Master of Science
University of Wollongong. Centre for Medical Radiation Physics
Griffin, Stewart Claude, MegaVoltage Cone Beam CT: Potential for Application in Radiation Therapy Treatment Planning, Master of Science thesis, University of Wollongong. Centre for Medical Radiation Physics, University of Wollongong, 2010. http://ro.uow.edu.au/theses/3398
The Siemens Artiste MVision integrated imaging system is an IGRT tool that enables the therapist to better align a patient before treatment commences with volumetric MV cone-beam imaging. This system can also possibly be used as a tool in tracking a patient’s daily dose over the course of their treatment.
The imaging system is required to achieve an adequate level in numerous performance categories compared to a traditional CT. These characteristics are investigated using a CATPHAN 504 imaging phantom and include, uniformity, resolution and contrast as well as a CT number to ED calibration. In order to track a patient’s daily dose accurately organ delineation and dose calculation must be achievable on the resulting images of a patient scan. A CIRS IMRT phantom was used to simulate a patient’s anatomy and enable dose calculation comparison between the MV and CT imaging systems. This was followed with a two patient pilot study including DVH comparison of critical organs and target volumes.
The system shows a non-uniform response to a uniform material, there is an over-correction of the cupping effect in which the centre region of the image is assigned a greater value compared to the traditional CT gold standard. The spatial resolution of the system is determined to be two line-pairs per cm and its contrastability for an object of fifteen mm is greater than one percent.
The phantom study has shown that the reliability of the CT number to electron density calibration has a significant effect on the accuracy of the dose calculation. The phantom study with CT-ED calibration resulted in differences ranging from 1.7% for a simple solid water only AP 10 cm x 10 cm beam through to 14.5% for the lateral 10cm x 10 cm beam passing through a significant portion of bone. With forced ED’s the same phantom scenario’s presented results varying from 0.8% for the simple solidwater AP 10 cm x 10 cm beam, through to 3.4% for the AP 10 cm x 10 cm scenario passing through a lung insert.
The patient’s daily dose is a measurable quantity, it is possible to track the doses received to each Organ at Risk and also the accumulated dose to the target, however the accuracy of such measurements at this time is in sufficient to be used clinically and further improvements will be required.