Comparison of CT and USG 3D images

I have data from CT and from USG 3D. The patient has a liver tumor. I am trying to compare tumor volume in both modalities. For CT I know that the computed volume is reliable but I am not sure what about USG 3D. Should I scale it somehow?

If the 3D ultrasound imaging system is calibrated then it should provide correct physical dimensions. Most ultrasound systems assume a certain speed of sound in tissue, which may differ from the actual speed, but this should not cause more than a few percent error.