Heavy Ion Transport Code System Research Articles

Radiation field characterization with emphasis on the collimator configuration at the compact neutron source RANS-II facility

ABSTRACT The RIKEN Accelerator-driven compact Neutron Source-II (RANS-II) based on the 7Li(p,n)7Be reaction with 2.49 MeV proton injection was installed in a small area of the experimental hall, where the scattered radiation is expected to strongly influence experiments. Therefore, we have begun to study the radiation characteristics of the RANS-II hall by simulation with the Particle and Heavy Ion Transport code System (PHITS) and by experiments to clearly identify the scattered component. In addition, to suppress the background, a collimator is studied with respect to its size, shape, and materials. With the use of the 30 mm borated polyethylene collimator, we found that the background becomes less than 1 of the extracted neutron beam intensity 1.0 m from the Li target. For the experiment, neutron suppression was characterized by measuring the neutron dose rate with a neutron dosimeter and a collimator. The suppression ability of the neutron beam was confirmed by comparing the results of simulations with those of experiments.

Development of the DICOM-based Monte Carlo dose reconstruction system for a retrospective study on the secondary cancer risk in carbon ion radiotherapy

Objective. A retrospective study on secondary cancer risk on carbon ion radiotherapy (CIRT) is ongoing at the Heavy Ion Medical Accelerator in Chiba (HIMAC). The reconstruction of the whole-body patient dose distribution is the key issue in the study because dose distribution only around the planning target volume was evaluated in the treatment planning system. Approach. We therefore developed a new dose reconstruction system based on the Particle and Heavy Ion Transport code System (PHITS) coupled with the treatment plan DICOM data set by extending the functionalities of RadioTherapy package based on PHITS (RT-PHITS). In the system, the geometry of patient-specific beam devices such as the range shifter, range compensator, and collimators as well as the individual patient’s body are automatically reconstructed. Various functions useful for retrospective analysis on the CIRT are implemented in the system, such as those for separately deducing dose contributions from different secondary particles and their origins. Main results. The accuracy of the developed system was validated by comparing the dose distribution to the experimental data measured in a water tank and using a treatment plan on an anthropomorphic phantom. Significance. The extended RT-PHITS will be used in epidemiological studies based on clinical data from HIMAC.

Development of a simple calculation tool of dose distributions in a phantom for boron neutron capture therapy

A simple dose calculation tool, SiDE, was developed for dose evaluation in a water phantom for boron neutron capture therapy, which makes the calculation time much shorter compared with the conventional particle transportation Monte Carlo codes and is applicable to any type of incident neutron spectra to the phantom. As the SiDE can not only calculate quantitatively the dose distribution in the phantom but also output dose indexes such as advantage depth and peak tumor dose, a comparison between different boron neutron capture therapy neutron sources can be easily performed. Consistency with a Monte Carlo transportation code was verified through comparison with the conventional dose calculation with the Particle and Heavy Ion Transport Code System, and the calculation time was nearly 1/90 in the SiDE. The dose distributions for a reactor and accelerator-based neutron sources were compared, and the differences were found to be small although large differences existed between the incident spectra.

Modeling and Analysis of Percentage Depth Dose (PDD) and Dose Profile of X-Ray Beam Produced by Linac Device with Voltage Variation

The Percentage Depth Dose (PDD) and dose profile of X-Ray output from a LINAC therapy device have been modeled and analyzed. The research was conducted by simulation method through the use of Particle and Heavy Ion Transport code System (PHITS) program. The LINAC therapy device modeled in this work refers to the Siemens Primus LINAC therapy device, which is operated at 6 MV, 10 MV and 18 MV voltages. Determination of PDD was carried at a depth of 0-30 cm and dose profile at a depth of 0-20 cm in a water phantom, placed at 100 cm from the source, which is exposed to a radiation field area of 10×10 cm 2 . Results from the modeling of the LINAC therapy device agrees with the actual X-ray apparatus and has produced Bremsstrahlung X-ray. It was found from the analysis of the PDD curve that the maximum doses are at the depth of 1.5 cm, 2.5 cm and 3.4 cm. The value of build up factor for each LINAC voltage agrees with the reference. Additionally,  the results of the analysis of the doses profile suggest that the X-ray output has good degree of uniformity. The flatness of dose profile occurs at the depth of 20 cm with percentage value of flatness at 1.6 %, 1.9 % and 1.2 %. The flatness values are all less than 2%. The flatness values shows ≤ 2 % deviation from reference value, which is below the tolerance range required in a measurement.

Dose estimation for cone-beam computed tomography in image-guided radiation therapy using mesh-type reference computational phantoms and assuming head and neck cancer

This study aimed to estimate the additional dose the cone-beam computed tomography (CBCT) system integrated into the Varian TrueBeam linear accelerator delivers to a patient with head and neck cancer using mesh-type International Commission on Radiological Protection reference computational phantoms. In the first part, for use as a benchmark for the accuracy of the Monte Carlo geometry of CBCT, Particle and Heavy Ion Transport code System (PHITS) calculations were confirmed against measured lateral and depth dose profiles using a computed tomography dose profiler. After obtaining good agreement, organ dose calculations were performed by PHITS using mesh-type reference computational phantom (MRCP) and irradiating the neck region; the effective dose was calculated utilising absorbed organ doses and tissue weighting factors for male and female MRCP. Substantially, it has been found that the effective doses for male and female MRCP are 0.81 and 1.06 mSv, respectively. As this study aimed to assess the imaging dose from the CBCT system used in image-guided radiation therapy, it is required to take into account this dose in terms of both the target organ and surrounding tissues. Although the absorbed organ dose values and effective dose values obtained for both MRCP males and females were small, attention should be paid to the additional dose resulting from CBCT. This study can create awareness on the importance of doses arising from imaging techniques, especially CBCT.

OPTIMIZATION AND VERIFICATION OF DOUBLE LAYER BEAM SHAPING ASSEMBLY (DLBSA) FOR EPITHERMAL NEUTRON GENERATION

The designs of Beam Shaping Assembly (BSA) for moderating fast neutron into epithermal neutron have been conducted. Some BSA models that are previously developed are still having problems in generating epithermal neutron. Instead, we propose designs of double layer beam shaping assembly (DLBSA) to produce epithermal neutron. Optimization of the Double Layer Beam Shaping Assembly (DLBSA) design was carried out using the genetic algorithm (AG) method using MCNPX and verified using the Particle and Heavy Ion Transport code System (PHITS). The optimization resulted in four configurations up to the 21st generation capable of producing epithermal neutron beams that comply with the IAEA standards. The best four configurations are obtained by combining: (1) Al with one of the CaF2, BiF3 or PbF2 materials as moderator, (2) Pb with Pb, Ni, or Bi as a reflector, (3) Ni with FeC, or C as collimator, (4) (FeC + LiF) as fast neutron filter, Cd or B4C as thermal neutron filter. Verification of the four optimum configurations of the DLBSA model using PHITS code shows that the epithermal neutron beam produced by DLBSA has met the IAEA standards.

Microdosimetric Modeling of Relative Biological Effectiveness for Skin Reactions: Possible Linkage Between In Vitro and In Vivo Data

Precise evaluation of the relative biological effectiveness (RBE) for skin reactions is indispensable for the treatment planning of particle therapy and boron-neutron capture therapy. The evaluation is also needed for the future radiation protection system for mixed radiation fields. Such RBE is often evaluated based on in vitro cell survival data, but its validity remains incompletely understood. This study aimed to develop a model for estimating RBE for skin reactions and dermal cell survival in the same framework and quantitatively discuss a possible link between them. The microdosimetric kinetic model, which was originally developed for estimating cell surviving fractions for various radiations, was improved to be capable of estimating the mean and uncertainty of RBE for skin reactions. The parameter used in the model was independently determined from in vitro measurements of dermal cell survival and in vivo measurements of skin reactions taken from 8 and 23 articles, respectively. In the parameter determination, the characteristics of the radiation fields employed in each measurement were reproduced in detail by the Particle and Heavy Ion Transport Code System. Our model quantitatively revealed that RBE for skin reactions tend to be higher than those for dermal cell survival. RBE of various monoenergetic radiations calculated from this model confirmed that the past evaluations made by the International Commission on Radiological Protection and the National Council on Radiation Protection and Measurements a few decades ago are still supported by recent experimental data. Our model can play important roles not only in medical physics for avoiding unnecessary skin reactions in particle therapy and boron-neutron capture therapy but also in radiation protection for future decision-making of the recommended RBE values.

Development of back-illuminated thin silicon diode applied to fast neutron sensor in active personal dosimeter

A back-illuminated thin Si diode was developed for fast neutron sensor in active personal neutron dosimeter for detecting fast neutrons below 1 MeV. Neutron response functions for the fast neutron sensor with the back-illuminated thin Si diode were evaluated based on measurements of monoenergetic neutron beams at the National Metrology Institute of Japan and Monte Carlo simulations using the Particle Heavy Ion Transport Code System in the neutron energy over 500 keV. The lower limit of neutron detection energy was decreased to 500 keV and neutron detection efficiencies were increased several times over other fast neutron sensors. It is possible to measure neutron doses in the neutron energy range of 500 keV–1 MeV using this novel fast neutron sensor, but the neutron doses were undetectable using other fast neutron sensors. The neutron detection efficiencies were closer to the personal dose equivalent conversion curve than those for other fast neutron sensors in this neutron energy range. Neutron doses of individuals employed at a nuclear facility were evaluated using the active personal neutron dosimeter, which consists of both the novel fast and slow neutron sensors.

Investigation on electron contamination of LINAC at different operating voltages using particle heavy ion transport code system (PHITS)

Research has been carried out to investigate the occurrence of secondary electron contamination in a linear accelerator (LINAC) machine. The research was conducted in a simulation using a Monte Carlo-based simulator, namely Particle and Heavy Ions Transport code System (PHITS). The simulation of the occurrence of secondary electron contamination was carried out based on the model of the LINAC Electa head that is operated at voltages of 6, 8, 10, 15, 18, and 25 MV, using a field area of 10 X 10 cm and SSD 100 cm. The simulation results show that electron contamination occurs due to the interaction of X-ray photons with the components of the LINAC head, namely the primary collimator, flattening filter, and secondary collimator. The secondary electron contaminants generated by the LINAC head components spread through the water phantom. The higher the operating voltage, the higher the secondary electron flux produced. The secondary electron contamination dose calculated in the water phantom shows that the higher the LINAC voltage, the higher is the dose received in the phantom.

Estimation of radioactivity and dose equivalent rate by combining Compton imaging and Monte Carlo radiation transport code

In a radiation environment, such as the decommissioning site of a nuclear power station, visualization of the distribution of radioactive substances and estimation of the dose equivalent rate around the site can help reduce the exposure dose of workers and plan their work. The author has developed a method of visualizing the existence of a radiation source using a gamma-ray imager, estimating its radioactivity, and estimating the dose equivalent rate around the source. A Compton camera, which is a gamma-ray imager, is used to visualize the existence of a137Cs radiation source and estimate its radioactivity, and a three-dimensional (3D) model of the region around the source is generated using a simultaneous localization and mapping device based on 3D light detection and ranging. Next, the dose equivalent rate around the source is calculated by importing the 3D model data and radioactivity information into a particle and heavy ion transport code system. The validity of the calculated dose equivalent rates is also confirmed by comparing them with values measured using a survey meter. This method can be used not only to simply visualize a source and calculate the dose equivalent rate around it but also to evaluate how addition of shielding or removal of contaminated objects can contribute to reducing the dose equivalent rate.

Barite concrete-based cement composites for 252Cf spontaneous neutron and 60Co/192Ir shielding based on Monte Carlo computation

Barite concrete composite materials have been investigated for 252Cf spontaneous neutron and 60Co/192Ir gamma sources’ shielding using Monte Carlo computational method. The Particle and Heavy Ion Transport code System (PHITS) was used to compute the shielding properties of three different materials (barite concrete, barite cement, and barite aggregate) used as structural walls in fixed neutron & gamma industrial radiography for Non-Destructive Testing applications. The obtained results displayed good properties of barite concrete in shielding spontaneous neutrons emitted from the 252Cf source, as the effective dose drops about 108 times in only 140 cm wall thickness, and it was found to be about 10 times more effective than other materials investigated. In addition, the investigated gamma shielding properties of the barite concrete showed a relatively smaller wall thickness compared to the ordinary concrete. The decision-making process based on the ALARA principle of dose limitation showed that the use of barite concrete in such facilities is more effective than the use of barite cement and barite aggregate, for both gamma and neutron radiography shielding design. To achieve an average value of 1 μSv/h, the obtained result shows that 80 cm of Barite concrete is needed, while 125 and 130 cm of barite cement and barite aggregate are needed, respectively to shield the Co-60 source. Meanwhile, 50 cm of wall made of barite concrete is sufficient to cut down the effective dose rate to 1 μSv/h (for 50 Ci and 55 cm for 150 Ci 192Ir), which is an appropriate design for the public area adjacent to the industrial radiographic facility. It was therefore concluded from the obtained data that barite concrete is the most effective shielding material for radioactive sources (60Co, 192Ir, and 252Cf) used in radiographic applications.

High-Energy Photon Attenuation Properties of Lead-Free and Self-Healing Poly (Vinyl Alcohol) (PVA) Hydrogels: Numerical Determination and Simulation.

This work numerically determined high-energy photon shielding properties of self-healing poly(vinyl alcohol) (PVA) hydrogels containing lead-free, heavy-metal compounds, namely, bismuth oxide (Bi2O3), tungsten oxide (WO3), and barium sulfate (BaSO4), through XCOM software packages. In order to understand the dependencies of the shielding properties of the hydrogels on filler contents and photon energies, the filler contents added to the hydrogels were varied from 0–40 wt.% and the photon energies were varied from 0.001–5 MeV. The results, which were verified for their reliability and correctness with those obtained from PHITS (Particle and Heavy Ion Transport code System), indicated that overall shielding performances, which included the mass attenuation coefficients (µm), the linear attenuation coefficient (µ), the half-value layer (HVL), and the lead equivalence (Pbeq), of the hydrogels improved with increasing filler contents but generally decreased with increasing photon energies. Among the three compounds investigated in this work, Bi2O3/PVA hydrogels exhibited the highest photon attenuation capabilities, determined at the same filler content and photon energy, mainly due to its highest atomic number of Bi and the highest density of Bi2O3 in comparison with other elements and compounds. Furthermore, due to possible reduction in self-healing and mechanical properties of the hydrogels with excessive filler contents, the least content of fillers providing a 10-mm sample with the required Pbeq value of 0.5 mmPb was investigated. The determination revealed that only the hydrogel containing at least 36 wt.% of Bi2O3 exhibited the Pbeq values greater than 0.5 mmPb for all photon energies of 0.05, 0.08, and 0.1 MeV (common X-ray energies in general nuclear facilities). The overall outcomes of the work promisingly implied the potential of PVA hydrogels to be used as novel and potent X-ray and gamma shielding materials with the additional self-healing and nonlead properties.

Proton range monitoring using 13N peak for proton therapy applications.

The Monte Carlo method is employed in this study to simulate the proton irradiation of a water-gel phantom. Positron-emitting radionuclides such as 11C, 15O, and 13N are scored using the Particle and Heavy Ion Transport Code System Monte Carlo code package. Previously, it was reported that as a result of 16O(p,2p2n)13N nuclear reaction, whose threshold energy is relatively low (5.660 MeV), a 13N peak is formed near the actual Bragg peak. Considering the generated 13N peak, we obtain offset distance values between the 13N peak and the actual Bragg peak for various incident proton energies ranging from 45 to 250 MeV, with an energy interval of 5 MeV. The offset distances fluctuate between 1.0 and 2.0 mm. For example, the offset distances between the 13N peak and the Bragg peak are 2.0, 2.0, and 1.0 mm for incident proton energies of 80, 160, and 240 MeV, respectively. These slight fluctuations for different incident proton energies are due to the relatively stable energy-dependent cross-section data for the 16O(p,2p2n)13N nuclear reaction. Hence, we develop an open-source computer program that performs linear and non-linear interpolations of offset distance data against the incident proton energy, which further reduces the energy interval from 5 to 0.1 MeV. In addition, we perform spectral analysis to reconstruct the 13N Bragg peak, and the results are consistent with those predicted from Monte Carlo computations. Hence, the results are used to generate three-dimensional scatter plots of the 13N radionuclide distribution in the modeled phantom. The obtained results and the developed methodologies will facilitate future investigations into proton range monitoring for therapeutic applications.

Physical dosimetric reconstruction of a case of large area back skin injury due to overexposure in an interventional procedure

To estimate the physical dose of skin and key organs in a case of overexposure during a cardiac interventional procedure. MethodsThe female patient aged 50 suffered from overexposure during cardiac interventional therapy in a hospital, Xinxiang city, Henan province, China in January 2020. The mesh-type phantom for the patient was constructed based on the adult mesh-type reference computational phantoms (MRCPs) released by the International Commission on Radiological Protection Publication 145 (ICRP145) and phantom deformation technology. Models of exposure scenario were constructed and simulated with particle and heavy ion transport code system (PHITS) according to exposure conditions. ResultsThe maximum absorbed dose of key organs/tissues under irradiation in posteroanterior (PA) and 30° left anterior oblique directions(LOA) was 632.4 and 305.6 ​mGy, respectively. The left lung, heart, and left mammary gland received a larger dose under both irradiation conditions. The ratio of the absorbed dose with and without shielding was calculated, and the relative difference in most organs was <1% between two directions. The iso-dose curve of the back skin revealed the distribution of the absorbed dose (0.1 −5.2 ​Gy). The dose estimate of key tissues/organs was higher than the conventional level, especially the local skin, up to 5.2 ​Gy. ConclusionsThe interventional procedure in this case resulted in a higher dose. Monte Carlo codes combined with the MRCPs can be employed to estimate physical dose to individuals in concrete irradiation scenarios.

Features of accelerator-based neutron source for boron neutron capture therapy calculated by particle and heavy ion transport code system (PHITS)

Accelerator-based neutron sources have been developed and installed in recent decades for boron neutron capture therapy (BNCT) in several clinical facilities. Lithium is one of the targets that can produce epithermal neutrons from the 7Li(p,n)7Be near-threshold reaction, and accelerator-based BNCT systems employing a Li target are promising for cancer treatment. The accurate evaluation of the characteristics of an accelerator-based neutron source is a key to estimating the therapeutic effects of the accelerator-based BNCT. Particle and Heavy Ion Transport code System (PHITS) is a general-purpose Monte Carlo code, which can simulate a variety of diverse particle types and nuclear reactions. The latest PHITS code enables simulating the generation of neutrons from the 7Li(p,n)7Be reactions by using the Japanese Evaluated Nuclear Data Library 4.0 high-energy file. Thus, the PHITS code can be adopted for dose estimation during treatment planning for the accelerator-based BNCT. In this study, we evaluated the neutron fluence using the PHITS code by comparing it to reference data. The subsequent neutron transport simulations were performed to evaluate the boron trifluoride detector responses and the recoiled proton fluence detected by a CR-39 plastic detector. These comparative studies confirmed that the PHITS code can accurately simulate neutrons generated from an accelerator using a Li target. The PHITS code has a significant potential for a detailed evaluation of neutron fields and for predicting the therapeutic effects of the accelerator-based BNCT.

Investigation of Dose Rate Distribution in an Experimental Hall of a RIKEN Accelerator-Driven Compact Neutron Source Based on the ⁹Be(p, n) Reaction With 7 MeV Proton Injection

To characterize the dose rate distribution in an experimental hall of a RIKEN accelerator-driven compact neutron source (RANS) based on the ⁹Be(p, n) reaction with 7 MeV proton injection, systematical measurements and calculations for neutron and gamma-ray dose rates by GEometry ANd Tracking (GEANT), Particle and Heavy Ion Transport code System (PHITS), and Monte Carlo N-Particle (MCNP) codes were performed. Calculations always underestimated measurements when proton beam loss effect was not considered. Relatively good agreements were observed among the different simulation codes. To explain the underestimations, the additional dominant neutron and gamma-ray sources due to proton beam loss were identified at the position around exit of the drift tube linac (DTL), made of copper, and the beam pipe from quadrupole (Q) magnets to steering (ST) magnets, made of aluminum, from measurements with placing collimators along linac. The beam loss fractions of 2&#x0025;&#x2013;3&#x0025; on copper and 1&#x0025; on aluminum, respectively, were the most appropriate estimation. In addition, we proposed the possible measures to reduce the measured total dose rate of <inline-formula> <tex-math notation="LaTeX">$3.8~\mu $ </tex-math></inline-formula>Sv/h at the operator position in the control room, with the addition of a wall at the entrance of experimental hall and extension of borated polyethylene (BPE) at the end of the beam. As a result, the dose rate became 2.5 times lower than the current one.

Study on Shielding Effectiveness of a Combined Radiation Shield for Manned Long Termed Interplanetary Expeditions

In the course of long-duration space missions, astronauts will be exposed to galactic cosmic rays (GCR) and solar particle events (SPE). The interaction with space radiation will lead to both severe and late effects on the crew members. Two countermeasures namely active and passive shields are available but both constitute some limitations. After briefly discussing active and passive methods for radiation shielding, this paper presents a novel combined radiation shield comprising an active shield attached with a polyethylene (PE) layer of 10 cm. The effectiveness of this PE layer as combined shield has been studied theoretically by Monte Carlo calculations code, Particle and Heavy Ion Transport code System (PHITS). While the cosmic rays will travel to the spaceships, in the active part, a superconducting toroid will deflect a specific portion of the incoming particles, after that a 10cm PE layer will provide the safeguard from the secondary particles that will be produced from the interaction with the active segment. The proposed superconducting toroid under certain conditions can theoretically deflect the lighter particles (Z< 3) up to 800 MeV/n, and for higher Z particles (Z<48) this cut-off energy is around 200 MeV/n. The particles that travel through the toroid coils will pose the additional dose to the crews. The proposed PE layer can reduce the absorbed dose rate of 1.9%, 7.3%, and 27.9% for proton, alpha, and carbon ion of 1GeV/n, respectively as calculated by the PHITS. Limitations of this type of shield are also briefly discussed.

Improvements in the particle and heavy-ion transport code system (PHITS) for simulating neutron-response functions and detection efficiencies of a liquid organic scintillator

ABSTRACT In this study, we simulated the neutron-response functions and detection efficiencies of a liquid organic scintillator using the particle and heavy-ion transport code system (PHITS). We incorporated the algorithm and database of the neutron-response simulation code SCINFUL–QMD into PHITS. Then, we updated the total, elastic, and inelastic cross-section data of the hydrogen and carbon nuclei for neutrons and developed a new scorer to analyze the light outputs from a scintillator. The calculation results of the neutron-response functions and the detection efficiencies were compared with results of SCINFUL–QMD, the previous PHITS with the new scorer, and the reported measurements. It was found that the improved PHITS successfully reproduced the results calculated by SCINFUL–QMD, except for around 150 MeV where a discontinuity in the detection-efficiency curve was observed in the SCINFUL–QMD values. Our results showed better agreement with the measured data than the results of the previous PHITS. The uncertainties of the detection efficiencies calculated by PHITS using the present extensions were estimated to be approximately 15% for neutrons in the energy region below 100 MeV.

MONTE-CARLO SIMULATION USING PHITS OF SECONDARY NEUTRONS PRODUCED IN-PATIENT DURING 16O ION THERAPY.

In hadrontherapy, oxygen ions 16O can be currently considered as an alternative to carbon ions 12C designed specifically for the treatment of deep and radioresistant tumors. Secondary particles, particularly neutrons constitute a serious problem of undesirable additional irradiation to surrounding healthy tissue. The objective of this study is to evaluate, by Monte-Carlo simulation [code Particle and Heavy Ion Transport code System (PHITS)], the contribution in terms of dose of secondary neutrons produced during interaction 16O ion of 300MeV u-1 in a soft tissue phantom. The dose of 16O ion, secondary particles and neutrons is evaluated, as well as the particle fluence and energy spectra of neutrons. The contribution to the total dose of secondary neutrons in a soft tissue phantom represents 0.1%. This dose, although apparently insignificant, is essential to conduct even more in-depth studies to understand the long-term effects of these secondary neutrons on the patient's body especially in pediatric case.

Assessment of Fetal Dose and Health Effect to the Fetus from Breast Cancer Radiotherapy during Pregnancy.

Decision for radiotherapy during the first trimester of pregnancy may occur, as patients may not realize their pregnancy at the very early stage. Since radiation dose can affect fetal development, the aim of this study was to evaluate fetal dose and associated deterministic effects and risks to the fetus from breast cancer radiotherapy of an 8-week pregnant patient. PHITS (Particle and Heavy Ion Transport code System) Monte Carlo simulation and the J-45 computational pregnancy phantom were used to simulate breast cancer radiotherapy from a 6 MV TrueBeam linear accelerator using the three dimensional-conformal radiotherapy (3D-CRT) technique with a prescribed dose to the planning target volume (PTV) of 50 Gy. Once the fetal dose was evaluated, the occurrence of the deterministic effects and risks for developing stochastic effects in the fetus were assessed using the recommendations of NCRP Report No. 174, AAPM Report No. 50, and ICRP Publication 84. The fetal dose was evaluated to be 3.37 ± 2.66 mGy, suggesting that the fetus was expected to have no additional deterministic effects, while the risks for developing cancer and malfunctions were similar to that expected from exposure to background radiation. The comparison with the other studies showed that accurate consideration of fetal position and size was important for dose determination in the fetus, especially at the early pregnancy stage when the fetus is very small.