Current Clinical Concerns in CT: Results : CT Interpretation

Liver Masses

1. Would you comment on differentiating FNH from adenoma by CT. What sort of comments do you put in your report of a hypodense (non-contrast), rapidly and uniformly enhancing (arterial phase) mass, which is isodense on portal phase imaging?

2. If a classic FNH is found in the liver on CT, with hypervascularization and central scar, is it necessary to do MRI to rule out fibrolamellar hepatocellular carcinoma? Some textbooks advise MR, and focus on the signal intensities of the central scar. This would mean that CT would never be the final imaging modality in FNH; although, fibrolamellar carcinoma is very rare compared to FNH. Would you advise biopsy in a classical CT of FNH?

3. What is your approach to one or more subcentimeter hypodense hepatic lesions, essentially too small to characterize, in patients with no known primary malignancy?

4. Do you have an algorithm for characterization of low density hepatic masses often detected on unenhanced CT studies. Is ultrasound or a repeat CT with IV contrast done? Does patient age, lesion size (< 1 cm, 1-2 cm, > 2 cm) or body habitus/weight influence which imaging modality you might use?

References
Hussain SM, Terkivatan T, Zondervan PE et al. Focal nodular hyperplasia: Findings at state-of-the-art MR Imaging, US, CT and pathologic analysis. Radiographics 2004; 24: 3-17.

• Summary: Following a discussion of the classification and pathology of FNH, this article reviews the appearance on MR, CT and US. This article provides CME credit.

Brancatelli G, Federal MP, Grazioli L, Blachar A, Peterson MS, Thaete L Focal nodular hyperplasia: CT findings with emphasis on multiphasic helical CT in 78 patients. Radiology 2001; 219: 61-68.

• Summary: This study describes the CT appearance of 124 pathology or clinically proven FNH masses. The masses were hypoattenuating in 40% and isoattenuating in 57% on precontrast scans, with 3% demonstrating increased attenuation prior to contrast administration. During arterial phase, FNH was hyperattenuating and hypervascular. An isoattenuating appearance was seen in 72% and increased attenuation in 22% during portal venous phase acquisitions. Of the 124 tumors, 50% demonstrated a central scar, more commonly identified in large lesions, but also depicted in approximately 30% of those less than 3 cm. Prior to contrast infusion, the scar appeared low attenuation in 54% and isoattenuating in 46%. After contrast infusion, the scar demonstrated decreased attenuation in 90% of lesions. An isoattenuating scar was identified in 6% and increased attenuation scar in 4% on the arterial phase acquisition.

Ruppert-Kohlmayr , Uggowitzer MM, Kugler C, Zebedin D, Schaffler G, and Ruppert GS. Focal Nodular Hyperplasia and Hepatocellular Adenoma of the Liver: Differentiation with Multiphasic Helical CT AJR 2001; 176(6): 1493 - 1498.

• Summary: This study compared FNH in 27 patients and with hepatocellular adenomas in 18, using triphasic contrast enhanced CT. A significant difference in enhancement was noted during the arterial phase, with FNH measuring 117.9 +/- 15 HU and adenomas 80 +/- 10 HU. A relative enhancement threshold of 1.6 identified 100% of the FNH lesions and included only 12% of the adenomas. Portal venous phase was not useful in discriminating between the two types of masses.

Robinson PJ, Arnold P and Wilson D. Small "indeterminate" lesions on CT of the liver: a follow-up study of stability. British Journal of Radiology 2003; 76: 866-874.

• Summary: In this study, 115 patients with known or suspected malignant disease who had "indeterminate" small liver lesions were followed up for 6-60 months with CT. CT characteristics including size, shape, edge, attenuation and internal structure were recorded. Seventy-nine percent were found to be stable and 21% changed in size (37/62 grew and 25/62 got smaller). Small size and a sharp edge were significantly associated with benignity; although, 10% of those less than 5 mm and 15% of those with a sharp edge were found to be unstable. When analyzed by size, 10% of lesions under 5 mm were unstable. The percentage of unstable lesions increased to 38% for those 10-15 mm. While heterogeneity and soft tissue attenuation were significantly associated with instability, only size and edge were found to be significant predictors of lesion outcome.

Blachar A, Federle M, Ferris J et al. Radiologists' Performance in the Diagnosis of Liver Tumors with Central Scars by Using Specific CT Criteria Radiology 2002;223:532-539.

• Summary: In this retrospective study, radiologists with varying degrees of experience reviewed CT scans of liver masses with a central scar, including FNH, fibrolamellar HCC and large hemangiomata. The authors identified clinical and radiologic features which aided in discrimination among the tumor types. On CT, fibrolamellar HCC (mean 14 cm) and hemangiomas (mean 11 cm) were larger. Fibrolamellar HCC often demonstrate a lobulated margin, as opposed to the smooth well-defined margin typical of a small FNH. Calcification can be seen in 35 to 68% of fibrolamellar HCC, usually centrally located. However, calcification is rare in FNH and large hemangioma. With respect to the scar, this study showed that a scar larger than 2 cm in size was indicative of fibrolamellar HCC, but sometimes seen in large hemangioma. Heterogeneous enhancement typical of fibrolamellar HCC was found to be useful as it contrasted to the homogeneous enhancement characteristic of FNH. On portal venous phase images, fibrolamellar HCC was most often hypo- or isoattenuating, but can be hyperattenuating. FNH is usually isoattenuating, and hemangioma is typically hyperattenuating. Statistical analysis showed that tumor size greater than 10 cm, nodular centripetal enhancement, invasion of hepatic vessels or bile ducts, scar size greater than 2 cm, heterogeneity, isoattenuation with blood vessels, extrahepatic metastases, calcification, surface lobulation, nearly isoattenuating on portal venous phase images and tumor capsule were all significant findings for distinguishing tumor types.