MIRD Phantom Research Articles

MCNP APPROACHES FOR DOSE RATES MODELING IN LABORATORY FOR NEUTRON ACTIVATION ANALYSIS AND GAMMA SPECTROMETRY AT OSTRAVA.

The Laboratory for Neutron Activation Analysis and Gamma Spectrometry at the VŠB-Technical University of Ostrava was equipped with the neutron generator MP320 operating on the principle of the deuterium-tritium fusion and producing 108 neutrons/s at maximum. To ensure radiation protection of radiation workers and public outside the laboratory, the concrete shielding was designed and its protection efficiency was validated by MCNP simulations. Three approaches to calculate the dose rates were compared. The dose rates were estimated for the ORNL MIRD phantom located at the relevant positions (Tally F6 and *F8) and using the MCNPX mesh tally feature with the new ICRP Publication 116 flux-to-dose conversion factors. It was proven that the Approach II in which the absorbed dose rates due to neutrons for all organs are computed using the cell tally F6 and the photon dose calculation is performed by the *F8 energy deposition tally is the most valuable one.

Evaluation of Organ Specific peripheral dose for Gamma knife 4C based on Monte Carlo

Introduction: Stereotactic Gamma Knife radiosurgery has been widely used for treating brain tumors. The scattered radiation outside of treatment field (peripheral dose) can induce the secondary cancer to specific organ. This paper investigated the absorbed dose to eyes, thyroid, heart, lung, breast and colon using a Monte Carlo technique for Mird phantom. We also study the effect of depth of tumor and helmet size on the peripheral dose. Materials and Methods: In this work, gamma knife 4C was simulated as 201 surfaces disc source modelling. Based on this model, parallel beams of uniformly distributed method emitted form 201 disk sources that correspond to inner aperture for helmet. The photon beams converged at the isocenter of gamma knife. The coordinates of centers of these discs as planes to be determined using Matlab software. The 201 disc sources simulated for 4, 8, 14 and 18 mm helmet and also various tumor depth using mcnpx. Results: Large differences were observed between the organs close to target volume such as eyes and thyroid respect to organ far from target volume. The eyes received the maximum dose and the left eye dose was more than right one. Significant differences weren’t observed for organ peripheral dose with increase helmet size. The peripheral organ doses were increased with decrease of isocenter point as result of decrease depth of tumor due to decrease scattered radiation. Conclusion: Treatment planning should be considered gamma knife peripheral peripheral doses for organs near to target volume specially in surface tumor.

Optimum neutron energy simulation in treatment of head and neck cancer at different depths in the BNCT method

Introduction: Recently head and neck cancer has pay attention to many researchers. Its therapeutic methods include surgery, chemotherapy, radiotherapy and Boron neutron capture therapy (BNCT). BNCT is better than conventional radiotherapy because it targets the tumor cell. This method involves two steps of infusion of stable 10B and then neutron radiation with a suitable intensity and energy. The BNCT in combination with boronphenylalanine (BPA) and borocaptate sodium (BSH) that was make using the epithermal neutron. BSH and PBA are used as 10B carriers. Epithermal neutrons reach to thermal transiting through tissues of the body. When 10B absorbed thermal neutrons, the α and 7Li particles produced in the 10B (n, α) 7Li reaction are of high linear energy. Transfer radiation have a short range of one cell diameter. Materials and Methods: Monte Carlo simulations were performed with MCNPX2.6 and RO31 MIRD phantom. The neutron source was employed the surface disk with 10 diameters and the range of energy was considered from 1ev-10Kev. The results of neutron and gamma dose at various depths was calculated using tally F4 and F6 in MCNPX2.6 code. Results: Relative Dose was obtained at various depths based on energy changes for gamma, fast and thermal neutron. The results of this study have shown increases of optimum energy as the tumor get deeper respect to the skin. In addition, an analytical relation was proposed for energy optimization with the position of the tumor. Conclusion: The optimum neutron energy dependence was investigated for neck tumor in different depths. These results provide useful information to the physicians to choice best optimum energy neutron beam in BNCT method.

Verification of evaluation accuracy of absorbed dose in the dose-evaluation system for iridium-192 brachytherapy for treatment of keloids

192Ir high-dose-rate superficial brachytherapy (HDR-SBT) has been delivered as a postoperative radiotherapy to prevent recurrence of keloids in Nippon Medical School Hospital. However, extra radiation exposure for organs at risk is a concern because high-energy gamma rays penetrate more deeply than electrons at the energies typically used for therapy. Therefore, a system which can evaluate absorbed dose in the affected area and radiation exposure to tissue and organs based on the actual geometry in HDR-SBT was developed using the PHITS Monte Carlo code and a MIRD-5 phantom in our previous work. In this study, the absorbed dose was measured in a simple geometry using a water-equivalent phantom and radio-photoluminescence glass dosimeters to verify the evaluation accuracy of absorbed dose calculated by the developed system. We find that, accounting for the transit dose component and the energy dependence analytically, the calculations by PHITS underestimate the absorbed dose by 10.2% to 19.0% (average 15.5% ± 1.9%) compared to the measurements. The sensitivity-correction procedure was considered to be the main reason for the difference between the measured and calculated absorbed dose of the dosimeters.

Photoneutron Dose Estimation in GRID Therapy Using an Anthropomorphic Phantom: A Monte Carlo Study.

Background:In the past, GRID therapy was used as a treatment modality for the treatment of bulky and deeply seated tumors with orthovoltage beams. Now and with the introduction of megavoltage beams to radiotherapy, some of the radiotherapy institutes use GRID therapy with megavoltage photons for the palliative treatment of bulky tumors. Since GRID can be a barrier for weakening the photoneutrons produced in the head of medical linear accelerators (LINAC), as well as a secondary source for producing photoneutrons, therefore, in terms of radiation protection, it is important to evaluate the GRID effect on photoneutron dose to the patients.Methods:In this study, using the Monte Carlo code MCNPX, a full model of a LINAC was simulated and verified. The neutron source strength of the LINAC (Q), the distributions of flux (φ), and ambient dose equivalent (H*[10]) of neutrons were calculated on the treatment table in both cases of with/without the GRID. Finally, absorbed dose and dose equivalent of neutrons in some of the tissues/organs of MIRD phantom were computed with/without the GRID.Results:Our results indicate that the GRID increases the production of the photoneutrons in the LINAC head only by 0.3%. The calculations in the MIRD phantom show that neutron dose in the organs/tissues covered by the GRID is on average by 48% lower than conventional radiotherapy. In addition, in the uncovered organs (by the GRID), this amount is reduced to 25%.Conclusion:Based on the findings of this study, in GRID therapy technique compared to conventional radiotherapy, the neutron dose in the tissues/organs of the body is dramatically reduced. Therefore, there will be no concern about the GRID effect on the increase of unwanted neutron dose, and consequently the risk of secondary cancer.

Evaluation of radiation exposure in Ir-192 brachytherapy for treatment of keloids

Nippon Medical School Hospital recently developed a high dose rate superficial brachytherapy (HDR-SBT) for keloids, which has been carried out since 2008. In this treatment, Ir-192 sources are placed on the affected area through flexible tubes, which has the advantage of providing high dose concentration to complicated shapes. Obviously, unnecessary radiation exposure to high-energy gamma rays must be reduced for vital organs around the affected area. We have developed a system which can evaluate the radiation exposure of tissues and organs in a MIRD-5 phantom using the PHITS Monte Carlo code when the prescribed dose was delivered uniformly to a flat surface based on the actual geometry in HDR-SBT. In the PHITS simulation, an affected area was assumed on the chest of the MIRD-5 phantom, then the dwell time at each dwell position was optimized to deliver 6 Gy uniformly over the affected area in the actual geometry with the lead shields and the bolus. In addition, the radiation exposures of each tissue and organ in the MIRD-5 phantom during the irradiation were calculated. The biologically equivalent dose of the thymus was the highest at 1145.60 mGy followed by the heart at 236.14 mGy and the breasts at 95.32 mGy.

Systematic investigation on the validity of partition model dosimetry for 90Y radioembolization using Monte Carlo simulation

We aimed to investigate the validity of the partition model (PM) in estimating the absorbed doses to liver tumour (), normal liver tissue () and lungs (), when cross-fire irradiations between these compartments are being considered. MIRD-5 phantom incorporated with various treatment parameters, i.e. tumour involvement (TI), tumour-to-normal liver uptake ratio (T/N) and lung shunting (LS), were simulated using the Geant4 Monte Carlo (MC) toolkit. 108 track histories were generated for each combination of the three parameters to obtain the absorbed dose per activity uptake in each compartment (, , and ). The administered activities, A were estimated using PM, so as to achieve either limiting doses to normal liver, or lungs, (70 or 30 Gy, respectively). Using these administered activities, the activity uptake in each compartment (, , and ) was estimated and multiplied with the absorbed dose per activity uptake attained using the MC simulations, to obtain the actual dose received by each compartment. PM overestimated by 11.7% in all cases, due to the escaped particles from the lungs. and by MC were largely affected by T/N, which were not considered by PM due to cross-fire exclusion at the tumour-normal liver boundary. These have resulted in the overestimation of by up to 8% and underestimation of by as high as −78%, by PM. When was estimated via PM, the MC simulations showed significantly higher for cases with higher T/N, and LS ⩽ 10%. All and by MC were overestimated by PM, thus were never exceeded. PM leads to inaccurate dose estimations due to the exclusion of cross-fire irradiation, i.e. between the tumour and normal liver tissue. Caution should be taken for cases with higher TI and T/N, and lower LS, as they contribute to major underestimation of . For , a different correction factor for dose calculation may be used for improved accuracy.

SU‐G‐IeP3‐10: Molecular Imaging with Clinical X‐Ray Sources and Compton Cameras

Purpose:The application of Compton cameras (CC) is a novel approach translating XFCT to a practical modality realized with clinical CT systems without the restriction of pencil beams. The dual modality design offers additional information without extra patient dose. The purpose of this work is to investigate the feasibility and efficacy of using CCs for volumetric x‐ray fluorescence (XF) imaging by Monte Carlo (MC) simulations and statistical image reconstruction.Methods:The feasibility of a CC for imaging x‐ray fluorescence emitted from targeted lesions is examined by MC simulations. 3 mm diameter water spheres with various gold concentrations and detector distances are placed inside the lung of an adult human phantom (MIRD) and are irradiated with both fan and cone‐beam geometries. A sandwich design CC composed of Silicon and CdTe is used to image the gold nanoparticle distribution. The detection system comprises four 16×26 cm2 detector panels placed on the chest of a MIRD phantom. Constraints of energy‐, spatial‐resolution, clinical geometries and Doppler broadening are taken into account. Image reconstruction is performed with a list‐mode MLEM algorithm with cone‐projector on a GPU.Results:The comparison of reconstruction of cone‐ and fan‐beam excitation shows that the spatial resolution is improved by 23% for fan‐beams with significantly decreased processing time. Cone‐beam excitation increases scatter content disturbing quantification of lesions near the body surface. Spatial resolution and detectability limit in the center of the lung is 8.7 mm and 20 fM for 50 nm diameter gold nanoparticles at 20 mGy.Conclusion:The implementation of XFCT with a CC is a feasible method for molecular imaging with high atomic number probes. Given constrains of detector resolutions, Doppler broadening, and limited exposure dose, spatial resolutions comparable with PET and molecular sensitivities in the fM range are realizable with current detector technology.

Monte Carlo investigation of prostate cancer ion – therapy by using SOBP technique in the GEANT4 toolkit and MCNPX code

Regarding the useful results concerned with an external radio-therapy in treatment of tumors, we consider in this paper a standard model of the human prostate phantom based on MIRD phantom for the Monte Carlo simulation in GEANT4 toolkit and also on MCNPX code for a prostate cancer treatment. We calculate the lateral as well as the dose profiles in the tumor region for both proton and alpha beams in a similar range, and finally having implemented the SOBP technique, we compare the results of the two beams in the corresponding codes used in this analysis.

Evaluation of organ doses and effective dose according to the ICRP Publication 110 reference male/female phantom and the modified ImPACT CT patient dosimetry.

We modified the Imaging Performance Assessment of CT scanners (ImPACT) to evaluate the organ doses and the effective dose based on the International Commission on Radiological Protection (ICRP) Publication 110 reference male/female phantom with the Aquilion ONE ViSION Edition scanner. To select the new CT scanner, the measurement results of the CTDI100,c and CTDI100,p for the 160 (head) and the 320 (body) mm polymethylmethacrylate phantoms, respectively, were entered on the Excel worksheet. To compute the organ doses and effective dose of the ICRP reference male/female phantom, the conversion factors obtained by comparison between the organ doses of different types of phantom were applied. The organ doses and the effective dose were almost identical for the ICRP reference male/female and modified ImPACT. The results of this study showed that, with the dose assessment of the ImPACT, the difference in sex influences only testes and ovaries. Because the MIRD‐5 phantom represents a partially hermaphrodite adult, the phantom has the dimensions of the male reference man including testes, ovaries, and uterus but no female breasts, whereas the ICRP male/female phantom includes whole‐body male and female anatomies based on high‐resolution anatomical datasets. The conversion factors can be used to estimate the doses of a male and a female accurately, and efficient dose assessment can be performed with the modified ImPACT.PACS number: 87.53.LY, 87.57.Q‐, 87.57.‐s

Conversion factors for determining organ doses received by paediatric patients in high-resolution single slice computed tomography with narrow collimation

Estimations of organ doses DT received during computed tomographic examinations are usually performed by applying conversion factors to basic dose indicators like the computed tomography dose index (CTDI) or the dose-length-product (DLP). In addition to the existing conversion factors for beam apertures of 5 mm or 10 mm, we present new DLP-DT conversion factors adapted to high-resolution CT (HRCT) examinations of infants and young children with beam apertures of the order of 1 mm and under consideration of bow tie filtration. Calculations are performed on mathematical MIRD phantoms for an age range from 0, 1, 5, 10, 15 up to (for comparison) 30 years by adapting PCXMC, a Monte Carlo algorithm originally developed by STUK (Helsinki, Finland) for dose reconstructions in projection radiography. For this purpose, each single slice CT examination is approximated by a series of corresponding virtual planar radiographies comprising all focus positions. The transformation of CT exposure parameters into exposure parameters of the series of corresponding planar radiographies is performed by a specially developed algorithm called XCT. The DLP values are evaluated using the EGSRay code. The new method is verified at a beam aperture of 10 mm by comparison with formerly published conversion factors. We show that the higher spatial resolution leads to an enhanced DLP-DT conversion factor if a small organ (e. g. thyroid gland, mammae, uterus, ovaries, testes) is exactly met by the chosen CT slice, while the conversion factor is drastically reduced if the chosen CT slice is positioned above or below the organ. This effect is utilized for dose-saving examinations with only a few single slices instead a full scan, which technique is applied in about 10% of all paediatric chest CT examinations.

Exposure for medical staff from patients during/after 89Sr therapy

89 Sr therapy patients. The Hp(10) for medical workers was measured using personal dosimeters and was found to be <1 μSv during the measurement period. Hp(0.07) for medical workers did not exceed 5 μSv per case during routine work, although the dose on the patient's skin surface was about 20 times that on the medical staff. To investigate the contribution of electrons to the measured dose, the dose on the patient's skin surface as obtained from uniformly distributed 89 Sr in a similar MIRD phantom was calculated using EGS5 code. The peak energy of the incident photons was approximately 0.6 MeV. The estimated Hp(10) and Hp(0.07) doses for the medical staff were 0.43 and 91 μSv/h, respectively. The contribution of electrons to the calculated Hp(0.07) was approximately 200 times that of photons. Thus, the high Hp(0.07) for medical staff is attributed to the strong contribution of electrons.

Bio-Distribution and Dosimetry of a Renal Agent in Normal Bangladeshi Subjects

Radiation absorbed dose estimation was performed on eleven normal patients who were in a process of routine diagnostic investigation of the renal function. Bio-kinetics and bio-distribution of 99mTc-DTPA in patients was evaluated by dual-head gamma camera imaging and blood-plasma sample counting method. Radiation dose estimations were performed using standard MIRD techniques and biodistribution of different organ was estimated by drawing region of interest (ROI) according to MIRD phantom model [1]. From the time-activity curves, cumulative activities and residence times of 99mTc- DTPA in the kidneys, brain, upper large intestine (ULI), small intestine (SI), lower large intestine (LLI), stomach, heart, liver, lung and remainder of the body was calculated. Using the information of residence times of the total body and urinary bladder voiding at 2.4 hours on MIRD 12 absorbed dose for the 99mTc-DTPA in different target organs of the body was measured [2]. The estimated average absorbed dose to the kidneys as a target organ in normal Bangladeshis are 5.71E-03 mGy/MBq of 99mTc-DTPA which is closer to the ICRP 53 and other recent published data. The calculated effective dose equivalent and effective dose was found 5.72E-03 mSv/MBq and 4.89E-03 mSv/MBq respectively. DOI: http://dx.doi.org/10.3329/bjmp.v4i1.14674 Bangladesh Journal of Medical Physics Vol.4 No.1 2011 21-26

Estimation of specific absorbed fractions for selected organs due to photons emitted by activity deposited in the human respiratory tract using ICRP/ICRU male voxel phantom in FLUKA

The ICRP/ICRU adult male reference voxel phantom incorporated in Monte Carlo code FLUKA is used for estimating specific absorbed fractions (SAFs) for photons due to the presence of internal radioactive contamination in the human respiratory tract (RT). The compartments of the RT, i.e. extrathoracic (ET1 and ET2) and thoracic (bronchi, bronchioles, alveolar interstitial) regions, lymph nodes of both regions and lungs are considered as the source organs. The nine organs having high tissue weighting factors such as colon, lungs, stomach wall, breast, testis, urinary bladder, oesophagus, liver and thyroid and the compartments of the RT are considered as target organs. Eleven photon energies in the range of 15 keV to 4 MeV are considered for each source organ and the computed SAF values are presented in the form of tables. For the target organs in the proximity of the source organ including the source organ itself, the SAF values are relatively higher and decrease with increase in energy. As the distance between source and target organ increases, SAF values increase with energy and reach maxima depending on the position of the target organ with respect to the source organ. The SAF values are relatively higher for the target organs with smaller masses. Large deviations are seen in computed SAF values from the existing MIRD phantom data for most of the organs. These estimated SAF values play an important role in the estimation of equivalent dose to various target organs of a worker due to intake by inhalation pathway.

Development of a web-based CT dose calculator: WAZA-ARI

A web-based computed tomography (CT) dose calculation system (WAZA-ARI) is being developed based on the modern techniques for the radiation transport simulation and for software implementation. Dose coefficients were calculated in a voxel-type Japanese adult male phantom (JM phantom), using the Particle and Heavy Ion Transport code System. In the Monte Carlo simulation, the phantom was irradiated with a 5-mm-thick, fan-shaped photon beam rotating in a plane normal to the body axis. The dose coefficients were integrated into the system, which runs as Java servlets within Apache Tomcat. Output of WAZA-ARI for GE LightSpeed 16 was compared with the dose values calculated similarly using MIRD and ICRP Adult Male phantoms. There are some differences due to the phantom configuration, demonstrating the significance of the dose calculation with appropriate phantoms. While the dose coefficients are currently available only for limited CT scanner models and scanning options, WAZA-ARI will be a useful tool in clinical practice when development is finalised.

Pediatric dosimetry for intrapleural lung injections of 32P chromic phosphate

Intracavitary injections of 32P chromic phosphate are used in the therapy of pleuropulmonary blastoma and pulmonary sarcomas in children. The lung dose, however, has never been calculated despite the potential risk of lung toxicity from treatment. In this work the dosimetry has been calculated in target tissue and lung for pediatric phantoms. Pleural cavities were modeled in the Monte Carlo code MCNP within the pediatric MIRD phantoms. Both the depth–dose curves in the pleural lining and into the lung as well as 3D dose distributions were calculated for either homogeneous or inhomogeneous 32P activity distributions. Dose–volume histograms for the lung tissue and isodose graphs were generated. The results for the 2D depth–dose curve to the pleural lining and tumor around the pleural cavity correspond well with the point kernel model-based recommendations. With a 2 mm thick pleural lining, one-third of the lung parenchyma volume gets a dose more than 30 Gy (V30) for 340 MBq 32P in a 10 year old. This is close to lung tolerance. Younger children will receive a larger dose to the lung when the lung density remains equal to the adult value; the V30 relative lung volume for a 5 year old is 35% at an activity of 256 MBq and for a 1 year old 165 MBq yields a V30 of 43%. At higher densities of the lung tissue V30 stays below 32%. All activities yield a therapeutic dose of at least 225 Gy in the pleural lining. With a more normal pleural lining thickness (0.5 mm instead of 2 mm) the injected activities will have to be reduced by a factor 5 to obtain tolerable lung doses in pediatric patients. Previous dosimetry recommendations for the adult apply well down to lung surface areas of 400 cm2. Monte Carlo dosimetry quantitates the three-dimensional dose distribution, providing a better insight into the maximum tolerable activity for this therapy.

Development of Female ATOM-MIRD Hybrid Voxel Model for Monte Carlo Dose Calculations

A hybrid-type female computational model was constructed by combining the tomographic voxel model of the ATOM adult female physical phantom with the mathematical organ models of the MIRD5 stylized phantom. The constructed model has 13 internal organs: 5 organs from the ATOM phantom and the other 8 organs from the MIRD5 phantom. The model was employed in Monte Carlo dose calculations with MCNPX to calculate organ doses, and the calculated values were compared with the values of Regina for reference external irradiation conditions.

Monte-Carlo simulation of uncertainty in the estimation of 125I in the thyroid

At the Bhabha Atomic Research Centre, a thin (76 mm diameter x 2 mm thickness) NaI (Tl) detector is used for the assessment of (125)I in the thyroid of the radiation workers engaged in the preparation of radio-immunoassay kits. The detector was calibrated using a REMCAL (radiation equivalent manikin calibration) phantom with a known amount of the (125)I activity filled in its thyroidal cavity. Since (125)I emits low-energy photons ranging from 27 to 35.4 keV, its detection efficiency depends on several parameters such as neck-to-detector distance, detector size, unknown tissue thickness overlying (OTT) the thyroid and the shape and size of the thyroid. To account for uncertainties introduced by these factors in the estimation of (125)I, a computer program based on the Monte Carlo photon transport techniques was developed. The program simulates the detector response and the corresponding detection efficiencies using two thyroid models: (1) revised MIRD head phantom and (2) Ulanvosky model. The program has been validated with experimental measurements carried out using a REMCAL phantom. The computed values of uncertainties due to placement errors (+/-0.5 cm) for different detector sizes, differences in the OTT of the thyroid (0.6-2.0 cm) and different thyroid shapes are presented in this paper. The computed values of the calibration factors, determined for the revised MIRD phantom, varied from 5.23 to 1.06 x 10(-2) counts per photon for detector distance of 3-12 cm and from 7.53 to 3.66 x 10(-2) counts per photon for OTT varying from 0.6 to 2.0 cm keeping the detector at a distance of 3 cm. This study shows that the variations in OTT constitute a major source of uncertainty. The computed uncertainties due to various parameters should be taken into account while estimating the thyroidal burden of (125)I in the radiation workers. The feasibility of using coincidence method for absolute determination of the (125)I activity in the thyroid is also discussed in this paper.

TU‐C‐304A‐03: Stylized MIRD Phantoms Should Be Replaced by Anatomically Realistic Phantoms: Discrepancies In Red Bone Marrow Doses From CT Scans

Purpose: To test the hypothesis that the stylized MIRD phantoms would cause significant error in the estimated red bone marrow (RBM) dose from CT scans in comparison with anatomically realistic phantoms. Method and Materials: The MC model of the CT scanner include the source geometry, movement, source energy spectrum, bow‐tie filter, as well and the beam shape. MCNPX 2.5.0 was used to simulate the RBM dose from various CT scanning procedures. To calculate the absorbed dose to the RBM as a function of photon fluence in the spongiosa and the photon energy, an F4 tally together with a set of DE/DF cards in MCNPX were used to score the photon fluence in MCNPX. The stylized MIRD phantom and the anatomically realistic RPI Adult Male and Adult Female phantoms were implemented in the MCNPX to determine organ doses using the same dose algorithm. Results: For all the cases studied, the RBM doses calculated using RPI adult phantoms were gearter than those obtained from MIRD‐ORNL phantoms. For the chest CT scan, the RBM dose ratio (RPI‐AM to MIRD‐ORNL) is about 1.50 (1.48–1.51), and RBM dose ratio of female phantoms is about 1.28 (1.24–1.32). For the abdominal‐pelvis CT scan, the RBM dose ratios are 1.30 (1.28–1.31) and 1.32 (1.28–1.38) for male and female phantom, respectively. These differences are mainly from the anatomical differences in the phantoms. Conclusion: As the RBM is not uniformly distributed in the human body, the homogeneous bone mixtures definition by MIRD phantoms underestimated the dose by as much as 50% in certain cases. This test concludes that the simplified MIRD phantoms used in existing CT dose software should and can be replaced by realistic phantoms. This is an opportunity to improve the anatomical realism and therefore the associated dose and risk assessments for patients who undergo CT examinations.

WE-B-351-02: Patient Dose Estimates From CT Examinations

Radiation dose from computed tomography has been an important issue for medical physicists for some time. This issue has increased in significance in the past several years due to advances in multidetector CT, which have resulted in increased utilization in areas such as pediatric CT, cardiac CT and even screening applications. Some of the key issues currently facing the medical physics community are assessing dose to patients from these various exams. One of the key building blocks to these assessments is the estimation of organ dose. The purpose of this presentation is to describe an approach to estimating organ doses to patients using Monte Carlo-based simulation methods. In this approach, both the scanner and patient are modeled in some detail and a CT exam may be simulated. The detailed modeled of the CT scanner is created by including information such as the source spectra and filtration, its geometry, beam collimation and the path that the source travels around the patient (such as the path during a helical CT exam). The development, testing and validation of these models will be discussed. Patient models generally fall into two categories. The first consists of geometric descriptions of organs (based on cones, cylinders, etc.) such as the MIRD phantom. The second consists of voxelized descriptions of patient anatomy that are created based on actual patient scans. In these, radiosensitive organs are identified in the image data to create a voxelized model of the patient geometry. For both types of models, there are challenges to create models representing patients of different sizes, ages and genders. Once both the scanner and patient are modeled, then different scan protocols can be simulated using a Monte Carlo based software package (such as MCNP or EGS). This involves selecting a scanner model, a patient model and then selecting a set of technical parameters, such as one would do for an actual scan — including body region being examined, etc. The Monte Carlo software then simulates the specified scan and tallies absorbed dose in each voxel or geometric unit of the patient model, which then allows the calculation of either mean organ dose or the distribution of dose within an organ. In this presentation, the results of this approach in several applications will be described, including: (a) dose to the fetus in pregnant women of early, middle and late gestational ages, (b) comparing dose to glandular breast tissue from thoracic CT scans both with and without tube current modulation. Educational Objectives: 1. Understand the Monte Carlo simulation based approach to estimating radiation dose to radiosensitive organs from CT scans. 2. Understand the current limitations of the Monte Carlo based approach. 3. Describe the results of some current applications of this approach to estimating fetal dose as well as breast dose reduction from tube current modulation.