Development of an integral test for image-guided radiotherapy

An integral system test was developed to determine the precision and accuracy of an image-guided radiotherapy system involving an x-ray volumetric imaging device mounted onto the gantry of a medical linear accelerator. The test was designed to interrogate the system components as a whole without deconstructing the individual sources of error. The integral system test was based on the imaging of an unambiguous stationary object in the treatment position and so took no account of patient related errors. An array of micromosfets interspersed within slices of a tissue equivalent phantom was developed as an imaging test object. It has previously been demonstrated that micro-mosfets have a very small active volume, are clearly visible on CT images, and produce no significant artifacts. In addition, the active volume of the micromosfets can be accurately inferred radiographically via the use of x-ray volumetric imaging. X-ray volumetric imaging was performed with the object in the treatment position, then reconstructed and transferred to a treatment planning system. With the phantom remaining undisturbed in the treatment position a series of treatment fields were designed to produce a series of fields with the leaf edge sweeping across active volume of the micromosfets. The fields were delivered with a micro-MLC to dosimetrically verify the position of the mosfets and compare with dose values produced by the treatment planning system. It was demonstrated that the systematic gantry flex could be accounted for by the imaging and delivery systems. For the delivery system small changes in leaf positions of the micro-MLC were required to account for gantry flex. The position of the micromosfets determined by the 50% dose position was on average (0.15 ± 0.13) mm away from the position determined radiographically for the x and y axes, and (1.0 ± 0.14) mm for the z axis. This implies that a margin of approximately 0.2 mm in the axial plane and 1.0 mm in the superior-inferior plane would be required at the delineation stage to ensure coverage of a tumor volume to account purely for imprecision in the image-guided radiotherapy system. The integral system test demonstrated that the image-guided radiotherapy system is capable, in the absence of patient motion, of imaging an object in the treatment position and delivering dose to that object with submillimeter accuracy.