PubMed · 7335865
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C A Seijo. A CT scanning protocol for a quality assurance program.. https://pubmed.ncbi.nlm.nih.gov/7335865/
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Linac-based radiosurgery has many advantages over the gamma knife, including low initial cost and no need of source replacement. On the other hand, most of the medical linacs currently in use were not originally designed to be applied for radiosurgery, and, therefore, careful quality assurance programs are required. In the gantry-head of a linac, a small CCD video camera is mounted in a position optically identical to that of the x-ray source. The video signal from the camera was digitalized to be evaluated for geometrical errors. A metal ball fixed to the stereotactic base frame via XYZ-sliding rods was used as a simulated target. Displacements of the target from the isocenter were measured during rotation of the gantry. Displacements in the gantry-rotation plane were satisfactorily small, while those perpendicular to it were maximal at gantry position angles of 0 degree and 180 degrees. This error night be caused by gravitational vending of the heavy gantry head. Although other major errors of the linac were within one millimeter, the center of coach rotation around the isocenter did not coincide with the center of gantry rotation, probably owing to gravitational vending. Special care should be taken when very small collimators are employed.
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A review of variations in block positions on port film vs. simulator film at The Nashua Regional Cancer Center, Nashua, NH, prompted the development of a method of measuring the accuracy of hot wire cut molds vs. a "test" film. The procedure is done at a minimum of once a year, or when accuracy of the block cuts are suspect. This simple test insures a high degree of confidence that the resulting cut molds are appropriate for the clinical situation and the need to recut blocks as a result of an out-of-tolerance Huestis Styro-former is kept to a minimum.