Study On Quality Control Technology For IMRT Dosimetry

Intensity modulated radiation therapy (IMRT) has become increasingly popular in the 21st century. A number of radiotherapy centers are now aiming for clinical implementation of IMRT because it can increase the gain ratio of radiotherapy. With high precision and complexity of IMRT delivery modalities, errors can occur in each stage of treatment planning and delivery. Even tiny errors may lead to serious accidents as demonstrated by a number of recent events, which aroused concerns of international organizations such as International Atomic Energy Agency (IAEA). Hence, it is essential to establish rigorous quality control (QC) protocol and perform it appropriately. As an advanced QC equipment, two-dimensional array (2D-array) detector controlled by computer can obtain real-time data quickly and reduce human mistakes, which has broad applications. To investigate the feasibility of using 2D -array for IMRT QC, this paper verified the relative dose of IMRT plans, and tested several key performance indexes of multileaf collimator (MLC) (such as leaf positioning accuracy, and leaf transmission, etc.).This paper investigated the applicability of 2D -array in verifying IMRT relative dose. Ten patient IMRT plans were executed on Octavius phantom and measured by Seven29 array. Consequently the measured and calculated dose distributions were compared and analyzed by y method. The agreement between the measurement and calculation can be explicitly displayed via dose profiles and isodose lines, as primary plan assessments. Quantitative y analysis (3% /3 mm criteria) showed that nine out of ten plans achieved passing rate of>90% and the lowest rate was 82.9%. Acceptance criteria could be met for nearly all measurement points.The dose distribution is largely determined by the performance of MLC. An experiment using 2D-array was designed to test positioning accuracy of segmented MLC (SMLC) for IMRT. The geometric errors were derived from dose deviations based on leaf edge function curves. The positioning accuracy of the tested 19 leaves was within ±0.1 mm, and the repeatability was within ±0.3mm. In addition, a new method was established to analyze the positioning accuracy of dynamic MLC. The leaf positioning error could be quantitatively calculated by comparing the slope of absorbed dose line and the leaf positioning calibration curve. The accuracy of this method is 0.1-0.2 mm. The average of intraleaf and interleaf transmission measured by 2D-array and film was within 2%, and the film can be used to quantitatively analyze the intraleaf and interleaf transmission.The research results showed that the simple and reliable method based on 2D-array is applicable in the IMRT quality control system. Some problems are left for further investigation, which encourage the development of new quality control technologies for IMRT dosimetry. The dosimetric characteristics of the 2D-array should be verified before measurement, and when necessary, other quality control equipments should be used together.