Age-dependent organ and effective dose coefficients for external photons: a comparison of stylized and voxel-based paediatric phantoms

Evaluation of the Correction Methods Using Age and BMI for Estimating CT Organ Dose Using a Radiophotoluminescence Glass Dosimeter and a Monte Carlo-based Dose Calculator

ABSTRACT

The size-specific dose estimates (SSDE) have been recommended to replace the volume computed tomography dose index (CTDIvol) because it takes patient size into account. On the other hand, organ dose is thought to be a more appropriate quantity in the radiation protection field due to its correlation with radiation risk. The web-based computed tomography (CT) dose calculator WAZA-ARIv2 only offers organ doses for adults with four different body shapes and for children with five different ages. Since the American Association of Physicists in Medicine (AAPM) offers the conversion factors for SSDE and the correlation of SSDE with organ dose has been demonstrated, implementation of the conversion table might improve the accuracy of WAZA-ARIv2. This study aimed to evaluate a body mass index (BMI)-based and age-based correction method for estimation of the organ dose by using a radiophotoluminescence dosimeter (RGD), an anthropomorphic phantom, and the dose calculator WAZA-ARIv2. RGDs were individually calibrated by using an ISOVOLT TITAN-320 x-ray generator. The ratio of the SSDE conversion factors (CFSSDE) was used as the comparison index. For the BMI-based correction method, the ratio of CFSSDE values for the adult phantoms was expected to be 1.065, and the average ratio of the organ doses for the adult phantoms was 1.163 ± 0.169. For the age-based correction method, the ratio of CFSSDE value for 5- and 10-y-old pediatric phantoms was expected to be 0.889, and the ratios of the organ doses were 0.866 ± 0.024 and 0.909 ± 0.047 for the WAZA-ARIv2 dosimetry system and RGD dosimetry system, respectively. In conclusion, both evaluations of the experimental results showed the consistency between WAZA-ARIv2 and the SSDE conversion factor table. Moreover, the importance of taking the measurement position into account when applying the mass attenuation coefficient was demonstrated according to this study.

Tabulation of organ dose conversion factors for terrestrial radioactivity monitoring program

Terrestrial radioactivity monitoring of ²³⁸U and ²³²Th series, and ⁴⁰K in soil is an essential practice for radioactivity and radiation measurement of a place. In conventional practice, only basic data can be in-situ measured using a survey instrument, for example radioactivity concentration in soil and ambient dose equivalent rate. For other physical quantities, for example organ absorbed dose and organ equivalent dose, the measurement is impossible to be performed and can only be computed using Monte Carlo radiation transport simulations. In the past, most of the works only focused on calculating air-kerma-to-effective dose conversion factors. However, the information on organ dose conversion factors is scarcely documented and reported. This study was conducted to calculate organ absorbed and tissue-weighted equivalent dose conversion factors as a result of exposure from terrestrial gamma radiation. Series of organ dose conversion factors is produced based on computations from Monte Carlo MCNP5 simulations using modelled gamma irradiation geometry and established adult MIRD phantom. The study found out that most of the radiation exposed organs absorb energy at comparable rates, except for dense and superficial tissues i.e., skeleton and skin, which indicated slightly higher values. The good agreement between this work and previous studies demonstrated that our gamma irradiation geometry and modelling of gamma radiation sources are adequate. Therefore, the proposed organ dose conversion factors from this study are reasonably acceptable for dose estimation in environmental radioactivity monitoring practices.

A comparative study on dispersed doses during photon and proton radiation therapy in pediatric applications

The Monte Carlo method was employed to simulate realistic treatment situations for photon and proton radiation therapy for a set of Oak Ridge National Laboratory (ORNL) pediatric phantoms for 15, 10, 5 and 1-year olds as well as newborns. Complete radiotherapy situations were simulated using the previously developed NRUrad input code for Monte Carlo N-Particle (MCNP) code package. Each pediatric phantom was irradiated at five different positions, namely, the testes, colon, liver, left lung and brain, and the doses in targeted organs (Dt) were determined using the track length estimate of energy. The dispersed photon and proton doses in non-targeted organs (Dd), namely, the skeleton, skin, brain, spine, left and right lungs were computed. The conversion coefficients (F = Dd/Dt) of the dispersed doses were used to study the dose dispersion in different non-targeted organs for phantoms for 15, 10, 5 and 1-year olds as well as newborns. In general, the F values were larger for younger patients. The F values for non-targeted organs for phantoms for 1-year olds and newborns were significantly larger compared to those for other phantoms. The dispersed doses from proton radiation therapy were also found to be significantly lower than those from conventional photon radiation therapy. For example, the largest F values for the brain were 65.6% and 0.206% of the dose delivered to the left lung (P4) for newborns during photon and proton radiation therapy, respectively. The present results demonstrated that dispersion of photons and generated electrons significantly affected the absorbed doses in non-targeted organs during pediatric photon therapy, and illustrated that proton therapy could in general bring benefits for treatment of pediatric cancer patients.

Stylized versus voxel phantoms: a juxtaposition of organ depth distributions

For external irradiation, the variability in organ dose estimation found between computational phantom generations arises particularly from the differences in organ positioning. This work represents the first effort to quantify the differences in organ depth below the body surface between a stylized and voxel phantom series. Herein, the revised Oak Ridge National Laboratory stylized phantom series and the University of Florida/National Cancer Institute voxel phantom series were compared. Both series include whole-body models of the newborn; the 1-, 5-, 10-, and 15-year-old; and the adult human. Organ depths from eight different directions applicable to external irradiation geometries were computed: antero-posterior, postero-anterior, left and right lateral, rotational, isotropic, cranial and caudal directions. Organ depths in the stylized phantoms were computed using a ray-tracing technique available through Monte Carlo radiation transport simulations in MCNP6. Organ depths in the voxel phantom were found using phantom matrix manipulation. Resultant organ depths for both series were plotted as distributions; available are twenty-four organs and two bone tissue distributions for each of six phantom ages and in each of the eight directional geometries. Quantitative data descriptors (e.g. mean and median depths) were also tabulated. For demonstration purposes, a literature review of relevant stylized versus voxel comparison works was performed to explore where the quantification of organ depth differences can provide further insight or evidence to study conclusions. The entire dataset of organ depth distributions and their data descriptors can be found in online supplementary files.

A model for estimating the total absorbed dose to the thyroid in Swedish inhabitants following the Chernobyl Nuclear Power Plant accident: implications for existing international estimates and future model applications

The time-integrated absorbed dose to the thyroid gland in the years after a fallout event can indicate the potential excess number of thyroid cancers among young individuals after a radionuclide release. Typical mean values of the absorbed dose to the thyroid have been calculated previously using reported data on radioiodine obtained from air sampling and dairy milk surveys in Sweden after the Chernobyl fallout, not including the contribution from ¹³⁴Cs and ¹³⁷Cs. We have developed a model for Swedish conditions taking these additional dose contributions into account. Our estimate of the average time-integrated absorbed dose to the thyroid, D _th,tot, during the first 5 years after fallout ranged from 0.5-4.1 mGy for infants and from 0.3-3.3 mGy for adults. The contribution to D _th,tot from ¹³¹I through inhalation and milk consumption varied considerably among different regions of Sweden, ranging from 9%-79% in infants, and from 4%-58% in adults. The external irradiation and exposure from the ingestion of ^134,137Cs in foodstuffs accounted for the remaining contributions to D _th,tot (i.e. up to 96% for adults). These large variations can be explained by the highly diverse conditions in the regions studied, such as different degrees of fractionation between wet and dry deposition, different grazing restrictions on dairy cattle, and differences in ^134,137Cs transfers through food resulting from differences in the local fallout. It is our conclusion that the main contribution to D _th,tot from nuclear power plant fallout in areas subjected to predominantly wet deposition will be from external exposure from ground deposition, followed by internal exposure from contaminated food containing the long-lived fission product ¹³⁷Cs and the neutron-activated fission product ¹³⁴Cs. The contribution from ^134,137Cs to the thyroid absorbed dose should thus be taken into account in future epidemiological studies.

Two-step validation of a Monte Carlo dosimetry framework for general radiology

The Monte Carlo technique is considered gold standard when it comes to patient-specific dosimetry. Any newly developed Monte Carlo simulation framework, however, has to be carefully calibrated and validated prior to its use. For many researchers this is a tedious work. We propose a two-step validation procedure for our newly built Monte Carlo framework and provide all input data to make it feasible for future related application by the wider community. The validation was at first performed by benchmarking against simulation data available in literature. The American Association of Physicists in Medicine (AAPM) report of task group 195 (case 2) was considered most appropriate for our application. Secondly, the framework was calibrated and validated against experimental measurements for trunk X-ray imaging protocols using a water phantom. The dose results obtained from all simulations and measurements were compared. Our Monte Carlo framework proved to agree with literature data, by showing a maximal difference below 4% to the AAPM report. The mean difference with the water phantom measurements was around 7%. The statistical uncertainty for clinical applications of the dosimetry model is expected to be within 10%. This makes it reliable for clinical dose calculations in general radiology. Input data and the described procedure allow for the validation of other Monte Carlo frameworks.

Pulmonary embolism in pregnant patients: Assessing organ dose to pregnant phantom and its fetus during lung imaging

PURPOSE

The purpose of this study was to provide updated radiation dose from diagnostic exams performed for pregnant patients suspected of pulmonary embolism (PE) using the recently developed BREP phantoms of pregnant woman and the fetus. Also to challenge the validity of current recommendations suggest that ventilation/perfusion (V/Q) single photon emission computed tomography (SPECT) vs. computed tomography pulmonary angiography (CTPA) should be considered for diagnosis of PE in radiosensitive groups such as pregnant women.

METHODS

The Monte Carlo calculations involving detailed geometrical simulation of pregnant women and the fetus were performed.

RESULTS

The results showed that when radiation dose to the fetus is of concern, CTPA is more appropriate at early stages causes 50%-97% lower fetal doses for the first two trimesters of pregnancy. While for gestational periods more than 6 months, V/Q SPECT leads to a 15% lower fetal dose and thus, is less hazardous. The fetal dose from CTPA increases with gestational age, while that from V/Q SPECT decreases. Furthermore, the maximum amount of fetal dose is received by fetal skeleton (i.e., on average about 1.8 and 3.9 times larger dose from SPECT and CT, respectively).

CONCLUSIONS

V/Q SPECT should not always be preferred for pregnant patients suspected of PE. This finding is in contrast with the guidance to choose the preferred modality based on the maternal effective dose. The reason of this issue was discussed in this paper based on chord length distributions (CLDs). The importance of considering fetal organs separately in MC calculations was also highlighted.

Dose coefficients for ICRP reference pediatric phantoms exposed to idealised external gamma fields

Organ and effective dose coefficients have been calculated for the International Commission on Radiological Protection (ICRP) reference pediatric phantoms externally exposed to mono-energetic photon radiation (x- and gamma-rays) from 0.01 to 20 MeV. Calculations used Monte Carlo radiation transport techniques. Organ dose coefficients, i.e., organ absorbed dose per unit air kerma (Gy/Gy), were calculated for 28 organs and tissues including the active marrow (or red bone marrow) for 10 phantoms (newborn, 1 year, 5 year, 10 year, and 15 year old male and female). Radiation exposure was simulated for 33 photon mono-energies (0.01-20 MeV) in six irradiation geometries: antero-posterior (AP), postero-anterior, right lateral, left lateral, rotational, and isotropic. Organ dose coefficients for different ages closely agree in AP geometry as illustrated by a small coefficient of variation (COV) (the ratio of the standard deviation to the mean) of 4.4% for the lungs. The small COVs shown for the effective dose and AP irradiation geometry reflect that most of the radiosensitive organs are located in the front part of the human body. In contrast, we observed differences in organ dose coefficients across the ages of the phantoms for lateral irradiation geometries. We also observed variation in dose coefficients across different irradiation geometries, where the COV ranges from 18% (newborn male) to 38% (15 year old male) across idealised whole body irradiation geometries for the major organs (active marrow, colon, lung, stomach wall, and breast) at the energy of 0.1 MeV. Effective dose coefficients were also derived for applicable situations, e.g., radiation protection or risk projection. Our results are the first comprehensive set of organ and effective dose coefficients applicable to children and adolescents based on the newly adopted ICRP pediatric phantom series. Our tabulated organ and effective dose coefficients for these next-generation phantoms should provide more accurate estimates of organ doses in children than earlier dosimetric models allowed.

COMPARISON OF MONOENERGETIC PHOTON ORGAN DOSE RATE COEFFICIENTS FOR STYLIZED AND VOXEL PHANTOMS SUBMERGED IN AIR

As part of a broader effort to calculate effective dose rate coefficients for external exposure to photons and electrons emitted by radionuclides distributed in air, soil or water, age-specific stylized phantoms have been employed to determine dose coefficients relating dose rate to organs and tissues in the body. In this article, dose rate coefficients computed using the International Commission on Radiological Protection reference adult male voxel phantom are compared with values computed using the Oak Ridge National Laboratory adult male stylized phantom in an air submersion exposure geometry. Monte Carlo calculations for both phantoms were performed for monoenergetic source photons in the range of 30 keV to 5 MeV. These calculations largely result in differences under 10 % for photon energies above 50 keV, and it can be expected that both models show comparable results for the environmental sources of radionuclides.

NEURAL NETWORK MODELLING OF CARDIAC DOSE CONVERSION COEFFICIENT FOR ARBITRARY X-RAY SPECTRA

In this article, an approach to compute the dose conversion coefficients (DCCs) is described for the computational voxel phantom 'High-Definition Reference Korean-Man' (HDRK-Man) using artificial neural networks (ANN). For this purpose, the voxel phantom was implemented into the Monte Carlo (MC) transport toolkit GEANT4, and the DCCs for more than 30 tissues and organs, due to a broad parallel beam of monoenergetic photons with energy ranging from 15 to 150 keV by a step of 5 keV, were calculated. To study the influence of patient size on DCC values, DCC calculation was performed, for a representative body size population, using five different sizes covering the range of 80-120 % magnification of the original HDRK-Man. The focus of the present study was on the computation of DCC for the human heart. ANN calculation and MC simulation results were compared, and good agreement was observed showing that ANNs can be used as an efficient tool for modelling DCCs for the computational voxel phantom. ANN approach appears to be a significant advance over the time-consuming MC methods for DCC calculation.

BODY SIZE-SPECIFIC EFFECTIVE DOSE CONVERSION COEFFICIENTS FOR CT SCANS

Effective dose from computed tomography (CT) examinations is usually estimated using the scanner-provided dose-length product and using conversion factors, also known as k-factors, which correspond to scan regions and differ by age according to five categories: 0, 1, 5, 10 y and adult. However, patients often deviate from the standard body size on which the conversion factor is based. In this study, a method for deriving body size-specific k-factors is presented, which can be determined from a simple regression curve based on patient diameter at the centre of the scan range. Using the International Commission on Radiological Protection reference paediatric and adult computational phantoms paired with Monte Carlo simulation of CT X-ray beams, the authors derived a regression-based k-factor model for the following CT scan types: head-neck, head, neck, chest, abdomen, pelvis, abdomen-pelvis (AP) and chest-abdomen-pelvis (CAP). The resulting regression functions were applied to a total of 105 paediatric and 279 adult CT scans randomly sampled from patients who underwent chest, AP and CAP scans at the National Institutes of Health Clinical Center. The authors have calculated and compared the effective doses derived from the conventional age-specific k-factors with the values computed using their body size-specific k-factor. They found that by using the age-specific k-factor, paediatric patients tend to have underestimates (up to 3-fold) of effective dose, while underweight and overweight adult patients tend to have underestimates (up to 2.6-fold) and overestimates (up to 4.6-fold) of effective dose, respectively, compared with the effective dose determined from their body size-dependent factors. The authors present these size-specific k-factors as an alternative to the existing age-specific factors. The body size-specific k-factor will assess effective dose more precisely and on a more individual level than the conventional age-specific k-factors and, hence, improve awareness of the true exposure, which is important for the clinical community to understand.

MONTE CARLO STUDY OF THE CARDIAC ABSORBED DOSE DURING X-RAY EXAMINATION OF AN ADULT PATIENT

The computational voxel phantom 'High-Definition Reference Korean-Man (HDRK-Man)' was implemented into the Monte Carlo transport toolkit Geant4. The voxel model, adjusted to the Reference Korean Man, is 171 cm in height and 68 kg in weight and composed of ∼30 million voxels whose size is 1.981 × 1.981 × 2.0854 mm³ The Geant4 code is then utilised to compute the dose conversion coefficients (DCCs) expressed in absorbed dose per air kerma free in air for >30 tissues and organs, including almost all organs required in the new recommendation of the ICRP 103, due to a broad parallel beam of monoenergetic photons impinging in antero-postero direction with energy ranging from 10 to 150 keV. The computed DCCs of different organs are found to be in good agreement with data published using other simulation codes. Also, the influence of patient size on DCC values was investigated for a representative body size of the adult Korean patient population. The study was performed using five different sizes covering the range of 0.8-1.2 magnification order of the original HDRK-Man. It focussed on the computation of DCC for the human heart. Moreover, the provided DCCs were used to present an analytical parameterisation for the calculation of the cardiac absorbed dose for any arbitrary X-ray spectrum and for those patient sizes. Thus, the present work can be considered as an enhancement of the continuous studies performed by medical physicist as part of quality control tests and radiation protection dosimetry.

Secondary neutron doses in proton therapy treatments of ocular melanoma and craniopharyngioma

Monte Carlo simulations were used to assess secondary neutron doses received by patients treated with proton therapy for ocular melanoma and craniopharyngioma. MCNPX calculations of out-of-field doses were done for ∼20 different organs considering realistic treatment plans and using computational phantoms representative of an adult male individual. Simulations showed higher secondary neutron doses for intracranial treatments, ∼14 mGy to the salivary glands, when compared with ocular treatments, ∼0.6 mGy to the non-treated eye. This secondary dose increase is mainly due to the higher proton beam energy (178 vs. 75 MeV) as well as to the impact of the different beam parameters (modulation, collimation, field size etc.). Moreover, when compared with published data, the assessed secondary neutron doses showed similar trends, but sometimes with sensitive differences. This confirms secondary neutrons to be directly dependent on beam energy, modulation technique, treatment configuration and methodology.

Organ doses, effective doses, and risk indices in adult CT: comparison of four types of reference phantoms across different examination protocols

PURPOSE

Radiation exposure from computed tomography (CT) to the public has increased the concern among radiation protection professionals. Being able to accurately assess the radiation dose patients receive during CT procedures is a crucial step in the management of CT dose. Currently, various computational anthropomorphic phantoms are used to assess radiation dose by different research groups. It is desirable to better understand how the dose results are affected by different choices of phantoms. In this study, the authors assessed the uncertainties in CT dose and risk estimation associated with different types of computational phantoms for a selected group of representative CT protocols.

METHODS

Routinely used CT examinations were categorized into ten body and three neurological examination categories. Organ doses, effective doses, risk indices, and conversion coefficients to effective dose and risk index (k and q factors, respectively) were estimated for these examinations for a clinical CT system (LightSpeed VCT, GE Healthcare). Four methods were used, each employing a different type of reference phantoms. The first and second methods employed a Monte Carlo program previously developed and validated in our laboratory. In the first method, the reference male and female extended cardiac-torso (XCAT) phantoms were used, which were initially created from the Visible Human data and later adjusted to match organ masses defined in ICRP publication 89. In the second method, the reference male and female phantoms described in ICRP publication 110 were used, which were initially developed from tomographic data of two patients and later modified to match ICRP 89 organ masses. The third method employed a commercial dosimetry spreadsheet (ImPACT group, London, England) with its own hermaphrodite stylized phantom. In the fourth method, another widely used dosimetry spreadsheet (CT-Expo, Medizinische Hochschule, Hannover, Germany) was employed together with its associated male and female stylized phantoms.

RESULTS

For fully irradiated organs, average coefficients of variation (COV) ranged from 0.07 to 0.22 across the four male phantoms and from 0.06 to 0.18 across the four female phantoms; for partially irradiated organs, average COV ranged from 0.13 to 0.30 across the four male phantoms and from 0.15 to 0.30 across the four female phantoms. Doses to the testes, breasts, and esophagus showed large variations between phantoms. COV for gender-averaged effective dose and k factor ranged from 0.03 to 0.23 and from 0.06 to 0.30, respectively. COV for male risk index and q factor ranged from 0.06 to 0.30 and from 0.05 to 0.36, respectively; COV for female risk index and q factor ranged from 0.06 to 0.49 and from 0.07 to 0.54, respectively.

CONCLUSIONS

Despite closely matched organ mass, total body weight, and height, large differences in organ dose exist due to variation in organ location, spatial distribution, and dose approximation method. Dose differences for fully irradiated radiosensitive organs were much smaller than those for partially irradiated organs. Weighted dosimetry quantities including effective dose, male risk indices, k factors, and male q factors agreed well across phantoms. The female risk indices and q factors varied considerably across phantoms.

Extension of RPI-adult male and female computational phantoms to obese patients and a Monte Carlo study of the effect on CT imaging dose

Although it is known that obesity has a profound effect on x-ray computed tomography (CT) image quality and patient organ dose, quantitative data describing this relationship are not currently available. This study examines the effect of obesity on the calculated radiation dose to organs and tissues from CT using newly developed phantoms representing overweight and obese patients. These phantoms were derived from the previously developed RPI-adult male and female computational phantoms. The result was a set of ten phantoms (five males, five females) with body mass indexes ranging from 23.5 (normal body weight) to 46.4 kg m(-2) (morbidly obese). The phantoms were modeled using triangular mesh geometry and include specified amounts of the subcutaneous adipose tissue and visceral adipose tissue. The mesh-based phantoms were then voxelized and defined in the Monte Carlo N-Particle Extended code to calculate organ doses from CT imaging. Chest-abdomen-pelvis scanning protocols for a GE LightSpeed 16 scanner operating at 120 and 140 kVp were considered. It was found that for the same scanner operating parameters, radiation doses to organs deep in the abdomen (e.g., colon) can be up to 59% smaller for obese individuals compared to those of normal body weight. This effect was found to be less significant for shallow organs. On the other hand, increasing the tube potential from 120 to 140 kVp for the same obese individual resulted in increased organ doses by as much as 56% for organs within the scan field (e.g., stomach) and 62% for those out of the scan field (e.g., thyroid), respectively. As higher tube currents are often used for larger patients to maintain image quality, it was of interest to quantify the associated effective dose. It was found from this study that when the mAs was doubled for the obese level-I, obese level-II and morbidly-obese phantoms, the effective dose relative to that of the normal weight phantom increased by 57%, 42% and 23%, respectively. This set of new obese phantoms can be used in the future to study the optimization of image quality and radiation dose for patients of different weight classifications. Our ultimate goal is to compile all the data derived from these phantoms into a comprehensive dosimetry database defined in the VirtualDose software.

Construction of an extended library of adult male 3D models: rationale and results

In order to best cover the possible extent of heights and weights of male adults the construction of 25 whole body 3D models has been undertaken. Such a library is thought to be useful to specify the uncertainties and relevance of dosimetry calculations carried out with models representing individuals of average body heights and weights. Representative 3D models of Caucasian body types are selected in a commercial database according to their height and weight, and 3D models of the skeleton and internal organs are designed using another commercial dataset. A review of the literature enabled one to fix volume or mass target values for the skeleton, soft organs, skin and fat content of the selected individuals. The composition of the remainder tissue is fixed so that the weight of the voxel models equals the weight of the selected individuals. After mesh and NURBS modelling, volume adjustment of the selected body shapes and additional voxel-based work, 25 voxel models with 109 identified organs or tissue are obtained. Radiation transport calculations are carried out with some of the developed models to illustrate potential uses. The following points are discussed throughout this paper: justification of the fixed or obtained models’ features regarding available and relevant literature data; workflow and strategy for major modelling steps; advantages and drawbacks of the obtained library as compared with other works. The construction hypotheses are explained and justified in detail since future calculation results obtained with this library will depend on them.

A comparison of organ doses between mathematical and voxel phantoms with the DS02 photon fluences

The purpose of this study is to quantify dosimetric differences if modern sophisticated voxel phantoms were used in the dosimetry system DS02 rather than the mathematical phantoms. The mathematical models (ADAM and EVA) and voxel phantoms (REX and REGINA) developed in Germany allow a useful comparison as they are very close in body weight, body height and organ masses. In this study, organ doses are calculated with published fluence-to-absorbed-dose conversion coefficients derived from those two model sets for unidirectional plane beam irradiation geometries, with DS02 photon energy spectra at various distances from the hypocentre in Hiroshima. Results showed that organ doses from mathematical models generally agree well with those from voxel phantoms except for a few organs at lateral irradiation geometries and eye lenses at antero-posterior irradiation, even though there were significant differences between the two phantom sets and various uncertainties in dose calculations.

Estimation of organ doses from kilovoltage cone-beam CT imaging used during radiotherapy patient position verification

PURPOSE

The purpose of this study was to develop a practical method for estimating organ doses from kilovoltage cone-beam CT (CBCT) that can be performed with readily available phantoms and dosimeters. The accuracy of organ dose estimates made using the ImPACT patient dose calculator was also evaluated.

METHODS

A 100 mm pencil chamber and standard CT dose index (CTDI) phantoms were used to measure the cone-beam dose index (CBDI). A weighted CBDI (CBDI(W)) was then calculated from these measurements to represent the average volumetric dose in the CTDI phantom. By comparing CBDI(W) to the previously published organ doses, organ dose conversion coefficients were developed. The measured CBDI values were also used as inputs for the ImPACT calculator to estimate organ doses. All CBDI dose measurements were performed on both the Elekta XVI and Varian OBI at three clinically relevant locations: Head, chest, and pelvis.

RESULTS

The head, chest, and pelvis protocols yielded CBDI(W) values of 0.98, 16.62, and 24.13 mGy for the XVI system and 5.17, 6.14, and 21.57 mGy for the OBI system, respectively. Organ doses estimated with the ImPACT CT dose calculator showed a large range of variation from the previously measured organ doses, demonstrating its limitations for use with CBCT.

CONCLUSIONS

The organ dose conversion coefficients developed in this work relate CBDI(W) values to organ doses previously measured using the same clinical protocols. Ultimately, these coefficients will allow for the quick estimation of organ doses from routine measurements performed using standard CTDI phantoms and pencil chambers.

Assessment of organ-specific neutron equivalent doses in proton therapy using computational whole-body age-dependent voxel phantoms

Proton beams used for radiotherapy will produce neutrons when interacting with matter. The purpose of this study was to quantify the equivalent dose to tissue due to secondary neutrons in pediatric and adult patients treated by proton therapy for brain lesions. Assessment of the equivalent dose to organs away from the target requires whole-body geometrical information. Furthermore, because the patient geometry depends on age at exposure, age-dependent representations are also needed. We implemented age-dependent phantoms into our proton Monte Carlo dose calculation environment. We considered eight typical radiation fields, two of which had been previously used to treat pediatric patients. The other six fields were additionally considered to allow a systematic study of equivalent doses as a function of field parameters. For all phantoms and all fields, we simulated organ-specific equivalent neutron doses and analyzed for each organ (1) the equivalent dose due to neutrons as a function of distance to the target; (2) the equivalent dose due to neutrons as a function of patient age; (3) the equivalent dose due to neutrons as a function of field parameters; and (4) the ratio of contributions to secondary dose from the treatment head versus the contribution from the patient's body tissues. This work reports organ-specific equivalent neutron doses for up to 48 organs in a patient. We demonstrate quantitatively how organ equivalent doses for adult and pediatric patients vary as a function of patient's age, organ and field parameters. Neutron doses increase with increasing range and modulation width but decrease with field size (as defined by the aperture). We analyzed the ratio of neutron dose contributions from the patient and from the treatment head, and found that neutron-equivalent doses fall off rapidly as a function of distance from the target, in agreement with experimental data. It appears that for the fields used in this study, the neutron dose lateral to the field is smaller than the reported scattered photon doses in a typical intensity-modulated photon treatment. Most importantly, our study shows that neutron doses to specific organs depend considerably on the patient's age and body stature. The younger the patient, the higher the dose deposited due to neutrons. Given the fact that the risk also increases with decreasing patient age, this factor needs to be taken into account when treating pediatric patients of very young ages and/or of small body size. The neutron dose from a course of proton therapy treatment (assuming 70 Gy in 30 fractions) could potentially (depending on patient's age, organ, treatment site and area of CT scan) be equivalent to up to approximately 30 CT scans.

Hybrid computational phantoms of the male and female newborn patient: NURBS-based whole-body models

Anthropomorphic computational phantoms are computer models of the human body for use in the evaluation of dose distributions resulting from either internal or external radiation sources. Currently, two classes of computational phantoms have been developed and widely utilized for organ dose assessment: (1) stylized phantoms and (2) voxel phantoms which describe the human anatomy via mathematical surface equations or 3D voxel matrices, respectively. Although stylized phantoms based on mathematical equations can be very flexible in regard to making changes in organ position and geometrical shape, they are limited in their ability to fully capture the anatomic complexities of human internal anatomy. In turn, voxel phantoms have been developed through image-based segmentation and correspondingly provide much better anatomical realism in comparison to simpler stylized phantoms. However, they themselves are limited in defining organs presented in low contrast within either magnetic resonance or computed tomography images-the two major sources in voxel phantom construction. By definition, voxel phantoms are typically constructed via segmentation of transaxial images, and thus while fine anatomic features are seen in this viewing plane, slice-to-slice discontinuities become apparent in viewing the anatomy of voxel phantoms in the sagittal or coronal planes. This study introduces the concept of a hybrid computational newborn phantom that takes full advantage of the best features of both its stylized and voxel counterparts: flexibility in phantom alterations and anatomic realism. Non-uniform rational B-spline (NURBS) surfaces, a mathematical modeling tool traditionally applied to graphical animation studies, was adopted to replace the limited mathematical surface equations of stylized phantoms. A previously developed whole-body voxel phantom of the newborn female was utilized as a realistic anatomical framework for hybrid phantom construction. The construction of a hybrid phantom is performed in three steps: polygonization of the voxel phantom, organ modeling via NURBS surfaces and phantom voxelization. Two 3D graphic tools, 3D-DOCTOR and Rhinoceros, were utilized to polygonize the newborn voxel phantom and generate NURBS surfaces, while an in-house MATLAB code was used to voxelize the resulting NURBS model into a final computational phantom ready for use in Monte Carlo radiation transport calculations. A total of 126 anatomical organ and tissue models, including 38 skeletal sites and 31 cartilage sites, were described within the hybrid phantom using either NURBS or polygon surfaces. A male hybrid newborn phantom was constructed following the development of the female phantom through the replacement of female-specific organs with male-specific organs. The outer body contour and internal anatomy of the NURBS-based phantoms were adjusted to match anthropometric and reference newborn data reported by the International Commission on Radiological Protection in their Publication 89. The voxelization process was designed to accurately convert NURBS models to a voxel phantom with minimum volumetric change. A sensitivity study was additionally performed to better understand how the meshing tolerance and voxel resolution would affect volumetric changes between the hybrid-NURBS and hybrid-voxel phantoms. The male and female hybrid-NURBS phantoms were constructed in a manner so that all internal organs approached their ICRP reference masses to within 1%, with the exception of the skin (-6.5% relative error) and brain (-15.4% relative error). Both hybrid-voxel phantoms were constructed with an isotropic voxel resolution of 0.663 mm--equivalent to the ICRP 89 reference thickness of the newborn skin (dermis and epidermis). Hybrid-NURBS phantoms used to create their voxel counterpart retain the non-uniform scalability of stylized phantoms, while maintaining the anatomic realism of segmented voxel phantoms with respect to organ shape, depth and inter-organ positioning.

Whole-body voxel phantoms of paediatric patients--UF Series B

Following the previous development of the head and torso voxel phantoms of paediatric patients for use in medical radiation protection (UF Series A), a set of whole-body voxel phantoms of paediatric patients (9-month male, 4-year female, 8-year female, 11-year male and 14-year male) has been developed through the attachment of arms and legs from segmented CT images of a healthy Korean adult (UF Series B). Even though partial-body phantoms (head-torso) may be used in a variety of medical dose reconstruction studies where the extremities are out-of-field or receive only very low levels of scatter radiation, whole-body phantoms play important roles in general radiation protection and in nuclear medicine dosimetry. Inclusion of the arms and legs is critical for dosimetry studies of paediatric patients due to the presence of active bone marrow within the extremities of children. While the UF Series A phantoms preserved the body dimensions and organ masses as seen in the original patients who were scanned, comprehensive adjustments were made for the Series B phantoms to better match International Commission on Radiological Protection (ICRP) age-interpolated reference body masses, body heights, sitting heights and internal organ masses. The CT images of arms and legs of a Korean adult were digitally rescaled and attached to each phantom of the UF series. After completion, the resolutions of the phantoms for the 9-month, 4-year, 8-year, 11-year and 14-year were set at 0.86 mm x 0.86 mm x 3.0 mm, 0.90 mm x 0.90 mm x 5.0 mm, 1.16 mm x 1.16 mm x 6.0 mm, 0.94 mm x 0.94 mm x 6.00 mm and 1.18 mm x 1.18 mm x 6.72 mm, respectively.