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Conclusions
The presented thesis work confirmed that, even if designed for imaging, aSi EPIDs are also useful for dosimetric applications. EPID data is digital, easy and fast to use in the clinical workflow. Despite these benefits, the EPID is underutilized for dosimetry, probably because issues related to the hardware, such as support arm backscatter, or to the interpretation of EPID results are still a limitation. Moreover, the use as dosimeter of the EPID aS1000 is not compatible with flattening filter free beams. Recently most producers released new EPIDs, such as the Varian aS1200, characterized by the promise to have solved most of the problems of their predecessors since they are build specifically for dosimetric, and not only imaging, purposes.
Several reports about recent radiation incidents increased pressure on safety authorities of the radiotherapy community to make in vivo dosimetry recommended, if not mandatory. In fact, pre- treatment plan verification is not able to detect errors due to, for instance, variation in patient position or anatomy. Moreover, accidentally treatment parameters can be modified between pre- treatment verification and treatment. In vivo treatment QA should be very time consuming, especially if detectors such as gafchromic or ionizing camera are used, and usually time is proportional to the number of patients treated. The use of EPID as a dosimetric tool doesn’t need more time for the positioning of the device or its reading. On the other hand, the analysis of the EPID results can take several minutes for patient. New automatic solutions are therefore of great interest, such as the software PerFraction introduced in this work. This automatic EPID-based system allows in vivo verification of the full 3D dose distribution during the treatment delivery. The system is able to correctly recalculate dose on patient CT and allows daily delivery check. Combining 3D in vivo dose measurements with cone beam CT (CBCT) imaging will serve together as a proof that also these more advanced treatment techniques are delivered as planned. To now, dose distribution is recalculated on the patient CT and not on the daily CBCT: it may be expected that a new approach, which takes into account the CBCT information, should be available soon. Bibliography
1- Wouter van Elmpt, Leah McDermott, Sebastiaan Nijsten, Markus Wendling, Philippe Lambin, and Ben Mijnheer. A literature review of electronic portal imaging for radiotherapy dosimetry. Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology, 88:289–309, Sep 2008.
2- Ann Van Esch, Tom Depuydt, and Dominique Pierre Huyskens. The use of an asi-based epid for routine absolute dosimetric pre-treatment verification of dynamic imrt fields. Radiotherapy and oncology: journal of the European Society for Therapeutic Radiology and Oncology, 71:223–34, May 2004.
Tesi di Specialità
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