Suitability of the MVCT of TomoTherapy and Siemens Oncor for Treatment Planning

The TomoTherapy Hi-Art II treatment system and the Siemens Oncor offer the possibility to apply an MVCT for checking the patient position prior to treatment. TomoTherapy has a 3.5 MV fan beam MVCT and the Oncor a 6 MV cone beam MVCT. It was investigated whether these two MVCTs are appropriate for contouring and dose calculation in treatment planning. The contrast-to-noise- ratio and the dose to the patient were analyzed to answer the question if contouring based on these MVCT data is possible. The clinical usage of TomoTherapy MVCT in cases of two hip protheses and a prostate carcinoma serves as a reference to this estimation. It was found, that image quality was sufficient for contouring on the Oncor MVCT data if a much higher dose to the patient (about a factor of 12) compared to the TomoTherapy MVCT is accepted. To investigate whether these MVCTs are appropriate for dose calculation, it was checked whether the relation between CT value and electron density depends on the location within the patient. The relation of the Oncor MVCT highly depends on the location, which results in large deviations of the planned dose. Such dependence does not exist for the TomoTherapy MVCT. But, because of an inaccuracy in the reconstruction, an artifact is visible in the center of all TomoTherapy MVCT slices. The impact of this artifact on the dose calculation was found to be negligible. It can be concluded, that the Oncor MVCT is appropriate for contouring but currently inappropriate for dose calculation, while the TomoTherapy MVCT can be used for treatment planning.