A Real-Time Monte Carlo System for Internal Dose Evaluation Using an Anthropomorphic Phantom with Different Shapes of Tumors Inserted

Transcription

1 A Real-Time Monte Carlo System for Internal Dose Evaluation Using an Anthropomorphic Phantom with Different Shapes of Tumors Inserted J. Wu, S. J. Chang, B. J. Chang, I. J. Chen, J. H. Chiu Health Physics Division, Institute of Nuclear Energy Research, P.O. Box 3-10 Lung-Tan Taiwan Abstract. In order to evaluate internal dose, MIRD system was proposed by Society of Nuclear Medicine in MIRDOSE3 computer software based on this concept is currently the most frequently used tool to calculate the organ dose in nuclear medicine community. Unfortunately it uses fixed anthropomorphic phantoms, so the geometric parameters of organs are short of individual characteristics. Although the tumor dose can be calculated by nodule module, the organ doses contributed from tumors are still not included. To conquer this problem and acquire more accurate dose information, a internal dose evaluation system based on real-time Monte Carlo simulation was developed. The INER anthropomorphic phantom was modified from the adult reference man proposed by Cristy and Eckerman. Different shapes of tumor models can be arranged everywhere within the phantom. And an input file for MCNP was then created automatically to simulate the S values of organs and tumors simultaneously. To verify this system, results show that 84% of all organs are within 0% of dose difference with MIRDOSE3. The ratio of thyroid dose is about 1.01 and the effective dose ratio is The self S value for a sphere tumor model inserted in abdomen is 5.916E-3 mgy/mbq s. And additional S values for normal organs are also provided. The agreement of S values with MIRDOSE3 results shows the validity of our system for most target organs with 131 I administration. An example of the inserted tumor model indicates the ability to provide more dosimetric information for patient-specific dose evaluation. Our real-time Monte Carlo internal dose evaluation system could be a good help for more accurate dose estimation. 1. Introduction According to the MIRD (Medical Internal Radiation Dosimetry) technique [1,], average absorbed dose of an organ can be calculated by the sum of the products of S value and cumulated activity from different source organs. MIRD pamphlets have been tabulated the S values for different kind of radioisotopes based on specific geometry of reference men, so the computation can be very fast by the assistance of computer programs. MIRDOSE3 [3-6] developed by Oak Ridge Associated University is this kind of software using pre-simulated data and user inputs of the information about residence time, which has been used wildly in the nuclear medicine community since In addition to normal organs, the dose of tumor can also be evaluated by nodule module in this program which uses the absorbed fractions for spheres published in MIRD Pamphlets 3 and 8 with different radius. However, the tumor sphere has no geometric relationship with reference men. So additional dose from tumors to adjacent organs and from organs to tumors can not take into account in MIRDOSE3, which cause the underestimation of dose in radiation protection and radioimmunotherapy purposes. Some studies addressed this problem by inserting a spherical tumor model into the anthropomorphic phantom built by their own and applying a real-time Monte Carlo simulation system like EGS4 or MCNP. MABDOSE [7] and DOSE3D [8] were this kind of software that S values for tumor as source and target organs could be calculated for the radiation protection of normal organs. However, if the tumor is not in regular shape, which happened all the time, the tumor model provided by these programs could cause some systematic errors. Another kind of approaches using CT or MRI images to create structural phantom [9-11] was mostly dedicated to radioimmunotherapy of tumor and provided 1

2 patient-specific dosimetry. The creation of such phantom is a rugged errand. It needs lots of man and computational power, and also the simulation is very time consuming. It would thus be of interest to combine all the advantages mentioned above and having more accurate dose estimation. In this study, INER anthropomorphic phantom was built based on the MCNP (Monte Carlo N-Particle) code with the ability of changing dimensions of each organ and body size. Different tumor models can be inserted into the phantom with different size and shape, so additional dose information would be provided. The goal is to establish a more accurate internal dose evaluation system using the INER phantom and real-time Monte Carlo simulation.. Materials and methods.1. Anthropomorphic phantom and tumor model The anthropomorphic phantom constructed by INER was mainly based on the adult phantom built by Cristy and Eclerman in ORNL. It is described in a right-hand coordinate system with the origin at the center of the base of the trunk section. All dimensions are in centimeters. The equations were slightly modified to avoid some problems in terms of particle loss and asymmetry in the combinatorial geometry. The arm bones were defined to maintain the consistency as a pair by: x y xz x x0 + ± µ z ) x x (1 where the ± sign indicates positive for the left side and negative for the right side, the µ sign is opposite. The fourth-degree equation of thyroid used in ORNL phantom cannot apply to the MCNP Monte Carlo simulation system because of the limitation of computational ability. Thus, the thyroid model developed by Yamaguchi [1] was adopted to instead of the original one. The equation was defined as two spheroids with equal volume. x ± 1.00 y z , ) ( where in the first term, the plus sign is used for right spheroid, the minus sign, for left spheroid. The total volume was maintained to 19.9 cm 3. A comparison between this model and ORNL model is shown in FIG. 1.

3 FIG. 1. The thyroid models from ORNL adult phantom (left) and INER phantom (right). Some other modifications were also done within the INER phantom, like the re-creation of skin model and solving the overlapping problem between the esophagus and the upper portion of spines. The height and weight of this phantom were 179 cm and 74 kg, respectively. And it is shown in FIG. with skin and muscle removed. FIG.. Representation of INER phantom with skin and muscle removed. The tumor models can be constructed and inserted into the INER phantom by a GUI computer program. Shapes including sphere, spheroid, cylinder, cone, and cuboid are available. If the tumor model overlaps with other organ, the superimposed volume can be chosen between these two compositions. Moreover, the volume will be re-calculated and the dose evaluation will be according to the new mass... Monte Carlo simulation The simulation of particle transport, based on MCNP (Monte Carlo N Particle) code, was recorded to calculate the specific absorbed fractions of 1 target organs and tumor models. The activity was uniformly distributed in the source organ. A graphic user interface which allowed users to input geometric parameters and inserted locations of tumor models was developed. Within this program, the radionuclide, the geometric parameters, simulation parameters and source distributions can also be selected to accord with the actual situation. An input file conformed to MCNP code was generated for automatic patch run. The S values were then derived from the SAFs based on the new mass of the target organs. To validate the INER phantom and the Monte Carlo simulation system, the administration of 131 I was performed. According to ICRP report 53 [13], five source organs including thyroid, stomach, small 3

4 intestine, kidneys, and bladder were chosen with the residence times of.53 d, 1.66 hr, 1.66 hr, 5.7 min, and 1.3 hr, respectively, when the uptake ratio of thyroid is 5%. The absorbed doses of 1 organs and the effective dose calculated according ICRP report 60 were compared with the MIRDOSE3. For non-penetrating radiation, the self-absorbed fraction was set to 1.0 and otherwise 0.0 in general situations. But for the hollow organs, the dose of the wall was half of the dose of content. For the bone marrow model, the marrow dose was set equal to the bone dose under the assumption that red marrow are uniformly distributed in the whole bone with the same the energy absorption coefficient of bone surface. The absorbed dose of gonads was represented by the mean dose of ovaries and testicles. There were events simulated for each source organ with the relative standard deviation less than 10%. It is within the reliable region of accuracy..3. Case study The additional doses of normal organs contributed by tumor model were studied. A 1-cm-radius sphere tumor model with the density of g/cm 3 was inserted into the INER anthropomorphic phantom just below stomach and pancreas (FIG. 3). The weight was 4.13 g. The S values were calculated by means of uniformly distributed of 131 I in tumor and compared with the data provided by the nodule module of MIRDOSE3. FIG. 3. A sphere tumor model with 1 cm radius inserted in the abdomen of INER phantom (pink object with an arrow). 3. Results 3.1. Program validation The absorbed dose and effective dose estimated from our system and MIRDOSE3 are shown with the ratios in Table I. The dose ratio of thyroid is This means the fourth-degree thyroid model can be replaced by two spheroids for the purposes of simplifying the geometry and accelerating the simulation. 84% of all organs are within 0% of dose difference, and the effective dose ratio is As expected, our system shows the accuracy of dose evaluation compared with MIRDOSE3. The major errors are coming from the mass difference and the accuracy of Monte Carlo simulation. 3.. Tumor model simulation The S values (S(X TUMOR)) contributed from the tumor are listed in Table II. Note that the data show the inverse relationship with the distance from the tumor and zero order relationship with mass because of the reciprocity principle. The self S value for the tumor is E-03 mgy/mbq s. The 4

5 data from MIRDOSE3 for 4 g and 6 g spheres are E-03 and E-03 mgy/mbq s, respectively. The results are in good agreement between them. 4. Discussion The advantages of using tabulated data to estimate internal dose are fast computation and many existed software packages, like MIRDOSE3 and NucliDose, can be used. Since the stationary anthropomorphic phantom can not be modified, the dose of tumors from normal organs and the dose of organs from tumors always been neglected. In our system, the tumor models with different shapes can be inserted into the INER phantom to evaluate the additional information of dose which is important to radiation protection and radiotherapy with real patients. In the application of radiation protection, this system is convenient to estimate the dose from tumors and the dose from organs and has reliable accuracy compared with MIRDOSE3. Table I. Comparison of absorbed dose (mgy/mbq) and effective dose (msv/mbq) using INER phantom and MIRDOSE3. Organ INER phantom MIRDOSE3 Ratio Gonad.91E E Red marrow 5.07E-0 7.8E Colon 4.39E E Lungs 7.06E E Stomach 4.4E E Bladder 4.40E E Breast.84E E Liver.33E E Esophagus 1.4E Thyroid 3.45E+0 3.4E Skin 6.3E E Adrenals 3.34E E Brain 1.5E E Large intestine 4.1E-0 5.3E Small intestine.3e-01.60e Kidneys 6.7E-0 6.0E Muscle 8.64E E Pancreas 4.91E-0 5.5E Spleen 3.66E-0 4.9E Thymus 1.06E E Uterus 5.0E-0 5.9E Effective dose 1.73E E source organ 5

6 When the body size of a patient is significant different to reference man, the modifiable organ size is favored. Based on the orthogonal radiographs, CT, MRI or sonograms, the geometry of organs can be derived with relative locations with each others. The INER phantom can be modified to adapt to that situation with more accurate dose evaluation. In the case of tumor in the left side of the lung, the dose received by left organs is considerably higher than that received by those on the right. Thus, it is important to have the information of each organ in a pair separately. With this system, pair organ, like lungs, kidneys, ovaries, testicles, and breasts, can be taken as two individual organs with more accurate dose distribution on radiation protection purpose. Moreover, the effects of different tumor models and the dose of tumor contributed from normal organs are still need to be examined for the purposes of radiation protection and radiotherapy. More precise tumor models and reference man would be helpful to have more accurate dose evaluations. Table II. The S values (mgy/mbq s) of target organs caused by tumor model as source organ. Organ Gonad Red marrow Colon Lungs Stomach Bladder Breast Liver Esophagus Thyroid Skin Adrenals Brain Large intestine Small intestine Kidneys Muscle Pancreas Spleen Thymus Uterus INER reference man 3.67E E E E-07 3.E E E-08.19E-07.89E E E E E E E E E E E E-08.79E-07 6

7 5. References 1. Snyder, W.S., Ford, M.R., Warner, G.G., Fisher, H.L., Estimates of absorbed fractions for mono-energetic photon sources uniformly distributed in various organs of a heterogeneous phantom. J. Nucl. Med., 10:5-5, (1969).. Society of Nuclear Medicine, S, Absorbed dose per unit cumulated activity for selected radionuclides and organs. MIRD Pamphlet 11, New York (1975). 3. Cristy, M., Mathematical phantoms representing children of various ages for use in estimates of internal dose. ORNL Report ORNL/TM-367 (1980). 4. Cristy, M., Eckerman, K., Specific absorbed fractions of energy at various ages from internal photons sources. ORNL Report ORNL/TM-8381 (1987). 5. Stabin, M., Watson, E., Cristy, M., Mathematical models and specific absorbed fractions of photon energy in the nonpregnant adult female and at the end of each trimester of pregnancy. ORNL Report ORNL/TM-1907 (1995). 6. Stabin, M.G., MIRDOSE: Personal computer software for internal dose assessment in nuclear medicine. J. Nucl. Med., 37: , (1996). 7. Johnson, T.K., MABDOSE: a generalized program for internal radionuclide dosimetry. Comput. Methods Programs Biomed. 7: , (1988). 8. Isabelle, C., Marcel, R., Jean, G., DOSE3D: EGS4 Monte Carlo code-based software for internal radionuclide dosimetry. J. Nucl. Med., 40: , (1999). 9. Sgouros, G., Chiu, S., Pentlow, K.S., Three-dimensional dosimetry for radioimmunotherapy treatment planning. J. Nucl. Med., 33: , (1993). 10. Giap, H.B., Macey, D.J., Podoloff, D.A., Development of a SPECT-based three-dimensional treatment planning system for radioimmunotherapy. J. Nucl. Med., 36: , (1993). 11. Kolbert, K.S., Sqouros, G., Scott, A.M., Implementation and evaluation of patient-specific three-dimensional internal dosimetry. J. Nucl. Med., 38: , (1993). 1. Yamaguchi, Y. JEUNESSE: A computer code to calculate photon external doses using age-specific phantoms. J. Jpn. Health Phys. 7:305-31, (199) (in Japanese). 13. International Commission on Radiological Protection, Radiation dose to patients from radiopharmaceuticals. Publication 53. Annals of the ICRP, 18, No. 1-4, Pergmon Press (1988). 7