Detection of liver metastasis: modality of choice?

Colorectal cancer is one of the most frequent cancers, and approximately 80% of patients with colorectal cancer have primary curative intended resection. The liver is the most common site of recurrence, and metachronous liver metastases affect 15–25% of patients during the first 5 years after curative intended resection of colorectal cancer. Liver resection is potentially curative for patients with metachronous metastases, and therefore accurate assessment of patients with liver metastases is essential in defining the appropriate treatment and in avoiding inappropriate surgery. Surgery is not performed when there is extrahepatic disease or when the extent of hepatic disease precludes complete eradication. Several comparative studies of the imaging modalities for the detection of liver metastases from colorectal cancer have been published over the years, with highly conflicting results. Some studies reveal results in favor of CT, others in favor of MRI, and these results depend mostly on the study design and the CT and MRI technique and contrast media policy used. Since the introduction of PET, several reports have concluded that PET has a sensitivity rate for the detection of liver metastases from colorectal cancer of above 90%. These high sensitivity rates have not been confirmed in newer prospective studies with surgical reference including IOUS. Multidetector CT, liver-specific magnetic resonance imaging contrast agents, and PET/CT have improved the performance of CT, MR, and PET imaging of liver metastases from colorectal cancer. In this issue of Acta Radiologica (1), MDCT, SPIOenhanced MRI, FDG-PET, and contrast-enhanced PET/CT are compared in a prospective blinded design in regard to the detection of liver metastases from colorectal cancer. Not surprisingly, and in accordance with the literature, the authors find that PET or PET/CT is the modality of choice regarding extrahepatic disease. Moreover, very interesting results appear. PET and contrast-enhanced PET/CT revealed the highest specificities, while MDCT and SPIOenhanced MRI the highest sensitivities. No significant differences in accuracy were found between the imaging modalities. A very important question raised by the authors is how small liver metastases less than 1 cm seen on MDCT or MRI, which are metabolically silent, should be handled. MDCT and SPIO-enhanced MRI detected 13 and 10 less-than-one-centimeterlarge liver metastases, respectively, and PET only one of 19. Using contrast-enhanced PET/CT, five of the 19 lesions were found. The same tendency was also observed for lesions larger than 1 cm. However, the differences between imaging modalities for these lesions were not so pronounced. These findings show that when reading a contrast-enhanced PET/CT, metabolically silent small lesions seen on MDCT cannot be declared benign. Furthermore, the findings also suggest that when PET/CT using low-dose CT for attenuation correction is performed, several liver metastases will not be found as low-dose CT used for attenuation correction will never find such small metastases. However, larger series are needed to confirm the results of this study (1), which makes for highly recommended reading.