Most patients who have primary liver cancer or liver metastases from colorectal cancer have poor survival rates because the cancer cannot be removed surgically. Conformal radiotherapy -- delineating the tumor with 3-D imaging and delivering a high dose of radiation to it while delivering less to surrounding normal tissue -- has been shown to improve local control of inoperable primary and metastatic liver cancers. MRI is reported to image liver cancers better than CT, but the difference in detecting tumor size has not been addressed. Knowing the spatial extent of the tumor is important for high-precision radiation planning. Dr. Laura A. Dawson, Jon-Paul Voroney, Kristy K. Brock and colleagues from the department of radiology at the University of Toronto in Ontario, recently compared MRI with CT for identifying liver cancer tumor volumes.As reported in the November issue of the International Journal of Radiation Oncology-Biology-Physics, the researchers performed CT and MRI scans on 26 patients afflicted with inoperable liver cancer. They obtained CT images of the gross tumor volume using a 16-slice scanner from General Electric Co. of Milwaukee, Wis. With the exception of one, the patients underwent scanning headfirst with a typical scanning time of 16 to 18 seconds. They received CT again using a contrast agent at 25 to 30 seconds postinjection during the arterial phase and again at 50 to 60 seconds during the venous phase. The researchers obtained MRI images of the gross tumor volume using a 1.5-tesla MRI simulator, also from GE. A minimum of six sequences were acquired in addition to a three-plane localizer sequence. A difference in the number of tumor foci was found in five patients. MRI revealed more tumor foci in three patients, while CT revealed more in two patients. There also were differences between the tumor defined on CT and MRI after the researchers resolved the geometric discrepancies between the imaging sessions with deformable registration. The average distance between the CT tumor surface and the MRI tumor surface was 3.7 mm. The median percentage of the tumor’s surface area, which differed by more than 5 mm between the two techniques, was 26 percent. The researchers concluded that an MRI-defined tumor volume can be significantly different from the volume defined by CT. Because these differences have an impact on the precision of radiotherapy, they recommend that MRI scans be evaluated in conjunction with CT scans because they can provide complementary information about liver tumor location.