Evaluation of tabletop materials for autocontouring in CT treatment planning

Computerized tomography (CT) treatment planning has been proven to be essential in precision radiotherapy. Typically CT planning requires a large number of CT scans (between 3 and 50 slices) to be entered into the treatment planning system. A significant amount of time in the planning process is devoted to outlining internal structures, organs, and the external patient contour in each CT slice. In principle, external contours could be generated easily using autocontouring routines; however, in reality it does not always provide a satisfactory contour due to the limitations of the CT tabletop having nearly the same CT number as that of the body structure. A solution to this problem is to create a large CT number gradient interface between the patient and CT tabletop by inserting a thin sheet of low CT number material. The optimum material for a tabletop was investigated from a range of low-density and low atomic number media. Various materials were studied by placing them underneath an unsliced humanoid phantom. A portion of the phantom abdomen was imaged and analyzed on a Picker Premier IQ CT scanner. Results indicate that the tabletop should be made of the material that has a CT number at least 10 times lower than the tissue in contact with the table. A simple and cost-effective method of avoiding failures in autocontouring is to place a thin sheet of low-density material such as cardboard or foam board on the tabletop. Such an insert creates a large CT gradient resulting in a significant improvement in the accuracy of the edge detection algorithm used for autocontouring. Detailed analysis is presented.