Calibration of a tomotherapeutic MVCT system

Megavoltage CT provides the ability to image the patient before, during or after a radiotherapy treatment. This allows one to verify not only the plac ement of a patient's external boundary, but also the locations of internal anatomy. In addition, the reconstructed MVCT values are potentially useful for treatment planning inhomogeneity corrections and dose reconstruction. T o this end, dosimetric calibration of the University of Wisconsin Tomothera py Benchtop MVCT system was investigated. It was found that MVCT values cor relate extremely well with electron density and that unlike kilovoltage CT, this correlation is well maintained for higher atomic number materials. Im provements of the order of 1% in the dosimetric calculations of high atomic number materials should be possible by deriving input images from MVCT as opposed to kVCT, and calibrating in terms of electron density, as opposed t o physical density.