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(Radiographics. 2002;22:1037-1039.)
© RSNA, 2002

EDUCATION EXHIBIT

Invited Commentary

Jay P. Heiken, MD

Mallinckrodt Institute of Radiology, Washington University School of Medicine, St Louis, Missouri


   Commentary
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Accurate characterization of the various nodular lesions that occur in the liver, particularly the cirrhotic liver, is among the most challenging imaging problems a radiologist encounters. The current nomenclature of hepatocellular nodules, published in 1995 by the International Working Party on Terminology of the World Congress of Gastroenterology (1), simplifies the previously used terminology and serves as a useful framework for understanding the stepwise process of hepatocarcinogenesis. In the preceding article, Hussain et al (2) provide an excellent overview of the current nomenclature of hepatocellular nodules and include a helpful correlation of the typical MR imaging features with the underlying histopathologic features.

Differentiation among the different types of hepatocellular nodules is important clinically because appropriate patient treatment depends on accurate lesion characterization. For example, FNH and hepatocellular adenoma are nodular liver masses that occur primarily in women of childbearing age and can have similar appearances. Whereas FNH has no malignant potential and rarely requires surgical resection, solitary adenomas are usually resected because they are classified as premalignant neoplastic nodules (1). In patients with cirrhosis, the distinction between dysplastic nodules and small HCCs has major clinical importance. Early detection of HCC (and distinction from dysplastic nodule) is critical because treatment of HCC is most effective when the tumor is small (3,4). In addition, the diagnosis of one or more HCCs in a patient with cirrhosis may affect that patient’s prioritization for liver transplantation.

The authors make the important point that "most small HCCs cannot be distinguished histologically from dysplastic nodules with certainty." This being the case, it is not difficult to understand that differentiation between dysplastic nodules and small HCCs on the basis of imaging characteristics is also extremely difficult and in many cases impossible. The authors also point out that "the appearance of new tumor vessels is important in the transformation of regenerative nodules into dysplastic nodules and small HCCs" and that "neovascularity within HCC can be used for early detection and characterization of these lesions with imaging." Indeed, multiple studies have demonstrated a sequential decrease in portal venous blood supply and increase in hepatic arterial blood supply with progression from low-grade dysplastic nodule to high-grade dysplastic nodule to overt HCC (5–8). Nevertheless, overlap exists in the blood supply patterns of the various types of hepatocellular nodules (9,10), making definitive diagnosis challenging.

Although the authors state that the signal intensity characteristics of dysplastic nodules are not well established, I believe a more detailed description of what is currently known about the MR imaging characteristics of dysplastic nodules and the difficulty in distinguishing them from small HCCs is useful. Dysplastic nodules are generally hypo- or hyperintense on T1-weighted images and iso- or hypointense on T2-weighted images (11–14). HCC may be hypo-, iso-, or hyperintense on T1-weighted images and iso- or hyperintense on T2-weighted images (15–17). Because the signal intensity characteristics of dysplastic nodules overlap significantly with those of small HCCs, most investigators have concluded that dysplastic nodules and HCC cannot be distinguished accurately at unenhanced MR imaging (11,12,18–22). A feature that can be helpful in distinguishing a dysplastic nodule from HCC at unenhanced MR imaging is the fact that dysplastic nodules are almost never hyperintense on T2-weighted images (23,24).

Because the main blood supply to dysplastic nodules is from the portal venous system and that to HCC is from the hepatic arterial system (5), HCC often can be distinguished from dysplastic nodule at dynamic gadolinium-enhanced MR imaging on the basis of identification of hepatic arterial phase enhancement (25). Dysplastic nodules generally do not enhance during the hepatic arterial phase. Nevertheless, dysplastic nodules do contain varying amounts of arterial blood supply (5–8,10), and a small percentage of them may demonstrate hepatic arterial phase enhancement (26,27). Although the pathophysiology of hepatic arterial phase enhancement of dysplastic nodules is unclear, one hypothesis is that these lesions contain increased numbers of unpaired arteries (ie, arteries unaccompanied by bile ducts) compared with regenerative nodules (28). Thus, neoplastic angiogenesis may account for hepatic arterial phase enhancement of some dysplastic nodules. An additional challenge in the characterization of nodular hepatic lesions is distinguishing HCC from other benign causes of hepatic arterial phase enhancement, including transient hepatic attenuation difference, arterioportal shunts, hepatic peliosis, fibrosis, and other poorly understood causes (29,30).

During the past 15 years, much has been learned about the imaging features and histopathologic features of nodular hepatic masses. In the preceding article, Hussain et al (2) present a useful review of these lesions including the typical MR imaging appearances and their histopathologic correlation. The authors conclude that "state-of-the-art MR imaging with a number of T1- and T2-weighted and multiphasic dynamic contrast-enhanced sequences facilitates detection and characterization of the majority of hepatic nodules, including regenerative nodules, dysplastic nodules, HCC, hepatocellular adenoma, and FNH." Although the typical MR imaging features can serve as general guidelines for distinguishing among these hepatic lesions, definitive diagnosis often is difficult. In particular, distinguishing between dysplastic nodules and HCC in patients with cirrhosis remains a major clinical challenge for radiologists.


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 Commentary
 References
 

  1. International Working Party. Terminology of nodular hepatocellular lesions. Hepatology 1995; 22:983-993.[CrossRef][Medline]
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Related Article

Benign versus Malignant Hepatic Nodules: MR Imaging Findings with Pathologic Correlation
Shahid M. Hussain, Pieter E. Zondervan, Jan N. M. IJzermans, Solko W. Schalm, Rob A. de Man, and Gabriel P. Krestin
RadioGraphics 2002 22: 1023-1036. [Abstract] [Full Text] [PDF]




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