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Experimental Study of Two-parameter Exponential Model for Dosimetric Verification of Intensity Modulated Radiotherapy

Author: CaoMingNa
Tutor: ZhouLingHong
School: Southern Medical University,
Course: Biomedical Engineering
Keywords: Dose Verification Exponential model The original scattering line separation IMRT
CLC: R730.5
Type: Master's thesis
Year: 2011
Downloads: 14
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Abstract

Radiation therapy with surgery, chemotherapy three tumor treatment. Domestic and international statistics show that about 70% of cancer patients require different degrees (radiotherapy alone or with surgery, drugs with treatment) received radiotherapy. The fundamental purpose of radiation therapy is to improve the treatment of cancer treatment gain ratio, that is, to maximize the improvement of local tumor control rate and reduce the radiation to the surrounding normal tissue complication probability. In recent years, radiotherapy technology continue to progress as a treatment technique, three-dimensional conformal radiation therapy (3-Dimensional Conformal Radiation Therapy, 3D-CRT) in the irradiation direction, the shape of the radiation field, and the target of the projector conforms to the shape, makes high the shape of the dose distribution in three dimensions is consistent with the shape of the target volume dose conformal radiation therapy at the same time meet the field points in the output of the dose rate can be adjusted according to the requirements of the target area and surface equal everywhere when called intensity modulated radiation therapy (Intensity Modulated Standardization of Breast Radiotherapy IMRT), its clinical application is more extensive. IMRT well follow the four principles of radiotherapy: accurate target dose to adjacent normal tissue radiation dose and target dose is evenly distributed to protect critical organs. IMRT plan design and implementation is a complex system, outlined, including CT positioning, target vital organs, accelerator physics parameters and multi-leaf collimator leaves to determine the location and place control, patient position settings, etc. accuracy, repeatability and stability of any small error could lead to treatment failure. ICRU 24 report pointed out that the primary the stove radical dose accuracy better than ± 5%, ± 5% deviation from the optimum dose of the target dose, it is possible tumor out of control or complications of the primary tumor, which may lead to the failure of the treatment plan. Therefore, strict quality assurance of IMRT the QA (QualityAssurance) and Quality Control QC (Quality Control) protection to achieve precise radiotherapy dose verification is a very important aspect of which. Beam fluence uniform distribution of conventional radiotherapy or three-dimensional conformal radiation therapy can be used to manually calculate check measurement point or points dose method to verify the accuracy of treatment planning and implementation of dose. IMRT many take advantage of the the MLC movement to intensity modulated, each radiation field to produce a certain number of sub-wild through the MLC's Games, and shape of each sub-field accelerator hops vary between them complex relationship, after all these sub-wild synthesis dose distribution is more complex, with three-dimensional conformal radiation therapy, clinical in order to ensure the IMRT beam output accuracy of the dose IMRT plan must be carefully dosimetry verification. The ultimate goal of radiation therapy is to give the maximum tumor irradiation normal tissue irradiation as low as possible to improve the patient's quality of life is the key. IMRT highly conformal and complicated process, each sub-field shape and accelerator hops; errors in any aspect of the course of treatment may lead to treatment failure, irreparable harm to the patient, in order to ensure that doses accuracy and patient safety, each implementation of the treatment plan should dose verification, IMRT efficiency is relatively low. Dosimetry verification consists of two aspects: the absolute dose verification and relative dose verification. The absolute dose verify IMRT plan transplantation (original tune loaded into the phantom IMRT plans did not move, just different from the original plan the implementation of the object is phantom through the treatment planning system, and the original the implementation of the scheme is that the patient, the treatment program implemented by the phantom through dose verification, then you can verify the implementation of the patient's body to the phantom is accurate), to produce a quality assurance program, select one - point, this point The measurement values ??consistent with the corresponding point on the dose and Click Quality Assurance Scheme irradiation. The relative dose verification refers to the the corresponding plane dose distribution measurement values ??consistent whether the distribution and the actual irradiation dose on a plane of this quality assurance program. Mode vivo dose measured based authentication is still the most common individual IMRT plan dosimetry verification methods, point dose verification and flat dose verify that the combination of more comprehensive verification of IMRT dose distribution. Film, thermoluminescence dosimeter, BANG gelatin dosimeter is dose proven an effective tool to use or analyze the cumbersome, time-consuming; 2D ionization chamber matrix and electron irradiation portal imaging device (Electronic Portal Image Device, EPID) is more advanced real-time two-dimensional verification system, you can quickly get a complex two-dimensional data, but EPID dosimetric characteristics of understanding is still relatively limited, and its dose verification needs further improvement and research. For scattering line in the IMRT embodiment is more obvious, and the MLC leaves transmittance and other issues, once the solutions proposed have improved Clarkson integral method (using TMR wild extrapolated to zero to calculate the distribution of the scattering line), based on the pen-shaped nucleus convolution algorithm. Also a lot of literature suggests that the use of a pen-shaped nucleus and Monte Carlo algorithm independent verification of the accelerator hops dose verification. Establish a two-parameter exponential model This model can be more easily separated from the original rays and the scattered rays, and to obtain the scattering line differential analysis, the distribution can be calculated based on the obtained original ray and scattered ray dose arbitrary emitted. The dose of the reference point of the field center axis. The model is based on the percentage depth dose (Percentage Depth Dose, PDD) or tissue phantom (Tissue Phantom Ratios, TPR) and relative output factor (Relative Output Factors), measurement data, to provide secondary treatment plan complex dose algorithm validation, the model generating the dose predicted values ??and the treatment plan the predicted value and the actual measured values ??are in good agreement, which can to some extent reduce IMRT dose verification should be carried out the number of times for each Example treatment plan, reducing IMRT quality control time. Experiment in PDD and relative output factor data measured from SIMEMS PRIMUS linear accelerator 6MV X-ray data, SSD, MU and MLC leaf position data from the treatment planning system. Application of two-parameter exponential model: self programming, fitting the two parameters in the model, the model; implementation of the the IMRT plan's patients with retinal fixed positioning mark, CT simulation, set the target prescription dose and the critical organ Limited design IMRT plans CADPLAN (helios). Mold body using the same conditions as the positioning markers and the scanning of the CT; reverse computing the resulting flux of the irradiation dose of the patient's IMRT treatment plan view and a corresponding rack angle parameters, treatment beds parameters, etc. All transplanted to the phantom CT reconstruction image data , thereby calculating the phantom validation on the planned dose distribution and the mold body planned dose of the measuring point of the ionization chamber; through the treatment plan generated read the MLC leaves the position, to obtain the shape of a different sub-field, according to the established two parameter exponential model the original ray scattering line dose distribution calculated central axis dose at different depths; press IMRT positioning requirements motif accurate positioning, then the implementation of IMRT verification plan, the simultaneous measurement of the center of each sub-wild The actual dose of the sum of the absorbed dose, with the planned values ??and the dose values ??generated by the model for comparison. The experimental results indicate that the dose has established the two-parameter exponential model to calculate the dose values ??and treatment plan prediction value comparison, the error is within the range of ± 3%, suggesting that this model is a simple to meet clinical requirements. Distinguish scattering of the treatment head and the mold body scattering, MLC's transmittance, etc. in the model, so the model the dependence on the distance, found a higher degree of dose ray central axis and its vicinity (0.5cm), in other locations ray dose error is quite large, Secondly, the model does not reflect the the MLC leaf accuracy and positioning accuracy problems, the clinical application of this model need to be further improved. The end of this article summarizes the problems existing in the work, and the prospect of future work.

Experimental Study of Two-parameter Exponential Model for Dosimetric Verification of Intensity Modulated Radiotherapy

Abstract

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