with head-and-neck tumors. Patients and methods In this work we investigated the absorbed dose distribution inside a solid water ® slab phantom with embedded high density material irradiated by a 6 MV photon beam of field size 10 × 10 cm. We evaluated the absorbed dose distribution with three different techniques: superposition algorithm, radiochromic film, and the fluence map Monte Carlo (FMMC) method. Results The results obtained with radiochromic film and FMMC were in good agreement (within ±5% of the dose) with one another. The superposition algorithm, which is often considered superior to other commercially available dose calculation algorithms, produced appreciably less accurate results than FMMC. In particular, downstream from the high density cerrobend inhomogeneity the superposition algorithm predicts a higher dose than the measurement does by at least 10–16% depending upon the size of the inhomogeneity and the distance from it. Upstream of the high density inhomogeneities the superposition algorithm predicts a lower than measured dose due to its failure to predict the dose enhancement close to the inhomogeneity interface. Conclusions The delivered dose downstream from a high density inhomogeneity would be significantly less than the prescribed dose calculated by the superposition algorithm. The FMMC method which is based on a hybrid of the superposition algorithm input fluence data and Monte Carlo can be a useful tool in predicting dose in the presence of high density (e.g. dental) materials.

Administered activity and metastatic cure probability during radioimmunotherapy of ovarian cancer in nude mice with 211At-MX35 F(ab′)2

To elucidate the therapeutic efficacy of alpha-radioimmunotherapy of ovarian cancer in mice. This study: (i) estimated the minimum required activity (MRA), giving a reasonable high therapeutic efficacy; and (ii) calculated the specific energy to tumor cell nuclei and the metastatic cure probability (MCP) using various assumptions regarding monoclonal-antibody (mAb) distribution in measured tumors. The study was performed using the alpha-particle emitter Astatine-211 (211At) labeled to the mAb MX35 F(ab')2. Animals were inoculated intraperitoneally with approximately 1 x 10(7) cells of the cell line NIH:OVCAR-3. Four weeks later animals were treated with 25, 50, 100, or 200 kBq 211At-MX35 F(ab')2 (n = 74). Another group of animals was treated with a nonspecific mAb: 100 kBq 211At-Rituximab F(ab')2 (n = 18). Eight weeks after treatment the animals were sacrificed and presence of macro- and microscopic tumors and ascites was determined. An MCP model was developed and compared with the experimentally determined tumor-free fraction (TFF). When treatment was given 4 weeks after cell inoculation, the TFFs were 25%, 22%, 50%, and 61% after treatment with 25, 50, 100, or 200 kBq (211)At-MX35 F(ab')2, respectively, the specific energy to irradiated cell nuclei varying between approximately 2 and approximately 400 Gy. As a significant increase in the therapeutic efficacy was observed between the activity levels of 50 and 100 kBq (TFF increase from 22% to 50%), the conclusion was that the MRA is approximately 100 kBq (211)At-MX35 F(ab')2. MCP was most consistent with the TFF when assuming a diffusion depth of 30 mum of the mAbs in the tumors.

Radiological Characterization of Semiconductor Materials in Field Effect Transistor Dosimeter by Monte Carlo Method

The use of semiconductor materials in radiation processing, radiation therapy and diagnostics, and detection of cosmic radiation motivated development of numerical methods for its radiological characterization. This paper presents the application of the Monte Carlo method using the FOTELP-2K4 code for radiological characterization of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) dosimeter. The advantages of MOSFET dosimeters include small size, immediate readout, and ease of use for a wide photon energy range. In order to determine the dosimeter response accurately, distribution of the absorbed dose in the MOSFET structure has been investigated. Our results show that the absorbed dose distribution calculated by the presented simulation model compares well with the published data.

SU‐FF‐T‐264: Impact of Heterogeneous Activity Distributions On the Tumor Absorbed Dose Distribution for An In‐Vivo Colorectal Cancer Model

Purpose: To compare the heterogeneous uptake distribution of intact and domain deleted monoclonal antibodies with simulated activity distributions used to derive the dosimetric impact of heterogeneous uptake. Method and Materials: A total of 4 tumors with approximate masses of 0.1 g (∼5mm diameter) and 0.5 g (∼10mm diameter) were chosen for this study. Tumor activity distributions were reconstructed in 3D using serially sectioned LS174T tumors grown on the flank of athymic nude mice. The mice received an injection of either 177Lu‐PA‐DOTA‐HuCC49 or 177Lu‐PA‐DOTA‐HuCC49dCH2 monoclonal antibody. Simulated distributions of similar dimensions were created for comparison. All distributions were normalized to 100 uCi/g uptake and convolved with dose point kernels for a short range (I131) and a long range (Y90) isotope. Compared were radial profiles for activity density and dose rate, mean dose and tumor central dose. Results: The activity distributions are much more erratic for tumor uptake compared to the simulated distributions. There is also variation in minimum and maximum activity density between tumors. The simulated distributions under‐estimated the tumor size dependence of the average dose for both isotopes by about 10%. The central dose for the small tumor were overestimated for Y90 and bracketed for I131. The central dose for the larger tumors was under‐estimated for both isotopes. Conclusion: The simulated distributions did not well represent the large variation in uptake observed in tumors. Compared to the simulated distributions, dose due to Y90 performed better for the 0.5 g tumors, but worse for the 0.1 g tumors and dose due to I131 performed better for the 0.5 g tumors and similar for the 0.1 g tumors. The heterogeneity of uptake in tumors are not well represented by a single simulated distribution because of the wide variation in local uptake.

Properties of absorbed dose distribution in heterogeneous media

Accuracy of calculation of the absorbed dose spatial distribution into patient body is an important task in the radiation treatment of cancer. The correct calculation determines radiotherapy effectiveness. Thus, researches are improving calculation methods permanently to achieve running speed and accuracy increasing of used algorithms of calculation routines. The algorithms of routines for calculations of absorbed dose radial distributions into homo- and heterogeneous medium irradiated by directional source of photons are described in the presented work [1, 2].

患者呼吸模擬システムを用いた呼吸同期照射法に対する線量評価

Respiratory-gated (RG) radiotherapy is useful for minimizing the irradiated volume of normal tissues resulting from the shifting of internal structures caused by respiratory movement. In this technique, although improvement in the dose distribution of the target can be expected, the actual absorbed dose distribution is not clearly determined. Therefore, it is important to clarify the absorbed dose at the tumor and at the evaluation points according to the patient's respiration. We have developed a phantom system that simulates patient respiration (TNK Co., Ltd.), to evaluate the absorbed dose and ensure precise RG radiotherapy. Actual patient respiratory signals were obtained using a respiratory synchronization and gating system (AZ-733V, Anzai Medical). The acquired data were then transferred to a phantom system driven by a ball screw to simulate the shifting of internal structures caused by respiratory movement. We measured the absorbed dose using a micro-ionization chamber dosimeter and the dose distribution using the film method for RG irradiation at expiratory phase by using Linac (PRIMUS, Toshiba Medical Systems Corp.) X-rays. When the distance of phantom movement was set to the average patient respiratory movement distance of 1.5 cm, we first compared absorbed dose with RG irradiation with a gating signal of 50% or less, and without RG irradiation. The absorbed dose at the iso-center was improved by 6.0% and 4.4% at a field size of 4x4 cm2, and by 1.3% and 0.7% at a field size of 5x5 cm2 with an X-ray energy of 6 MV and 10 MV, respectively. There was, however, no dose change at a field size of 10x10 cm2 and 15x15 cm2. When the gating signal was reduced to 25% and 10%, absorbed dose was also improved. With regard to the flatness of the dose profile, no changes in dose distribution were observed in the lateral direction, e.g., beam flatness was within 1.4% and 1.6% at field sizes of 5x5 cm2 and 10x10 cm2, respectively, with an X-ray energy of 6 MV. In the cranial-caudal direction, the dose profile was relatively large even if a gating signal of 50% was applied, i.e., 8.1% and 10.4% at field sizes of 5x5 cm2 and 10x10 cm2, respectively. Beam flatness without RG was much worse, i.e., 37.8% and 38.2%, at field sizes of 5x5 cm2 and 10x10 cm2, respectively. In both cases, the dose was insufficient in the expiratory direction. Although RG radiotherapy is quite useful, the margins in the inspiratory and expiratory phases should be considered based on the level of gating signal and field size in order to formulate appropriate radiotherapy planning in terms of the shifting of internal structures. To ensure accurate radiotherapy, the characteristics of the RG irradiation technique and the radiotherapy equipment must be clearly understood when this technique is to be employed in clinical practice.

Virtual commissioning of a treatment planning system for proton therapy of ocular cancers

The virtual commissioning of a treatment planning system (TPS) for ocular proton beam therapy was performed using Monte Carlo (MC) simulations and a model of a double-scattering ocular treatment nozzle. The simulations produced both the input data required by the TPS and the dose distributions to validate the analytical predictions from the TPS. An MC simulation of a typical ocular melanoma treatment was compared with the TPS predictions, revealing generally good agreement in the absorbed dose distribution. However, in the depth-dose profiles, differences >5% existed in the proximal region of all validation cases considered. Comparison of the radiation coverage at or above the 90% dose level, showed that MC calculated coverage was 82% and 68% of the coverage calculated by the TPS in two planes intersecting the tumour.

106Ru/106Rh Plaque and proton radiotherapy for ocular melanoma: a comparative dosimetric study

The objective of this study was to perform comparative dosimetric studies of both 106Ru/106Rh plaque brachytherapy and external beam proton therapy proposed for ocular treatments at the University of Texas M. D. Anderson Cancer Center, Houston, TX, USA. These modalities were also compared with traditional 125I plaque brachytherapy. Using a standardised eye model with a representative ocular melanoma tumour, the relative dose distributions within the tumour and surrounding tissue were calculated using the Monte Carlo code MCNPX. Published absorbed dose distributions benchmarked the Monte Carlo models. Results indicate that the proton beam provided superior dose uniformity within the tumour volume, whereas the dose distribution from 106Ru/106Rh was more heterogeneous. Relative to 125I COMS plaque, both 106Ru/106Rh and protons have shown more confined dose distributions to the tumour volume in this situation, thus sparing other critical ocular structures. For protons, it has been shown that only doses lower than the maximum dose are delivered outside the tumour volume. Depending on the clinical situation, this may aid in the sparing of critical structures located in the sclera and optic disc boundary. The Monte Carlo model's statistical uncertainties of the mean dose estimates for the 106Ru/106Rh plaque and proton beam were 3 and 2.5%, respectively.

The ICRU (International Commission on Radiation Units and Measurements): its contribution to dosimetry in diagnostic and interventional radiology

The ICRU (International Commission on Radiation Units and Measurements was created to develop a coherent system of quantities and units, universally accepted in all fields where ionizing radiation is used. Although the accuracy of dose or kerma may be low for most radiological applications, the quantity which is measured must be clearly specified. Radiological dosimetry instruments are generally calibrated free-in-air in terms of air kerma. However, to estimate the probability of harm at low dose, the mean absorbed dose for organs is used. In contrast, at high doses, the likelihood of harm is related to the absorbed dose at the site receiving the highest dose. Therefore, to assess the risk of deterministic and stochastic effects, a detailed knowledge of absorbed dose distribution, organ doses, patient age and gender is required. For interventional radiology, where the avoidance of deterministic effects becomes important, dose conversion coefficients are generally not yet developed.

Development of Three Kinds of Tissue Substitutes for a Physical Phantom in Neutron Dosimetry

Three kinds of tissue substitutes were synthesized in order to develop a physical phantom for neutron dosimetry. The optimal elemental compositions and densities were determined using a Monte Carlo simulation method, based on absorbed dose distributions of neutrons of a few MeV in various materials. The characteristics of the synthesized tissue substitutes were verified by absorbed dose measurements using monoenergetic neutron sources and a 252Cf neutron source. It was found that the developed tissue substitutes, NAN-, HAI- and HONE-JAERI, have tissue equivalence to actual tissues within an accuracy of 10% for neutrons in most of the common fast neutron fields.