| Together with surgery and chemotherapy, radiotherapy plays an important role in oncology, both in the definitive and palliative aspects of treatment. It treats with about 60%-70% patients suffering from cancer and cures 40% of them. The purpose of radiotherapy is to achieve the best treatment, which is dependent on maximizeing the dose to the tumor while minimizeing the dose to surrounding normal tissue and organs at risk from radiation.Radiotherapy is one of the most important methods for tumor treatment and engages computer radiotherapy treatment planning system as its key tool.In this thesis, we focused on one of the key problems:dose calculation, which is crucial to develop a radiotherapy treatment planning system.With the development of oncology, the improvement of precise technology on radiotherapy has never stopped. In 1949, Swedish scientists Leksell proposed the concept of radiosurgery using stereotactic techniques, delivery high dose beam ray to the target and destroyed tumor in only one time, then the technology was named stereotactic radiosurgery (SRS), and it had be used in clinical application in 1955. In 60s of last century, Japanese scientists via a motor-driven collimator control the shape of beam, making the beam cross-section coincided with target projection in BEV derection. The early 70s, American scientists began to realize the importance of intensity modulated radiotherapy (IMRT), and studied on it. Late 80s early 90s, the United States, Germany and other countries had developed a commercial X-knife; Sweden had developed third generation of y-knife system; In the same period "Cyber Knife"came out. In 1995,the conception was proposed which use dynamic MLC in IMRT by Spirou et al and started to apply in prostate cancer treatment. In the new century, with the development of radiation physics, radiation biology, clinical oncology, medical imaging and other related disciplines, radiation therapy technology has undergone tremendous changed. Precision radiation therapy is to "precise positioning, accurate planning and precise treatment " for the characteristics of the new radiotherapy technology collectively. Precise radiation therapy in cancer treatment technology enables high accuracy, high dose, high efficacy and low damage the modern radiation patterns possible.Precise radiotherapy compared with conventional radiotherapy technology has great progress, include:(1) Commonly precise radiotherapy used CT, MRI, PET and other advanced imaging technology for determining the target and vital organs, and conventional radiation therapy commonly used X-ray simulator images;(2) Modern radiotherapy always together with TPS.It is necessary to reconstruct and display image datas in three-dimensional is dependent by designed treatment plan.and 3-d display about dose distribution, and conventional radiation therapy techniques are only for 2-D treatment plan design;(3) Modern radiotherapy commonly adopt mask, vacuum pads, head ring to combine with 3-D coordinate system, in order to repeat target position accurately. Generally conventional radiotherapy techniques such as surface lesions compared outlined rough positioning methods. (4) Morden radiotherapy uses multiple non-coplanar radiation field or multiple non-coplanar arc radiation radiation, rather than conventional radiotherapy techniques use single or two fied irradiation, three field irradiation;(5) Precise radiotherapy enables high dose area and target shape more consistent, and to ensure that inside target and the surface dose equal everywhere. That is to achieve the appropriate dosage conformal, conventional radiotherapy can not be achieved.Dose calculation is the criterion to evaluate the merits of the treatment planning, optimization and inverse treatment planning is also based on a treatment plan design. treatment planning system is developing, while the rapid development of its core content of the dose calculation models and methods are constantly evolving. The development mainly from the dose calculation model,which improve from correction-based to model-based dose calculation to changes in the pattern and the mutual penetration between them. Correction-based dose calculation model which is based on field data under the conditions of the standard measurement of depth dose, off-axis ratio, scatter factor, appended to the necessary amendments and combined with the physical conditions to reflect the actual radiation dose distribution in radiotherapy..As computer storage capacity and computing speed improving, more accurate dose calculation methods continue to be used in treatment planning system. Such methods are generally focus on dose calculation model,which take into account the interaction in the human body entirely, in order to make a perfect agreement between the dose distribution based on theoretical and experimental measurements.In this paper, two categories of methods described in detail, focusing particularly on several model-based dose calculation methods analysis and research. By following,:Dose distribution in the phantom is separated into the primary and scattered contribution of radiation. Such method include Clarkson method, differential scattering air ratio method, Day's law.Monte Carlo is to use the principles of statistical methods to simulate a large number of single-energy photons and interact in the transport process in the phantom. Photons incident a medium, through the interaction of particle in the medium, and transfer a part or entire of ionizing radiation energy. Advantage of the method it applies mainly in the phantom arbitrary geometry and a variety of secondary particles on.Another accurate model of dose calculation is convolution. In this model, the radiation dose calculated by energy fluence convolute with a point spread function or pencil beam kernel. Convolution method for distribution of any luminous radiation field, so this method is particularly suitable for irregular field calculations. In addition, because the convolution can be done by fast Fourier transform, convolution operation which makes the speed has been significantly enhanced.Through the several model-based dose calculation methods were analyzed. Convolution is selected for TPS, and the principles of the model is researched deeply. The method has been improved on the basis and evolved accuracy of model. Convolution model as one of the mainstream models currently applied to TPS, When assume the kernel is an invariate with depth change,the model is simplified a plane fluence distribution convolute wth a pencil beam kernel. Dose distribution in the plane which plumb to incident beam at a certain depth calculated by convolution. For pencil beam kernel used in model, according to Fermi theory, in the case of small angle scattering, can be expressed as two independent one-dimensional product in perpendicular directionOne-dimensional pencil beam kernel can be derived from measured broad beam profile.Then it can be used to reconstruct beam profiles of various field sizes for photons.For the studies we have done so far,the agreement between measured profiles and calculated profiles is excellent.This method has the advantage that the kernel thus obtained include all physical processes as well as other perturbations such as scattering from various components of the treatment machine head and variation of energy spectrum with depth and are therefore more realistic. Futhermore, the require data, namely, central axis depth dose and beam profiles,are routinely measured data and readily available. But there are still some problems about this method. Use a single pencil beam kernel for reconstruct dose distribution of field profiles,it is will fail because the affect of field size and energy spectrum.,lead to deviation of measurement and calculaion. Based on the number of kernesl from the measurement data,a kernel array according to field size and depth is builded up. In the process of calculation, accordance with the target of radiant conditions fit an approximate kermel, while ensuring the calculation speed, the maximum guarantee the accuracy of the model calculations. The experiment confirmed that satisfactory results can be achieved is a feasible solution.At the end of this thesis, we also discuss some problems which haven't been completed. In addition, we prospect some work which we'll do in the future. |
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