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Abstract
(1) In order to characterize the beam quality, depth dose distributions for γ and X -rays were 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) were calculated and the calculated data was compared with the available published measured data. The results are useful to verify the accuracy of the present calculations.
(2) The contributions of γ and X-rays with different energy intervals were performed. At the low depth, the contribution of the high energy photons is the maximum and the contribution decreases with the depth. On the contrary, the low energy photons contribute more and more as the depth increases.
(3) The three dimensional Monte Carol Nuclear Particle (MCNP-4B) code used in the present study shows accurate results and proven to be suitable for calculation of the dose distribution in the human-body using different radiation sources with different activities (Co60 radiation therapy unit with average -ray energy 1.25 MeV and Linac 6 MeV X-rays).
(4) The good agreement between the calculated and measured dose distributions in different organs during a case of actual breast and urinary bladder indicate that the Alderson Rando Phantom (ARP) and the thermoluminescent dosimeter (TLD) provide good tools for radiotherapy practice.
(5) In the present work, 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.
(6) In case of breast radiotherapy, two treatment fields were considered to maximize the dose at the center of the breast and minimize the dose at the surrounding healthy organs. Treatment Planning Systems (TPSs) calculate the absorbed dose at the pre- described points at which TLD detectors were inserted inside the phantom.
(7) During breast radiotherapy, the difference between measured (M) and calculated (C) doses relative to the measured [(M-C)/M] in different places ranged from -0.376 to +0.269. The measured and calculated doses have the same behaviour in different organs.
(8) Depth dose distribution for gamma rays from Co60 in the human equivalent model decreases rapidly with the distance from the skin surface. The dose at the point 0.5 cm has the maximum value. The build up region is clear from zero up to 0.5 cm then the dose decrease rapidly with depth inside the model. Variation between the measured and calculated doses at 11.5 cm and 15.5 cm is due to the methodology, i.e., using water phantom instead of the tissue equivalent phantom. However, good agreement between measured and calculated doses was found .
(9) The mathematical model used in the present work was shielded with 0.5 cm lead apron to reduce the received or scattered radiation to the non target organs of patients under giving breast radiation therapy. The average calculated doses over the specific volume were decreased from 28.6 to 22.1, 26.8 to 12.3, 21.3 to 16.2, 14 to 11.3 and 8.9 to 7.3 cGy in right breast, left lung, thyroid gland, liver and right lung, respectively. The shield apron was found to reduce the dose of scattered radiation by almost 11 % up to 46% at different sites.
(10) The whole trunk region of the mathematical model was covered with 3 cm-thick fat layer to elucidate the impact of obesity on radiation dose distribution during breast radiotherapy. About 18.6% decrease in the dose ratio received by all tissues around