الفهرس 
INTRODUCTION AND AIM OF THE WORKOnly 14 pages are availabe for public view
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Abstract
Radiation therapy is the use of certain type of energy (originated from ionizing radiation) to kill cancer cells and shrink tumors. According to the recommendations of international institutes of radioprotection, an increasing attention is being paid to the patient protection during cancer radiotherapy treatments. External radiotherapy radiation may cause some side effects on the surrounding normal cells. It is very essential to have theoretical model for the human body to study the effect of the radiation exposure with any radiation source and for different exposure time. So, many models were put for this purpose. Now, Monte Carlo techniques are widely used in radiation protection field for its high accurate results and also for its ability to deal with the nature of radiation transport.
The present study is planned to measure and assess the radiation doses received by the target organ and the surrounding healthy sensitive organs in external radiation treatment. This will make it possible to produce almost any desired dose to the tumor volume and to deliver the dose which has the highest probability to cure the patient without inducing severe complications in normal tissues. For this purpose we used the three dimensional Monte Carlo Nuclear Particles Code (MCNP). We designed also the mathematical adult human-equivalent model according to Snyder et al.(1974) with certain modifications in some organs. This model has different densities and chemical compositions for lungs, skeleton, and soft tissues. Monte Carlo computer calculations of the photon spectra and the ratios of dose equivalent at the surfaces and in some of the internal organs were performed.
Great interest was directed towards the breast and bladder because these two organs are the most liable to be affected by cancer in women and men, respectively in Egypt. Before calculation of the dose distribution in the mathematical model, it is necessary to characterize the source and the beam quality. In this respect, depth dose distribution was calculated by MCNP-4B code and measured by ion-chamber using water phantom (80 x 80 x 80 cm3). Moreover, beam profiles in different depths for square field (10x10cm2) was calculated and the calculated data was compared with the available published measured data.
In case of breast radiotherapy, radiation dose distribution in the breast and the surrounding organs was measured using human-phantom. During experimental practice, The Alderson Rando Phantom (ARP) was irradiated following the experimental set up in an actual breast radiotherapy. Two treatment fields (directions) were considered in this process to maximize the dose at the center of the target (breast) and minimize it at the surrounding (non target) healthy organs. The LiF Thermoluminescent Dosimeters (TLD) were put at different places inside the phantom and used to verify dose measurements. TPS (Treatment Planning System) calculates the absorbed dose at the pre-described points at which the TLD detectors were inserted inside the phantom.
A Co60 source is used in this experiment as a source of gamma rays with average energy 1.25 MeV. The Co60 source is in the form of a cylinder of diameter 2.0 cm and 2.0 cm length with its circular end facing the phantom. The daily fraction (df) dose was 200 cGy during the treatment session. The source to surface skin distance (SSD) is 80 cm. After irradiation process, the TLDs were removed and were readout by TLD reader (Harshaw 4000, USA) at Radiation Protection Department, Atomic Energy Authority.
All the experimental setup information was fed to the MCNP-4b code to calculate the dose distributions at the selected pointes. The depth dose distribution was performed also. Both measured and calculated data were compared.
From the comparison between the measured and the calculated results, it was found that the difference between the measured (M) and calculated (C) doses relative to the measured [(M-C)/M] ranged from -0.376 up to 0.269 The comparison showed also that the calculated and the measured data have the same behavior in different organs. This means that there is a good agreement between measured and calculated data. Moreover, depth dose distribution was found to decrease rapidly with the distance from the surface skin and the comparison between calculated and measured depth dose gives a good agreement.
Once the theoretical measurements of the radiation doses was validated by comparison with measured one, the effect of some factors such as shielding, obesity and breasts size on dose distribution can be studied. This will be of great importance in obtaining the optimum conditions in the radiotherapy practice. For example, the mathematical model was shielded (except at the passageway of the treatment radiation field) with 0.5 cm lead apron to clarify the impact of shielding on the radiation doses received by the non target organs during treatment. Shielding was found to greatly reduce the radiation doses at different non target organs by 11% to 46%.
On the other hand, the dose distribution in different organs during a case of actual bladder radiotherapy was measured using same human-phantom. During experimental practice, The ARP was irradiated following the experimental set up in an actual bladder radiotherapy case. Four treatment fields (directions) were considered in this process. The TLD were also used to verify dose measurements.
6 MeV X- ray beam is used in this experiment. The daily fraction (df) dose was 200 cGy during the treatment session. The source to surface skin distance (SSD) is 100 cm. All the experimental information was fed to the MCNP-4B code to calculate the dose distributions at the selected pointes. The measured and calculated data were compared.
In order to obtain the optimum conditions for bladder radiotherapy, the impact of filling of the urinary bladder on dose distribution was studied. The volume of bladder model was doubled and filled with water then the doses distribution was calculated at the same selected sites. The only greatest change was found in uterus, where the received dose by it decreased from 68.4% to 18.3% of the given dose. Moreover, more attenuation of the four treating radiation beams occur after crossing the filled bladder.
The comparison indicates that there is good agreement between the measured and calculated doses in the target organ (bladder) and the surrounding healthy organs. Comparison between the calculated and the measured results for a single beam only also showed a good agreement between both calculated and measured results.
The comparison between measured and calculated depth dose distribution for 6 MeV X-ray was carried out. The depth dose distributions for the anterior, posterior, right, and left fields were performed, and the result gives a clear description for the radiation behavior inside the human equivalent model.
In conclusion, The MCNP model used in the present study shows accurate results and is proven to be suitable for applications in radiation protection field. It can be also extended to study the dose assessment in the human body in case of external exposure for any type of radiation sources with different activities.