Dosimetric Characteristics Of Photon Beam In The Medium Interface Region

Dose accuracy is one of the core issues during Radiation therapy. Research shows that more than 5% of the dose deviation may cause serious complications or lead to failure of tumor control. All in the course of radiation therapy to achieve the dose accuracy object. Dose calculation deviation of the radiation treatment planning system should not exceed 3%.However, the human body's structure is complicated. There are a variety of organizations and cavity structures, such as soft tissue, bone, lung and esophagus, etc. They have different physical and dosimetric properties. Especially in the interface region between different tissues, different media of both side will affect he absorbed dose. At the time, the radiation-sensitive parts of some organs locate in the region, which require a higher accuracy of dose calculation. Treatment planning system often used to calculate dose with a large number of approximation and simplification methods. It's easy to produce calculation errors in the medium interface region, which can lead to wrong estimation of doses to tumor and normal tissue.So, it is necessary to research the interface problem. Understand the change characteristics of photon beam absorbed dose in the medium interface region correctly. Analysis deeply the accuracy of the in different treatment planning system algorithm, to ensure radiation therapy carry out properly.In this study, Monaco Monte Carlo planning system with XVMC algorithm of Elekta Companies is used to calculate absorbed dose of photon beam in inhomogeneous media. Obtain the percentage depth dose curves of beam central axis under different conditions, to analyze the changing characteristics of absorbed dose in the medium interface region.At the same time, based on the same interface and beam conditions, recalculate with the"Clarkson"algorithm of"Precise Plan"planning system and"Collapsed Cone Convolution"algorithm of"Pinnacle"planning system. Analyze the accuracy of dose calculation algorithm in the two planning systems in the medium interface region. We can get following conclusions: When photon beam interacts with the medium interface, the trend of percentage depth dose curve will be changed. In the experiment, shape of PDD curves express"Bulge"effect near the entering interface of"Bone Phantom"and near the outgoing interface of"Lung Phantom". On the contrary, Shape of curves shows"Concave"effect near the entering interface of"Lung Phantom"and near the outgoing interface of"Bone Phantom". How much curves change is associated with the beam energy and field size and location of the interfaceIn the"Bone Phantoms","Clarkson"algorithm and"Collapsed Cone Convolution"algorithm is similar to the results, and good conformity with the XVMC algorithm. In the"Lung Phantoms", the calculation results of"Collapsed Cone Convolution"algorithm are closer to that of Monte Carlo algorithm. When photon beam energy is high and field size is small, Clarkson algorithm can produce more than 11% of the dose deviation.The results of this research project have guiding significance for clinical practice of radiation therapy.