Conceptus radiation dose assessment from fluoroscopically assisted surgical treatment of hip fractures

Our aims in the present study were to (a) provide normalized dose data for the estimation of the conceptus dose from fluoroscopically assisted surgical treatment of hip fractures carried out during all trimesters of pregnancy and (b) estimate the conceptus radiation dose and risks associated with fluoroscopy during a typical treatment of hip fracture performed on a pregnant woman. Conceptus doses normalized to entrance surface dose (ESD) or dose area product (DAP) were obtained with the help of anthropomorphic phantoms simulating pregnancy in the three trimesters of gestation. ESD and conceptus dose measurements were carried out using thermoluminescent dosimeters. DAP to conceptus dose conversion factors were estimated for the first, second and third trimesters of gestation. Conceptus dose data normalized to ESD were also estimated to investigate whether these conversion factors may be used for procedures carried out in x-ray units not equipped with a DAP meter. Fluoroscopically assisted surgical treatments were performed in 18 women. The projections involved in these procedures are (a) posteroanterior (PA) and (b) lateral crosstable 45 degrees (LC). Radiation doses for a potential conceptus were estimated by using normalized dose data obtained in phantoms. The results consist of tabulated dose data normalized to DAP or ESD for the estimation of a conceptus dose. An important finding of this study was that the total DAP of a procedure, instead of the individual DAP values of each projection, could be used for the accurate estimation of the conceptus dose. Conceptus doses calculated using dose data normalized to ESD are about 23% higher compared to those estimated using data normalized to DAP. This discrepancy may be attributed to the contribution of scattering radiation from PA projection to ESD measurement of LC projection and vice versa. Therefore, dose data normalized to ESD do not provide accurate conceptus dose estimation. Doses normalized to DAP showed a dependence on (a) tube potential and (b) tube filtration. Data are provided to extent the doses normalized to DAP for the standard spectrum to other tube voltages and filtrations. The maximum dose for a potential conceptus was 0.425 mGy for a patient irradiated for 50 seconds during the PA projection and for 40 seconds during the LC projection. Although the total duration of fluoroscopy is usually less than 2 minutes during a typical procedure, screening time as long as 14 minutes has been reported in the literature for treatment of complex fractures. The relationship between conceptus dose and fluoroscopy time found in the current study showed that, in these cases, the radiation dose received by a conceptus may exceed 1 mGy. In conclusion, an accurate estimation of conceptus doses associated with fluoroscopically assisted surgical treatment of hip fractures can be made using the DAP normalized dose data provided in this study. Conceptus doses from a typical procedure is less than 1 mGy during all trimesters.     

Normalized dose data for upper gastrointestinal tract contrast studies performed to infants

The aim of the current study was to (a) provide normalized dose data for the estimation of the radiation dose from upper gastrointestinal tract contrast (UGIC) studies carried out to infants and (b) estimate the average patient dose and risks associated with radiation from UGIC examinations performed in our institution. Organ and effective doses, normalized to entrance skin dose (ESD) and dose area product (DAP) were estimated for UGIC procedures utilizing the Monte Carlo N-particle (MCNP) transport code and two mathematical phantoms, one corresponding to the size of a newborn and one to the size of a 1-year-old child. The validity of the MCNP results was verified by comparison with dose data obtained in physical anthropomorphic phantoms simulating a newborn and a 1-year-old infant using thermoluminescence dosimetry (TLD). Data were also collected from 25 consecutive UGIC examinations performed to infants. Study participants were (a) 12 infants aged from 0.5 to 5.9 months (group 1) and (b) 13 infants aged from 6 to 15 months (group 2). For each examination, ESD and dose to comforters were measured using TLD. Patient effective doses were estimated using normalized dose data obtained in the simulation study. The risk for fatal cancer induction was estimated using appropriate coefficients. The results consist of tabulated dose data normalized to ESD or DAP for the estimation of patient dose. Conversion coefficients were estimated for various tube potentials and beam filtration values. The mean total fluoroscopy time was 1.26 and 1.62 min for groups 1 and 2, respectively. The average effective dose was 1.6 mSv for group 1 and 1.9 mSv for group 2. The risk of cancer attributable to the radiation exposure associated with a typical UGIC study was found to be up to 3 per 10 000 infants undergoing an UGIC examination. The mean radiation dose absorbed by the hands of comforters was 47 microGy. In conclusion, estimation of radiation doses associated with UGIC studies performed to infants can be made using the normalized dose data provided in the current study. Radiation dose values associated with UGIC examinations carried out to infants are not low and should be minimized as much as possible.     

Influence of z overscanning on normalized effective doses calculated for pediatric patients undergoing multidetector CT examinations

The purpose of this study was to evaluate the effect of z overscanning on normalized effective dose for pediatric patients undergoing multidetector-computed tomography (CT) examinations. Five commercially available mathematical anthropomorphic phantoms representing newborn, 1-, 5-, 10-, and 15-year-old patients and the Monte Carlo N-Particle (MCNP, version 4C2) radiation transport code were employed in the current study to simulate pediatric CT exposures. For all phantoms, axial and helical examinations at 120 kV tube voltage were simulated. Scans performed at 80 kV were also simulated. Sex-specific normalized effective doses were estimated for four standard CT examinations i.e., head-neck, chest, abdomen-pelvis, and trunk, for all pediatric phantoms. Data for both axial and helical mode acquisition were obtained. In the helical mode, z overscanning was taken into account. The validity of the Monte Carlo results was verified by comparison with dose data obtained using thermoluminescence dosimetry and a physical pediatric anthropomorphic phantom simulating a 10-year-old child. In all cases normalized effective dose values were found to increase with increasing z overscanning. The percentage differences in normalized data between axial and helical scans may reach 43%, 70%, 36%, and 26% for head-neck, chest, abdomen-pelvis, and trunk studies, respectively. Normalized data for female pediatric patients was in general higher compared to male patients for all ages, examined regions, and z overscanning values. For both male and female children, the normalized effective dose values were reduced as the age was increased. For the same typical exposure conditions, dose values decreased when lower tube voltage was used; for a 1-year-old child, for example, the effective dose was 3.8 times lower when 80 kV instead of 120 kV was used. Normalized data for the estimation of effective dose to pediatric patients undergoing standard axial and helical CT examinations on an multidetector CT system were calculated. This data was found to depend strongly on CT acquisition mode and exposure parameters as well as patient age and sex. The effective dose from a pediatric CT scan performed in axial mode was always considerably lower compared to the corresponding scan performed in helical mode, due to the additional tissue regions exposed to the primary beam in helical examinations as a result of z overscanning.     

Calculation of patient dose and somatic dose index due to roentgen examinations

In order to estimate the risk involved in x-ray examination for an individual or for a population, doses resulting from x-ray examinations are needed for organs particularly sensitive to radiation, i.e., the thyroid gland, the lungs, the breasts, and the bone marrow. A dose calculation program is presented based on a semiempirical formula involving exposure, backscatter factor, relative depth dose, and off-axis attenuation. Results have been compared with experimental measurements and Monte Carlo calculations presented in the literature. Doses to any point, organ doses, energy imparted, and somatic dose index can be calculated in phantoms of different sizes. The agreement between these and Monte Carlo results is probably sufficient for radiation protection risk estimates.     

Radiation dose in diagnostic radiology: Monte Carlo simulation studies

We applied Monte Carlo calculations to determine the radiation dose absorbed in water phantoms. Monoenergetic incident x-ray beams with energies from 15 to 100 keV and phantom thicknesses from 5 to 20 cm were considered in this study. We calculated the spatial distributions of energy absorption in the phantom, the rad/R conversion factors, the average rad/R conversion factors, and the scatter-to-primary ratios of absorbed dose. We also compared the relative absorbed doses under various imaging conditions when the transmitted radiation produced a given optical density on radiographic film. The information provided will be useful for the estimation of radiation doses in various radiographic procedures.     

The effect of z overscanning on radiation burden of pediatric patients undergoing head CT with multidetector scanners: a Monte Carlo study

The purpose of this study was to investigate the effect of z overscanning on eye lens dose and effective dose received by pediatric patients undergoing head CT examinations. A pediatric patient study was carried out to obtain the exposure parameters and data regarding the eye lens position with respect to imaged volume boundaries. This information was used to simulate CT exposures by Monte Carlo code. The Monte Carlo N-Particle (MCNP, version 4C2) radiation transport code and five mathematical anthropomorphic phantoms representing newborn, 1-, 5-, 10-, and 15-year-old patient, were employed in the current study. To estimate effective dose, the weighted computed tomography dose index was calculated by cylindrical polymethyl-methacrylate phantoms of 9.7, 13.1, 15.4, 16.1, and 16.9 cm in diameter representing the pediatric head of newborn, 1-, 5-, 10-, and 15-year-old individuals, respectively. The validity of the Monte Carlo calculated approach was verified by comparison with dose data obtained using physical pediatric anthropomorphic phantoms and thermoluminescence dosimetry. For all patients studied, the eye lenses were located in the region -1 to 3 cm from the first slice of the imaged volume. Doses from axial scans were always lower than those from corresponding helical examinations. The percentage differences in normalized eye lens absorbed dose between contiguous axial and helical examinations with pitch=1 were found to be up to 10.9%, when the eye lenses were located inside the region to be imaged. When the eye lenses were positioned 0-3 cm far from the first slice of region to be imaged, the normalized dose to the lens from contiguous axial examinations was up to 11 times lower than the corresponding values from helical mode with pitch=1. The effective dose from axial examinations was up to 24% lower than corresponding values from helical examinations with pitch=1. In conclusion, it is more dose efficient to use axial mode acquisition rather than helical scan for pediatric head examinations, if there are no overriding clinical considerations.     

On the use of Monte Carlo-derived dosimetric data in the estimation of patient dose from CT examinations

The purpose of this work was to investigate the applicability and appropriateness of Monte Carlo-derived normalized data to provide accurate estimations of patient dose from computed tomography (CT) exposures. Monte Carlo methodology and mathematical anthropomorphic phantoms were used to simulate standard patient CT examinations of the head, thorax, abdomen, and trunk performed on a multislice CT scanner. Phantoms were generated to simulate the average adult individual and two individuals with different body sizes. Normalized dose values for all radiosensitive organs and normalized effective dose values were calculated for standard axial and spiral CT examinations. Discrepancies in CT dosimetry using Monte Carlo-derived coefficients originating from the use of: (a) Conversion coefficients derived for axial CT exposures, (b) a mathematical anthropomorphic phantom of standard body size to derive conversion coefficients, and (c) data derived for a specific CT scanner to estimate patient dose from CT examinations performed on a different scanner, were separately evaluated. The percentage differences between the normalized organ dose values derived for contiguous axial scans and the corresponding values derived for spiral scans with pitch = 1 and the same total scanning length were up to 10%, while the corresponding percentage differences in normalized effective dose values were less than 0.7% for all standard CT examinations. The normalized organ dose values for standard spiral CT examinations with pitch 0.5-1.5 were found to differ from the corresponding values derived for contiguous axial scans divided by the pitch, by less than 14% while the corresponding percentage differences in normalized effective dose values were less than 1% for all standard CT examinations. Normalized effective dose values for the standard contiguous axial CT examinations derived by Monte Carlo simulation were found to considerably decrease with increasing body size of the mathematical phantom used. When the body-mass index was increased from 23.0 to 32.7 kg/m2 discrepancies in patient effective dose were up to 34%. The error in estimating effective dose from a CT exposure performed on a specific CT scanner using Monte Carlo data derived for a different CT scanner was estimated to be up to 25%. A simple method was proposed and validated for the determination of scanner-specific normalized dosimetric data from data derived from Monte Carlo simulation of a specific scanner. In conclusion, computed tomography dose index (CTDI) to effective dose conversion coefficients derived by Monte Carlo simulation of axial CT scans may provide a good approximation of corresponding coefficients applicable in helical scans. However, the use of Monte Carlo conversion coefficients for the estimation of patient dose from a CT examination involves a remarkable inaccuracy when the body size of the mathematical anthropomorphic phantom used in Monte Carlo simulation differs from the body of the patient. Therefore, separate sets of Monte Carlo dosimetric CT data shall be generated for different patient body sizes. Besides calculation of different sets of Monte Carlo data for each commercially available scanner is not necessary, since scanner specific data may be derived with acceptable accuracy from the Monte Carlo data calculated for a specific scanner appropriately modified for the different CTDI(W)/CTDI(air) ratio.     

Comparisons of point and average organ dose within an anthropomorphic physical phantom and a computational model of the newborn patient

Pediatric radiographic examinations yield medical benefits and/or diagnostic information that must be balanced against potential risk from patient radiation exposure. Consequently, clinical tools for measuring internal organ dose are needed for medical risk assessment. In this study, a physical phantom and Monte Carlo simulation model of the newborn patient were developed based upon their stylized mathematical expressions (ORNL and MIRD model series). The physical phantom was constructed using tissue equivalent substitutes for soft tissue, lung, and skeleton. Twenty metal-oxide-semiconductor field effect transistor (MOSFET) dosimeters were then inserted at three-dimensional positions representing the centroids of organs assigned in the ICRP's definition of the effective dose. MOSFET-derived point estimates of organ dose were shown to be in reasonable agreement with Monte Carlo estimates for representative newborn head, chest, and abdomen radiographic exams. Ratios of average organ dose assessed via MCNP simulations to the MOSFET-derived point doses (point-to-organ dose scaling factors, SF(POD)) are tabulated for subsequent use in clinical irradiations of the newborn phantom/MOSFET system. Values of SF(POD) indicate that MOSFET measurements of point dose for in-field exposures need to be adjusted only to within 10% to report volume-averaged organ dose. Larger adjustments to point doses are noted for organs out-of-field. For walled organs, point estimates of organ dose at the content centroid are shown to underestimate the average wall dose when the organ is within the primary field: SF(POD) of 1.19 for the stomach (AP chest exam), and SF(POD) of 1.15 for the urinary bladder (AP abdomen exam).     

Radiation doses and detriment from chest x-ray examinations

Radiation dose distributions for chest x-ray examinations have been measured in a Rando phantom for three views (AP, PA and lateral) as a function of kVp. On the basis of these data, the relationship between the surface dose, energy imparted and the effective dose equivalent have been determined. The mean energy imparted in a typical chest examination (PA + lateral views at 100 kVp) is 1.7 mJ and the corresponding value of the effective dose equivalent, HE, is 42 muSv. The measured radiation doses associated with chest x-rays were compared with the predictions of Monte Carlo calculations. The average difference between Monte Carlo and measured data for the HE was only about 16%. Demographic features (age/sex) of patients undergoing chest x-rays were investigated, and a population irradiation factor (PIF) introduced to estimate the radiation detriment to this population. The probability of expressed radiation-induced detriment to the patient population from chest x-ray examinations was computed to be about one half of that expected for a normal adult (working) population receiving the same dose. The radiation risk associated with chest x-ray examinations for this population was estimated to be less than 0.3 fatal cancers plus serious genetic disorders in the first two generations per million patient examinations.     

Experimental validation of Monte Carlo calculations with a voxelized Rando-Alderson phantom: a study on influence parameters

The development and improvement of techniques for an accurate dose assessment in medical physics is an important task. In this study, we focus on the validation of Monte Carlo calculations, by comparing organ doses assessed experimentally with thermoluminescent detectors in the Rando-Alderson phantom with doses calculated for a voxelized model of the same phantom for some typical x-ray procedures. A detailed study has been performed to identify the key parameters that affect the determination of organ doses. Initially, TLD measurements were up to 65% higher than the calculated values. After the corrections made on TLD energy dependence, TLD angular dependence, material composition and field size and position, most differences between measurements and calculations are within 15%. For organs far away from the field the difference is about 30%.     

Characterization of a novel micro-irradiator using Monte Carlo radiation transport simulations

Small animals are highly valuable resources for radiobiology research. While rodents have been widely used for decades, zebrafish embryos have recently become a very popular research model. However, unlike rodents, zebrafish embryos lack appropriate irradiation tools and methodologies. Therefore, the main purpose of this work is to use Monte Carlo radiation transport simulations to characterize dosimetric parameters, determine dosimetric sensitivity and help with the design of a new micro-irradiator capable of delivering irradiation fields as small as 1.0 mm in diameter. The system is based on a miniature x-ray source enclosed in a brass collimator with 3 cm diameter and 3 cm length. A pinhole of 1.0 mm diameter along the central axis of the collimator is used to produce a narrow photon beam. The MCNP5, Monte Carlo code, is used to study the beam energy spectrum, percentage depth dose curves, penumbra and effective field size, dose rate and radiation levels at 50 cm from the source. The results obtained from Monte Carlo simulations show that a beam produced by the miniature x-ray and the collimator system is adequate to totally or partially irradiate zebrafish embryos, cell cultures and other small specimens used in radiobiology research.     

Reduction of the gamma-ray component from 252Cf fission neutron source--optimization for biological irradiations and comparison with MCNP code.

Gamma-rays contribute 33% of the absorbed dose from an unfiltered 252Cf fission neutron source. To reduce this gamma-ray component and to enable radiobiological experiments at as high a dose rate as possible, Monte Carlo calculations for several filter materials (Al, Fe, Pb and concrete) have been made using MCNP neutron and photon transport code version 4a. A lead filter of thickness 4 cm was found to reduce the gamma-ray component to 6.7% of the total dose whilst reducing the neutron dose by only about 10%. Such a filter was installed at the MRC 252Cf neutron irradiation facility and dosimetric measurements were made using a TE-TE chamber and a 7LiF(Mg, Cu, P) TLD. Monte Carlo simulations agree with experimental measurements of neutron and gamma-ray doses within 6%. V79-4 Chinese hamster cells were irradiated with lead-filtered and unfiltered neutrons and also with 60Co gamma-rays at two dose rates. The survival fraction obtained for each radiation was consistent with the reduced gamma-ray dose. The relative biological effectiveness for neutrons alone, corrected for gamma-ray effects, was found to be 9.2 +/- 3.4 from the initial slopes and 3.1 +/- 0.5 at 10% survival, both relative to the acute gamma-rays.     

Organ and effective doses in pediatric patients undergoing helical multislice computed tomography examination

As multidetector computed tomography (CT) serves as an increasingly frequent diagnostic modality, radiation risks to patients became a greater concern, especially for children due to their inherently higher radiosensitivity to stochastic radiation damage. Current dose evaluation protocols include the computed tomography dose index (CTDI) or point detector measurements using anthropomorphic phantoms that do not sufficiently provide accurate information of the organ-averaged absorbed dose and corresponding effective dose to pediatric patients. In this study, organ and effective doses to pediatric patients under helical multislice computed tomography (MSCT) examinations were evaluated using an extensive series of anthropomorphic computational phantoms and Monte Carlo radiation transport simulations. Ten pediatric phantoms, five stylized (equation-based) ORNL phantoms (newborn, 1-year, 5-year, 10-year, and 15-year) and five tomographic (voxel-based) UF phantoms (9-month male, 4-year female, 8-year female, 11-year male, and 14-year male) were implemented into MCNPX for simulation, where a source subroutine was written to explicitly simulate the helical motion of the CT x-ray source and the fan beam angle and collimator width. Ionization chamber measurements were performed and used to normalize the Monte Carlo simulation results. On average, for the same tube current setting, a tube potential of 100 kVp resulted in effective doses that were 105% higher than seen at 80 kVp, and 210% higher at 120 kVp regardless of phantom type. Overall, the ORNL phantom series was shown to yield values of effective dose that were reasonably consistent with those of the gender-specific UF phantom series for CT examinations of the head, pelvis, and torso. However, the ORNL phantoms consistently overestimated values of the effective dose as seen in the UF phantom for MSCT scans of the chest, and underestimated values of the effective dose for abdominal CT scans. These discrepancies increased with increasing kVp. Finally, absorbed doses to select radiation sensitive organs such as the gonads, red bone marrow, colon, and thyroid were evaluated and compared between phantom types. Specific anatomical problems identified in the stylized phantoms included excessive pelvic shielding of the ovaries in the female phantoms, enhanced red bone marrow dose to the arms and rib cage for chest exams, an unrealistic and constant torso thickness resulting in excessive x-ray attenuation in the regions of the abdominal organs, and incorrect positioning of the thyroid within the stylized phantom neck resulting in insufficient shielding by clavicles and scapulae for lateral beam angles. To ensure more accurate estimates of organ absorbed dose in multislice CT, it is recommended that voxel-based phantoms, potentially tailored to individual body morphometry, be utilized in any future prospective epidemiological studies of medically exposed children.     

Assessment of different computational models for generation of x-ray spectra in diagnostic radiology and mammography

Different computational methods based on empirical or semi-empirical models and sophisticated Monte Carlo calculations have been proposed for prediction of x-ray spectra both in diagnostic radiology and mammography. In this work, the x-ray spectra predicted by various computational models used in the diagnostic radiology and mammography energy range have been assessed by comparison with measured spectra and their effect on the calculation of absorbed dose and effective dose (ED) imparted to the adult ORNL hermaphroditic phantom quantified. This includes empirical models (TASMIP and MASMIP), semi-empirical models (X-rayb&m, X-raytbc, XCOMP, IPEM, Tucker et al., and Blough et al.), and Monte Carlo modeling (EGS4, ITS3.0, and MCNP4C). As part of the comparative assessment, the K x-ray yield, transmission curves, and half value layers (HVLs) have been calculated for the spectra generated with all computational models at different tube voltages. The measured x-ray spectra agreed well with the generated spectra when using X-raytbc and IPEM in diagnostic radiology and mammography energy ranges, respectively. Despite the systematic differences between the simulated and reference spectra for some models, the student's t-test statistical analysis showed there is no statistically significant difference between measured and generated spectra for all computational models investigated in this study. The MCNP4C-based Monte Carlo calculations showed there is no discernable discrepancy in the calculation of absorbed dose and ED in the adult ORNL hermaphroditic phantom when using different computational models for generating the x-ray spectra. Nevertheless, given the limited flexibility of the empirical and semi-empirical models, the spectra obtained through Monte Carlo modeling offer several advantages by providing detailed information about the interactions in the target and filters, which is relevant for the design of new target and filter combinations and optimization of radiological imaging protocols.     

Monte Carlo study of TLD measurements in air cavities

Thermoluminescent dosimeters (TLDs) are used for verification of the delivered dose during IMRT treatment of head and neck carcinomas. The TLDs are put into a plastic tube, which is placed in the nasal cavities through the treated volume. In this study, the dose distribution to a phantom having a cylindrical air cavity containing a tube was calculated by Monte Carlo methods and the results were compared with data from a treatment planning system (TPS) to evaluate the accuracy of the TLD measurements. The phantom was defined in the DOSXYZnrc Monte Carlo code and calculations were performed with 6 MV fields, with the TLD tube placed at different positions within the cylindrical air cavity. A similar phantom was defined in the pencil beam based TPS. Differences between the Monte Carlo and the TPS calculations of the absorbed dose to the TLD tube were found to be small for an open symmetrical field. For a half-beam field through the air cavity, there was a larger discrepancy. Furthermore, dose profiles through the cylindrical air cavity show, as expected, that the treatment planning system overestimates the absorbed dose in the air cavity. This study shows that when using an open symmetrical field, Monte Carlo calculations of absorbed doses to a TLD tube in a cylindrical air cavity give results comparable to a pencil beam based treatment planning system.     

Determination of the tissue attenuation factor along two major axes of a high dose rate (HDR) 192Ir source.

Quantitative information on photon scattering around brachytherapy sources is needed to develop dose calculation formalisms capable of predicting dosimetric parameters with minimal empiricism. Photon absorption and scatter around brachytherapy sources can be characterized using the tissue attenuation factor, defined as the ratio of dose in water to water kerma in free space. In this study, the tissue attenuation factor along two major axes of a high dose rate (HDR) 192Ir source was determined by TLD measurements and MCNP Monte Carlo calculations. A calculational method is also suggested to derive the tissue attenuation factor along the longitudinal source axis from the factor along the transverse axis, using published anisotropy data as input. TLD and Monte Carlo results agreed with each other for both source axes within the statistical uncertainty (approximately +/- 5%) of Monte Carlo calculations. Comparison with published data, available only for the transverse source axis, also showed good agreement within +/- 5%. The shape and magnitude of the tissue attenuation factor are found to be remarkably different between the two axes. The tissue attenuation factor reaches a maximum value of about 1.4 at 8 cm from the source along the longitudinal source axis, while a maximum value of about 1.04 occurs at 3-4 cm from the source along the transverse axis. The calculated tissue attenuation factor along the longitudinal source axis generally reproduced the TLD and Monte Carlo results within +/- 5% at most radial distances.     

Comparison of EGS4 and MCNP4b Monte Carlo codes for generation of photon phase space distributions for a Varian 2100C

Monte Carlo based dose calculation algorithms require input data or distributions describing the phase space of the photons and secondary electrons prior to the patient-dependent part of the beam-line geometry. The accuracy of the treatment plan itself is dependent upon the accuracy of this distribution. The purpose of this work is to compare phase space distributions (PSDs) generated with the MCNP4b and EGS4 Monte Carlo codes for the 6 and 18 MV photon modes of the Varian 2100C and determine if differences relevant to Monte Carlo based patient dose calculations exist. Calculations are performed with the same energy transport cut-off values. At 6 MV, target bremsstrahlung production for MCNP4b is approximately 10% less than for EGS4, while at 18 MV the difference is about 5%. These differences are due to the different bremsstrahlung cross sections used in the codes. Although the absolute bremsstrahlung production differs between MCNP4b and EGS4, normalized PSDs agree at the end of the patient-independent geometry (prior to the jaws), resulting in similar dose distributions in a homogeneous phantom. EGS4 and MCNP4b are equally suitable for the generation of PSDs for Monte Carlo based dose computations.     

Scattered dose to thyroid from prophylactic cranial irradiation during childhood: a Monte Carlo study

The purpose of this study was to estimate the scattered dose to thyroid from prophylactic cranial irradiation during childhood. The MCNP transport code and mathematical phantoms representing the average individual at ages 3, 5, 10, 15 and 18 years old were employed to simulate cranial radiotherapy using two lateral opposed fields. The mean radiation dose received by the thyroid gland was calculated. A 10 cm thick lead block placed on the patient's couch to shield the thyroid was simulated by MCNP code. The Monte Carlo model was validated by measuring the scattered dose to the unshielded and shielded thyroid using three different humanoid phantoms and thermoluminescense dosimetry. For a cranial dose of 18 Gy, the thyroid dose obtained by Monte Carlo calculations varied from 47 to 79 cGy depending upon the age of the child. Appropriate placement of the couch block resulted in a thyroid dose reduction by 39 to 54%. Thyroid dose values at all possible positions of the radiosensitive gland with respect to the inferior field edge at five different patient ages were found. The mean difference between Monte Carlo results and thyroid dose measurements was 9.6%.     

An EGS4-ready tomographic computational model of a 14-year-old female torso for calculating organ doses from CT examinations.

Fifty-four consecutive CT scans have been used to construct a tomographic computational model of a 14-year-old female torso suitable for the determination of organ doses from CT. The model, known as ADELAIDE, is in the form of an input file compatible with user codes based on XYZDOS.MOR from the readily available EGS4 Monte Carlo radiation transport code. ADELAIDE's dimensions are close to the Australian averages for her age so the model is representative of a 14-year-old girl. The realistic anatomy in the model differs considerably from that in Cristy's 15-year-old mathematical computational model by having realistically shaped organs that are appropriately located within a real external contour. Average absorbed dose to organs from simulated CT examinations of the chest and abdomen have been calculated for ADELAIDE using EGS4 within a geometry specific to the General Electric Hi-Speed Advantage CT scanner and using an x-ray spectrum calculated using data from the scanner's x-ray tube. The simulations include the scanner's beam shaping filter and patient table. It is suggested that the resulting values have fewer possible sources of uncertainty than organ doses derived from dose coefficients calculated for a MIRD style model with mathematical anatomy and a spectrum that may not match that of the scanner. The organ doses were normalized using the scanner's CTDI measured free-in-air and an EGS4 simulation of the CTDI measurement. Effective dose to the torso from 26-slice chest and 24-slice abdomen examinations (at 120 kV, 200 mAs, 7 mm slices) is 4.6 +/- 0.1 mSv and 4.3 +/- 0.1 mSv respectively.     

Determination of dosimetrical quantities used in microbeam radiation therapy (MRT) with Monte Carlo simulations

Microbeam radiation therapy (MRT) is being performed by using an array of narrow rectangular x-ray beams (typical beam sizes 25 microm X 1 cm), positioned close to each other (typically 200 microm separation), to irradiate a target tissue. The ratio of peak-to-valley doses (PVDR's) in the composite dose distribution has been found to be strongly correlated with the normal tissue tolerance and the therapeutic effect of MRT. In this work a Monte Carlo (MC) study of the depth- and lateral-dose profiles in water for single x-ray microbeams of different shapes and energies has been performed with the MC code PENELOPE. The contributions to the dose deposition from different interaction types have been determined at different distances from the center of the microbeam. The dependence of the peak dose, in a water phantom, on the microbeam field size used in the preclinical trials, has been demonstrated. Composite dose distributions for an array of microbeams were obtained using superposition algorithms and PVDR's were determined and compared with literature results obtained with other Monte Carlo codes. The dependence of the PVDR's on microbeam width, x-ray energy used, and on the separation between adjacent microbeams has been studied in detail.