| The GZP3 high dose rate (HDR) 60Co brachytherapy unit product is a two-dimensional radiation therapy equipment and the high dose rate brachytherapy has been widely used in the treatment of cervical cancer. According to the reviewed report of American Association of Physicists in Medicine (AAPM), all medical instruments should be corrected and measured its dosimetric parameters before they are put into clinical application.The high dose rate brachytherapy is a term used to describe the short distance treatment of cancer using radiation from small, encapsulated radionuclide sources. The sources are placed in body cavities close to the tumour volume or implanted within the tumour volume, and they give the tumor high dose irradiation. In the current commercial treatment planning system, the calculation of dose distribution around the radioactive source is generally in the uniform water environment.American Association of Physicists in Medicine (AAPM) TG-43 revised the calculation model of the dose distribution around a linear radiation source in brachytherapy, and recommended the calculation mode of the radiation source dosimetry parameters can be obtained by experiment or Monte Carlo method in homogeneous aqueous medium. However, for those surrounding organs at risk of esophageal cancer patients’ peripheral, including the spinal cord, sternum, trachea, lung, their mass density, electron density and elemental composition are different from those of water. While the commercial treatment planning system neglects these differences in the dose calculation. In addition, different organ geometric anatomical position may influence the accuracy of dose calculation, and ultimately affect the dose volume histogram (DVH) accuracy. DVH is an important evidence to estimate radiation therapy planning.Therefore, this research mainly discussed two aspects of these problems.First, according to the report of TG43U1, AAPM recommended using Monte Carlo method to simulate GZP3 high dose rate brachytherapy dosimetry parameters, including the dose rate constant, geometric function, radial dose function and anisotropy function of 60Co radiation source in homogeneous aqueous medium.Second, in order to account for the presence of tissue inhomogeneities in the patient anatomy around the oesophagus, their location and cross sectional dimensions were derived by calculating the average of corresponding measurements on the computed tomography (CT) transversal images utilized for the brachytherapy plan, then a three-dimensional model is established. The MCNP5 is used in patient-equivalent phantom geometry and uniform water medium phantom geometry respectively, and we compared the results in terms of dose volume histograms of esophagus, spinal cord, sternum and lung respectively.In order to validate the correctness and feasibility of this system and the method of research, we use MCNP5 to simulate the dosimetric parameters of 192Ir radioactive sources and compared with the data of Daskalov GM.The study found that the presence of patient inhomogeneities does not alter the delivery of the planned dose distribution to the planning treatment volume (PTV). Regarding the PTV, the presence of the surrounding inhomogeneities does not affect the dose distribution. This is due to the predominance of the primary dose component at distances close to the source where the PTV is situated. In the case of the sternum bone, the common practice of consider the patient geometry as a homogeneous water medium which calculations underestimating dose relative to the calculated dose results in the inhomogeneous patient-equivalent geometry up to 1.1%. This, however, is due to the interference of the air filled trachea between the implanted catheter and the sternum that results in negligible attenuation and is not accounted for by the common practice. Regarding the spinal cord, the common practice which assumes a homogeneous water geometry does not predict the increased attenuation within the bone material that results in a corresponding dose decrease within the spinal cord Consequently, an overestimation of dose in the spinal cord of up to 16.08% by the common practice is observed relative to the calculated dose results in the inhomogeneous patient-equivalent geometry.In clinical trial, the maximum prescription dose of PTV is limited by the amount dose of those organs at risk. In the treatment of radiation therapy of esophageal cancer patients, it is important to protect the spinal cord, whose absorbed dose is seriously affected the dose of PTV. From the above analysis, we can conclude that as esophageal cancer program target area complement exposure, the spinal cord dose is undervalued in the commercial treatment planning system whose dose distribution calculations are based on the dosimetric parameters obtained in the uniform water model. So we can consider increasing the dose of PTV.We obtained some 60Co dosimetric parameters which can be used as original data in the HDR treatment planning system, which is applicable. |
PDF Full Text Request