In this work, the influences of the soil densities and the radionuclide depth distributions(RDD) on the Full Energy Peak Efficiency (FEPE) calculation of the in-situ gamma rayspectrometer using the In Situ Object Counting Systems (ISOCS) software were studied. The data of the RDDs at the sites were investigated by using laboratory HPGe gamma spectrometer. Six different RDDs of 40K, 226Ra and 232Th were found at four studied sites with radionuclide deposition moving from surface to deeper positions. The results show that FEPE values vary strongly for the different RDDs, especially for the low gamma ray energies. Use of the uniform model for calculating FEPEs can result in noticeable errors from 29% to 101% for the realistic RDD of the exponential form (surfaceradionuclide deposition), negative variations from 14% to 30% for the realistic RDD of having a radionuclide deposition at the 30 cm depth, and negligible variations of less than 5 % for the realistic RDD of quasi uniform form in the range of gamma ray energies of interest.
The dose distribution calculation is one of the major steps in radiotherapy. In this paper the Monte Carlo code MCNP5 has been applied for simulation 15MV photon beams emitted from linear accelerator in a case of lung cancer of the General Hospital of Kien Giang. The settings for beam directions, field sizes and isocenter position used in MCNP5 must be the same as those in treatment plan at the hospital to ensure the results from MCNP5 are accurate. We also built a program CODIM by using MATLAB® programming software. This program was used to construct patient model from lung CT images obtained from cancer treatment cases at the General Hospital of Kien Giang and then MCNP5 code was used to simulate the delivered dose in the patient. The results from MCNP5 show that there is a difference of 5% in comparison with Prowess Panther program – a semi-empirical simulation program which is being used for treatment planning in the General Hospital of Kien Giang. The success of the work will help the planners to verify the patient dose distribution calculated from the treatment planning program being used at the hospital.
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