Introduction: A comparison of cytogenetic methods in the evaluation of occupationally received radiation dose by medical radiation workers and a study of adaptive response to subsequent higher radiation exposure in this group. Materials and Methods: Two cytogenetic methods, including chromosomal aberration analysis and cytokinesis – blocked micronucleus (CBMN) were carried out. The cytogenetic response of blood lymphocytes of 24 medical radiation worker, before and after one and two Gy gamma irradiation, was compared with the control group. Results: The frequencies of background the micronuclei assay and different chromosomal aberrations in radiation workers were more than those of non – exposed individuals (P<0.05). After one and two Gy in vitro irradiation of blood samples, these frequencies were lower for radiation worker than for the controls (P<0.05). Conclusion: Results indicate that in spite of considering a maximum permission dose by radiation workers, background level of cytogenetic defects in this group is higher than that of control group, but because of the adaptive response in cells wich were exposed to chroinic low doses of ionizing radation workers is lower than that of control group. The similarity of qualitative results in both methods, makes it possible to substitute simpler methods instead of complicated and time consuming ones.

Interaction of ionizing radiation causes ionization and excitation in atoms and molecules at biological system and production a number of deleterious and abnormal productions in a living system. Having advers effects of ionizing radiations necessitate exact determination of administrative amounts and patient doses, in order to prevent impermissible exposures of patients and theirs surroundings. Accurate calculation of radiation dose requires accurate data about the physical deacay characteristics of the radionuclide and its distribution in the body. On the other hand, the biologic function of different tissues of human body is different among persons. Therefore, there is the variability of the biological response of different individuals to the same radiation dose. How does on compute the total dose the patient form the time of administration (t=0) to the time when the dose rate has finally been reduced to zero In this article, our goal is the calculation and determination of dose in patients and providing the protectoral survey. In nuclear med clear procedures it is almost impossible to measure the radiation dose directly using any kind of radiation detection. Instead, this has to be calculated by using a variety of physical and biological data and special mathematical equations. An approach to determined radiation doses is the absorbed fraction method. This is the proceeded in four steps: calculation of the rate of energy emission by radionuclide in the source volume (s), calculation of the energy absorption by the target volume (T), calculation of the average dose rate (dD/dt), and calculation of the average dose D. That finally results dose. The overall expression may be used.                                        D(T¬S)=(1.44).F.A0.T1/2(eff) .s(T¬S) radWhere S factors is the dose rate form I mCi activity. It depends only on the phisical data that biokinetics of the radionuclidic distribution is more complex, and the uptake in the oragan is instantaneous, on the other hand, the disapperance of the radioactivity from the source doesn't have a single exponential term. In the case of a radiopharmaceutical with complex biokinetics, for radiation dose valculations, cumulated activity A needed for radiation dose calculations is determined by either graphic methods or multiexpontial curve-fitting methods. Then, Cumulated activity A in each organ may be calculated from the actual biokinetic data:The physical data such as radiations emitted by a radionuclide and the absorbed fractions for various source and target combinations of a standard main into on single term, 'S" factor, for radionuclide’s used in nuclear medicine, dose rate is nearly equivalent to the absorbed dose and can uses the principle of reducing exposure to the body, that is distance, time, and adequate shielding, to provide the ALARA.E(R) = (N.GAMMA/d2). em(linear)++x.tTherefore, it may provide necessarily safety in nuclear medicine by observing of protection principles and by mindfully application of radioisotopes and this increase the advantage of using ionizing radiation against on deleterious effects. 

Neutronic calculations of the 150 KW heavy water research reactor have been done. The most important parameters in these calculations are the effective multiplication factor (Keff), the flux distribution, the negative reactivity coefficients of fuel burn up, moderator temperature and void effect.The optimum position for target irradiation has been obtained, based on the flux.The effect of target dimensions on the Keff has been studied for different suitable target materials. The central thimble tube was filled with various materials, such as heavy water light water, polythelene, graphite, beryllium, nitrogen and air. Their conditions being the same, the effects of these filling materials on Keff and the flux distribution have been studied and thus the best filling material has been chosen. By fixing the irradiation, filling material and some other parameters related to radioisotope production, some target activity in (n , y), (n , p), (n , 2n), (n , a ) channels, irradiated in the central thimble reactor core for a specified period of time, have been calculated.

Nowadays, mammography is the Only existing method for screening breast cancer. Besides diagnostic limitations of mammography, this procedure may increase cancer risk in predisposed individuals -duello their exposure to ionizing radiation. Structural changes in chromosomes can activate oncogenes and deactivate tumor suppressor genes and therefore constitute an important mechanism for inducing cancer. The extent of chromosomal damages induced. by ionizing radiation constitutes a criterion to evaluate radio sensitivity and predisposition to cancer. In order to estimate this radio sensitivity, cytogenesis response of lymphocytes from 30 breast cancer patients before radiotherapy and chemotherapy to 3 Gray gamma-irradiation was compared with 30 healthy individuals. The assessment was peformed using micronuclei assay, a reliable method to examine cytogenetic effects of physico-chemical agents.Lymphocytes were stimulated with a mitogen 6 hour after irradiation.Cytochalasin-B was added 24 hours after the stimulation to inhibit the cytokines is of ymphocytes and in the 90th hour cells were harvested. Micronuclei count per 1000 binuclear lymphocytes showed that. the average number of spontaneous and co/Tested micronuclei (after irradiation and subtraction of .spontaneous micronuclei) was higher in breast cancer patients than control group (P<0.001). Considering the values higher than "Mean + 2SD" in control group, 33 percent of patients and 3 percent of control group have shown high radio sensitivity. Regarding the low incidence of mutation in highly penetrant genes ATM and BRCA1/2, a large number of patients with breast cancer may carry low penetrating genes that increase susceptibility to breast cancer. Cytogenetic studies may be used in future to screen individuals with risk of contracting breast cancer and to protect predisposed individuals from it.

Since the discovery of radioadaptive response in 1984, a worldwide effort has being carried out to clarify the basic mechanism of this phenomenon. Substantive evidence show that when human, animal and plant cells are exposed to low doses of an agent that induce DNA damages, they often become less sensitive to the detrimental effects of higher doses of those agents administered subsequently. This phenomenon has been termed an adaptive response. There is growing evidence for the existence of radioadaptive response. On the other hand, some investigators have reported entirely negative results in individuals with chromosome instabilities as well as healthy persons. Additionally, there are reports indicating that low-level pre-irradiation of human lymphocytes can be problematical; for example in radiotherapy, the induction of radioadaptive response in cancerous cells can be a great problem. In our previous studies we observed a young healthy non-responder who showed a severe hypersensitivity to radiatin after a low dose treatment. We selected this hypersensitive donor for a follow-up study. To assess whether this kind of hypersensitivity to ionizing radiation is a temporary situation or not, we evaluated the frequency of radiation-induced chromosome aberratins during a 6 month follow-up. Peripheral blood lymphocytes of this donor were exposed to an adapting dose of 10 cGy X-rays followed by a challenge dose of 3 Gy X-rays with different radiation qualities. Our blood donor did not show adaptive response in any experiment. In this regard, considering the lack of radioadaptive response in all experiments, it may be concluded that the lack of of such a response is not a temporary situation. Despite the fact that we do not know the frequency of individuals who show such an extraordinary synergistic effect, it can be concluded that possible implications of radio adaptive response in the estimation of risks of low-level radiation exposure may be more problematical.

The presence of natural background radiation and environmental radioactivity is due to distribution of radionuclides in the earth and cosmic rays. These causes exposure to biological area and human body. In most countries natural background radiation has been measured repeatedly, but there is no accurate and complete information in Iran. The contents of natural radio nuclides (uranium, actinium, and thorium) as well as the thin layer of atmosphere in the higher altitude regions are reasons of high level of human exposure. The aim of this survey was focused on determining the current background radiation of Chaharmahal and Bakhtiari province, which is the highest altitude region in Iran. The outdoors" environmental monitoring exposure rate of radiation was measured in 100 randomly selected regions using a portable Geiger-Muller detector. Eight measurements were made for each region and averaged as exposure rate of natural background radiation of that region. The mean exposure rate was found to be 28.2±µR/h and an overall outdoor dose rate was 49 nGy/h which is higher than the worldwide mean value of 44 nGy/h as reported in UNSCEAR, 1998. The annual effective dose equivalent was found to be 0.49 mSv/y. The results showed that there is a good correlation between altitude and exposure rate. In relation to the altitude and geological structure of the province the results also showed that the higher altitude regions has the higher natural background radiation level.