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Conformal Three Dimensional Radiotherapy The objective of radiation therapy is to deliver a high dose to the tumor to achieve a high level of local control while keeping the dose to neighboring healthy tissues as low as possible. These two conflicting demands have challenged radiation oncologists, physicists and radiobiologists ever since radiation was used in cancer therapy. In conventional radiation therapy (also known as two dimensional radiation therapy), the dose distribution can only be evaluated in a single, usually transverse, central plan although the tumor and the neighboring tissues bear complex three dimensional relationships to one another. To ensure that the tumor receives an adequate dose, a margin is included around the tumor volume to account for the uncertainties in tumor delineation and dosimetry, deviations due to patient movement. This safety margin often limits the delivery of a high dose to the target volume because of the tolerance of the normal tissues included in the irradiation. Three dimensional (3D) conformal radiation therapy is a sophisticated irradiation technique which allows a high dose delivered to the tumor while keeping the dose to the adjacent normal tissues below tolerance. While conformal therapy has been around since the mid-60’s, it is only recently that the technique has become popular in the radiation therapy community due to the explosive advancement in computer technology. 3D conformal therapy combines modern imaging technology (such as CT and MRI) for accurate tumor delineation with state-of-the-art computer planning systems for treatment planning and the latest development in linear accelerator technology for precise and sophisticated dose delivery. Hence the tumor can be irradiated to a high dose without increasing morbidity. The Department of Radiation Oncology at the University of Arizona is equipped with two computerized dual energy linear accelerators (Siemens MD2 series, 6MV and 10 MV x-rays), one single energy (Varian Clinac series, 6 MV x-rays) accelerator and a state-of-the-art 3D treatment planning system (TheraPlan Plus). Both Siemens linear accelerators are equipped with a record-and-verify system (Impac) for verification of setup parameters and a real-time portal imaging system (BeamView Plus) which will be implemented for treatment verification. One of the MD2 machines also is equipped with a multi-leaf collimator (MLC) which allows implementation of the most complex conformal therapy. The implementation of the complex 3D conformal techniques represents a collaborative effort combining the medical specialty of radiation oncologists with the technical expertise of the physicists and the engineers, the treatment planning skills of the dosimetrists and the careful execution of the treatment plans by the radiotherapists. Just as importantly, there are cases for which greater technical sophistication, will be of no benefit to the patient. The more basic approaches are retained for these cases. A new Elekta accelerator will be in use shortly; with this linac we will implement intensity modulated radiotherapy (IMRT) which represents a level of treatment planning and delivery more advanced than 3-D conformal therapy. |
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