Analysis and verification of the clinical dose of radiotherapy

• Main Authors: An-Cheng Shiau, 蕭安成
• Other Authors: Wei-Peng Kuan
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/98281135577048086404

博士 === 國立陽明大學 === 生物醫學影像暨放射科學系 === 100 === According to the report of the Department of Health of ROC in 2010, malignant tumor is the leading cause of death in Taiwan, accounting for 28.4% of all deaths. Obviously, malignant tumors pose the greatest threat to the public’s health. The main strategies of treatment of malignant tumors are surgery, chemotherapy and radiation therapy. For radiation therapy, approximately 1.2 million patients receive radiation treatment each year and about 67 medical departments provide radiotherapy services. Due to the demand for medical services and treatment quality, radiation treatment technology progressed rapidly in recent years, these include intensity modulated radiation therapy (IMRT), image-guided radiation therapy (IGRT), Tomotherapy, CyberKnife, Gamma Knife, volumetric intensity modulated arc therapy (VMAT), and RapidArc, Other advanced medical equipments and treatment techniques have also been introduced. Moreover, the country's first proton therapy center will begin to provide medical services in the near future. In order to ensure the quality of patient care, advanced medical services need careful quality assurance for dose measurements and plan assessments. The principle of radiation therapy is that the radiation energy released by the interactions of high energy radiation with human tissues causes damage to the cell's DNA, so that it can not continue to divide and proliferate. Through complex treatment planning, effects of radiation doses on malignant tumors and normal tissues can be assessed in order to confirm the feasibility of the treatment and the expected treatment effect. Upon confirmation of the treatment plan, various treatment parameters are sent to the treatment machine and precise radiation dose is delivered to the patient for the treatment. The effect of radiation therapy is based on the theory of radiation biology. The dose-response curve shows that in the middle of the curve where the greatest slope resides, 5% change of the dose will affect the tumor control rate by 10 % to 20%, or change the normal tissue complication rate by 20% to 30%. Therefore, for radiation therapy, accurate control of the given dose is very important. Radiation therapy uses high-energy ionizing radiation. For different purposes and situations, the absorbed dose is measured with different dosimeters and methods in order to obtain appropriate and reliable results. For the measurement of absolute dose, ionization chamber is one of the most appropriate choices. However, under conditions where the chamber cavity theory is not valid, clinical patient dose must be measured with other dosimeters. Thermoluminescent dosimeter (TLD), with a variety of commercially available shapes to choose from, is flexible in its use depending on the needs of the experiment and the reliability of the measurement. But the uncertainty of the TLD measurement is usually limited to about 5%. The readout procedure of TLD is cumbersome and time-consuming, and the experiment should be assessed carefully. Alternativelyt, the film is also a convenient dosimeter. Gafchromic ® EBT film, specifically designed for radiation dose measurement purposes, is different from the traditional silver bromide film in that the EBT film contains the acrylic monomers which undergoes polymerization reaction after radiation exposure. Owing to the fact that the amount of polymerization reaction is proportional to the absorbed dose, EBT film is suitable for measureing the radiation dose. EBT film can be operated under normal indoor light and without the film processing process, thus improving the accuracy and reliability of measurement. EBT film has a thickness of only 0.234 mm and can be cut into various sizes, making it a very convenient and reliable tool for clinical dosimetry. In this study, depending on the situation of the clinical dose distribution and the measurement purposes, ionization chamber, TLD and EBT film were used for dose verification. The results of measurements will serve as the basis to affirm or to modify the treatment plan and to assess the treatment effects. IMRT is the most widely used radiotherapy technique today, usually delivered with a multi-leaf collimator (MLC). Hung et al. have shown that the dose error caused by the leaf position error is approximately 6.9% per mm. Sharpe et al. also showed that for a 1 × 1 cm2 field, the position error of 1 mm of the leaf will result in about 8% of the dose error. Dose errors caused by the MLC with different methods of dose delivery, namely dynamic or static (step and shoot), are different. The American Association of Physicists in Medicine (AAPM) Radiation Therapy Committee recommended that the accuracy of the relative position of the MLC leaves be less than 1 mm at the isocenter plane. Because of the complexity and the dose uncertainty of the IMRT treatment plan, dose verification is important. In this study, analysis and verification of the clinical treatment doses for different threatment systems and techniques were performed. Accurate estimation of the surface and the shallow doses is very important for radiation therapy. When the tumor invades near the surface, accurate assessment of the dose to the tumor is needed so that adequate dose can be given. On the other hand, when the tumor is located at a deeper position, appropriate lowering the dose to the shallow region can prevent severe skin effects to the patient. However, the assessment and the adjustment are based on the ability to accurately determine the surface and shallow dose in the process of radiation therapy. Although advanced computer treatment planning system (TPS) can calculate doses accuratly in most of the treatment area, but for the surface and buildup region, where there is a lack of charged particle equilibrium, the accuracy of dose calculation remains dubious. TLD and EBT film were used and the experiments were designed in this study to measure and to analyze the surface and superficial doses for head and neck and breast cancer patients employing different clinical treatment techniques. Hypofractionation has been used more fquently in recent years and stereotactic body radiotherapy (SBRT) is likely to become a method of choice for the treatments of lung and liver. SBRT entails the techniques of stereotactic radiosurgery (SRS) and 3-D image-guided radiotherapy. For small volume lesions, SBRT delivers a fraction dose 3-15 times higher than that of a conventional treatment. With a smaller fraction number, more effective damages to the target can be achieved owing to the much higher fraction dose and the high precise localization. For SRS or SBRT, field sizes of 0.4-5 cm are used typically. However, dose verification for small field sizes is difficult. Farmer chamber usually used for output calibration in a conventional treatment system is not suitable for small field size calibration due to the large cavity volume. Additionally, for megavoltage photon beams, dose measurement with ion chamber for field sizes less than 3 cm has a further complication of the lack of charged particle equilibrium (CPE). Gamma knife, Cyber knife and X-knife are included in this study. Different dosimeters with different sensitive volumes including the Farmer (0.6 cc), Semiflex (0.125 cc), Pinpoint (0.015 cc), semiconductor (area of 1 mm2 and thickness of 2.5 m) and EBT film were used in this study. By using detectors with different spatial resolutions, the differences in dose measurements were analyzed for the small field size irradiation. Dose measurement tools calibrated by the national radiation standard laboratory were used to measure the planning doses and compare with the commercial systems used in the clinical departments. Based on this study, procedures of dose measurements for small field sizes will be developed. The dose accuracy of SRS and SBRT and the knowledge of dose measurements in the situation of lack of CPE can be improved. Radiation therapy aims to deliver a high and uniform dose to the tumor in the patient. The delivered doses are closely related to the tumor control probability and the normal tissue complication rate. The radiation therapy societies generally agree that for the radiation therapy process the overall dose uncertainty must be controlled below 5%. Therefore, dose verification has become very important especially for new treatment equipments and techniques. This is achieved by using the appropriate measurement tools to acquire the measurement data for the clinical dose in question. Radiation therapy has entered the millimeter era, there may be only a few millimeters of distance separating the treatment target and the critical organs. Although time-consuming, dose verification ensures the integrity of treatment planning and the robustness of radiation therapy technologies.