A feasibility study on the use of phantoms with statistical lung masses for determining the uncertainty in the dose absorbed by the lung from broad beams of incident photons and neutrons.

Computational models of the human body have gradually become crucial in the evaluation of doses absorbed by organs. However, individuals may differ considerably in terms of organ size and shape. In this study, the authors sought to determine the energy-dependent standard deviations due to lung size of the dose absorbed by the lung during external photon and neutron beam exposures. One hundred lungs with different masses were prepared and located in an adult male International Commission on Radiological Protection (ICRP) reference phantom. Calculations were performed using the Monte Carlo N-particle code version 5 (MCNP5). Variation in the lung mass caused great uncertainty: ~90% for low-energy broad parallel photon beams. However, for high-energy photons, the lung-absorbed dose dependency on the anatomical variation was reduced to <1%. In addition, the results obtained indicated that the discrepancy in the lung-absorbed dose varied from 0.6% to 8% for neutron beam exposure. Consequently, the relationship between absorbed dose and organ volume was found to be significant for low-energy photon sources, whereas for higher energy photon sources the organ-absorbed dose was independent of the organ volume. In the case of neutron beam exposure, the maximum discrepancy (of 8%) occurred in the energy range between 0.1 and 5 MeV.

Fetal and maternal dose assessment for diagnostic scans during pregnancy.

Despite the concerns about prenatal exposure to ionizing radiation, the number of nuclear medicine examinations performed for pregnant women increased in the past decade. This study attempts to better quantify radiation doses due to diagnostic nuclear medicine procedures during pregnancy with the help of our recently developed 3, 6, and 9 month pregnant hybrid phantoms. The reference pregnant models represent the adult female international commission on radiological protection (ICRP) reference phantom as a base template with a fetus in her gravid uterus. Six diagnostic scintigraphy scans using different radiopharmaceuticals were selected as typical diagnostic nuclear medicine procedures. Furthermore, the biokinetic data of radioiodine was updated in this study. A compartment representing iodide in fetal thyroid was addressed explicitly in the biokinetic model. Calculations were performed using the Monte Carlo transport method. Tabulated dose coefficients for both maternal and fetal organs are provided. The comparison was made with the previously published fetal doses calculated for stylized pregnant female phantoms. In general, the fetal dose in previous studies suffers from an underestimation of up to 100% compared to fetal dose at organ level in this study. A maximum of difference in dose was observed for the fetal thyroid compared to the previous studies, in which the traditional models did not contain the fetal thyroid. Cumulated activities of major source organs are primarily responsible for the discrepancies in the organ doses. The differences in fetal dose depend on several other factors including chord length distribution between fetal organs and maternal major source organs, and anatomical differences according to gestation periods. Finally, considering the results of this study, which was based on the realistic pregnant female phantoms, a more informed evaluation of the risks and benefits of the different procedures could be made.

Evaluation of dose conversion coefficients for an eight-year-old Iranian male phantom undergoing computed tomography.

In order to construct a library of Iranian pediatric voxel phantoms for radiological protection and dosimetry applications, an Iranian eight-year-old phantom was constructed from a series of CT images. Organ and effective dose conversion coefficients to this phantom were calculated for head, chest, abdominopelvis and chest-abdomen-pelvis scans at tube voltages of 80, 100 and 120 kVp. To validate the results, the organ and effective dose conversion coefficients obtained were compared with those of the University of Florida eight-year-old voxel female phantom as a function of examination type and anatomical scan area. For a detailed study, depth distributions of organs together with the thickness of surrounding tissues located in the beam path, which are shielding the internal organs, were determined for these two voxel phantoms. The relation between the anatomical differences and the level of delivered dose was investigated and the discrepancies among the results justified.

Neutrons applications in cancer treatment and in specific diagnostics.

The major effect of ionizing radiation in cells is to destroy the ability of cells to divide by damaging their DNA strands. Extensive researches are leading to an understanding that the characteristics of high LET radiations such as fast neutrons and low LET radiations like protons, photons and electrons are different; because of different types of their interactions with tissue. Low LET radiations mostly damage tissue by producing free radicals. Oxygen has an effect of enhancing free radical formation in cells. Indeed hypoxic cells, which exist in malignant tumors, are radio resistant under irradiation with low LET radiations. In contrast, neutron interacts with tissue primarily via nuclear interactions, so its biological effectiveness is not affected on the presence of oxygen. The required dose to kill the same number of cancerous cells by neutrons is about one third in comparison with photons. Clinical reports show that a full course of treatment with neutrons consists of 12 treatment sessions, compared to 30-40 treatments with photons or electrons. In conclusion, in this review we describe which cancers or tumors could be better treated with neutrons. We also refer to whether neutrons could be used for diagnosis.

Investigation of activation rate uniformity in a prompt-gamma rays IVNAA facility.

The preliminary design studies for developing a prompt-gamma rays in vivo neutron activation analysis facility are in construction at Neutron Activation Research Centre of Ferdowsi University of Mashhad. In this work, activation rate distribution is studied in different bilateral configurations in order to reduce the CV of activation rate distribution throughout the body to less than 10%. It means that the maximum RMS deviation from the mean value of distribution must be smaller than 0.5%. The best case includes two pairs of (241)Am-Be neutron sources. Each pair (with 70 cm distance between two sources) is positioned within up and down 40 x 110 cm(2) area graphite collimators. Also, four moderator/reflector objects are added to transversal body surfaces with 5 cm thick water and 3 cm thick graphite. The proper thickness of polyethylene pre-moderators is chosen 8 mm. The RMS deviation of distribution is 0.3% with the mean activation rate of 1.85 x 10(-6) cm(-3).

Evaluation of specific absorbed fractions from internal photon sources in ORNL analytical adult phantom.

To design a diagnostic or therapeutic irradiation programme, there is a need to estimate the absorbed dose. In this investigation, specific absorbed fractions (SAFs) were calculated based on Cristy and Eckerman's analytical adult phantom, by MCNP4C Monte Carlo code. SAFs were estimated with uncertainty <3%, for about 600 source organ-target organ pairs at 12 photon energies (these data are available at http://www.um.ac.ir/~mirihakim). Then these results were compared with Cristy and Eckerman's, which were based on direct Monte Carlo, reciprocity principle and point source kernel methods. Also, agreements and disagreements between them for different states were discussed.