Optimization Problems in External Beam Radiotherapy Public
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Intensity-modulated radiation therapy (IMRT) using a medical linear accelerator (linac) is a commonly used technique in external beam radiotherapy. The goal of IMRT planning and delivery is to optimize the therapy so that the prescribed radiation dose conforms as closely as possible to the target volume while minimizing the dose delivered to normal tissues. During the past decades, a number of medical physicists have devoted themselves to formulating and solving IMRT optimization problems. In this thesis, we will present several new schemes which improve on present methods. One of problems is that even though reducing the radiation pencil-beam size may lead to a significant improvement in dose conformity, there will be an increase in the time needed for the dose calculation and plan optimization. Eventually, the time requirement becomes unacceptable in a clinical setting. We have developed and evaluated a post-optimization refinement (POpR) method, which makes fine adjustments to the multileaf collimator (MLC) leaf positions enhancing the spatial precision and improving the plan quality without a significant impact on the computational burden. Organ motion during radiation delivery is another problem in optimized IMRT delivery. Tumor tracking using a dynamic MLC is a technique that repositions and reshapes the radiation beam to track the tumor movement. A dose-rate regulated tracking (DRRT) method has been proposed to cope with irregularities of breathing when delivering preprogrammed tracking plans. We investigated the effectiveness and dosimetric accuracy of DRRT in IMRT delivery. It was verified that, using DRRT, preprogrammed four dimensional (4D) IMRT plans can be delivered accurately and efficiently. Intensity-modulated arc therapy (IMAT) is a rotational IMRT technique by using a conventional linac with a dynamic MLC on a rotating gantry. In comparison to fixed-gantry IMRT, IMAT provides a more efficient approach to deliver conformal doses to targets. We developed a practical 4D planning scheme of IMAT using 4D computed tomography (4D-CT) for tumor tracking with a MLC. We also verified the feasibility of 4D-IMAT delivery. The temporal, geometric and dosimetric accuracies of the 4D-IMAT delivery was investigated by using electronic portal imaging, diode measurement and log files from linac control system.
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