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Science to Practice: Should Biopsy Be Performed in Potential Liver Donors When Unenhanced CT Shows an Unacceptable Degree of Steatosis for Transplantation?

Sezione di Radiologia
Istituto di Ricovero e Cura a Carattere Scientifico
70013 Castellana Grotte (Bari)
Italy
gbranca{at}yahoo.com

SUMMARY

Park et al (1) have demonstrated that unenhanced CT can accurately depict moderate to severe (ie, 30%) macrovesicular steatosis, thereby allowing avoidance of biopsy in potential living liver donors who have an unacceptable degree of steatosis for transplantation. Biopsy will still be needed in donors with macrovesicular steatosis of less than 30% at unenhanced CT to rule out occult chronic liver disease and more severe steatosis that is undetected at CT.

THE SETTING

Assessment of hepatic fat (steatosis) is important in preoperative living donor liver evaluation because steatosis places the graft at risk for dysfunction. Although liver biopsy remains the reference standard for detection and quantification of hepatic steatosis, it is not a risk-free procedure. Therefore, an alternative and noninvasive means of diagnosing steatosis would be desirable. In the current issue of Radiology, Park et al (1) describe the utility of unenhanced computed tomography (CT) in the identification of patients with an unacceptable degree of macrovesicular steatosis for liver donation.

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Because CT shows an inverse relationship between hepatic fat content and hepatic attenuation, unenhanced CT may provide a noninvasive test for identifying hepatic steatosis. The reliability of using absolute CT attenuation to identify fat deposition is limited, however, because variations in body size, body shape, and instruments will affect the attenuation value. Because the spleen acts as an internal control and eliminates factors such as fluctuations in kilovolt peak, the liver-to-spleen attenuation ratio (CT_L/S) and the attenuation difference between the liver and spleen (CT_L–S) have been demonstrated to limit errors of variation in CT attenuation values. Because splenic attenuation is normally 8–10 HU less than liver attenuation, lower attenuation values for the liver relative to the spleen indicate hepatic fatty infiltration. Park et al (1) compared CT_L/S, CT_L–S, and blood-free hepatic parenchymal attenuation (CT_LP) with the results of liver biopsy in 154 potential liver donors. Of note, the authors performed CT and biopsy on the same day, thereby overcoming temporal variations in the degree of steatosis. The use of a receiver operating characteristic curve analysis demonstrated that unenhanced CT can be used to accurately differentiate mild (<30%) macrovesicular steatosis from moderate to severe (30%) macrovesicular steatosis, with areas under the receiver operating characteristic curve ranging from 0.991–0.995. Cutoff values of 0.9 for CT_L/S, –7 for CT_L–S, and 58 for CT_LP provided balanced sensitivity and specificity. Lower attenuation values (ie, 0.8, –9, and 42 for CT_L/S, CT_L–S, and CT_LP, respectively) increased the specificity of unenhanced CT to 100% for patients with macrovesicular steatosis of less than 30%. The authors point out, however, that in 18%–27% of patients (depending on the index values used), unenhanced CT results suggest hepatic suitability for transplantation in contradistinction to liver biopsy—that is, CT results suggested that macrovesicular steatosis was less than 30% when in fact it was 30% of greater at liver biopsy when lower attenuation values were used.

THE PRACTICE

Clinical Use:
Owing to the heterogeneous distribution of fat within the liver, liver biopsy is subject to sampling error and may not precisely reflect the degree of steatosis in other liver segments. Park et al (1) overcome this potential limitation of liver biopsy by showing that unenhanced CT is able to noninvasively measure fat content in each part of the liver through the placement of multiple regions of interest. Liver biopsy can, therefore, be postponed until CT attenuation indices indicate levels of steatosis that are compatible with donation; after such levels have been reached, liver biopsy can then be used for confirmation. If CT attenuation indices show macrovesicular steatosis of 30% or greater, biopsy is not needed because the donor is not acceptable for transplantation. In addition to determining the presence and extent of hepatic steatosis, liver biopsy also allows detection of any underlying histologic liver damage, such as hepatitis, fibrosis, or increased hepatic iron, that would adversely affect the recipient allograft and donor remnant. Death due to congenital lipodystrophy that was not diagnosed before transplantation has been reported (2).

Future Opportunities and Challenges:
The role of biopsy in the triage of living liver donors remains controversial. According to a survey published in 2003, 26% of liver transplantation programs in the United States did not perform liver biopsies in prospective donors, while the other programs performed it inconsistently (3). Lack of consensus of recommendations for screening potential liver donors for steatosis raises the need for evidence-based guidelines. The study by Park et al (1) should alert the imaging and transplantation communities to the potential of using unenhanced CT to correctly identify individuals with severe macrovesicular steatosis, thereby avoiding the risks of biopsy in this subgroup of patients. Hopefully, additional research on this important question will stem from this work.

FOOTNOTES

See also the article by Park et al in this issue.

References

1. Park SH, Kim PN, Kim KW, et al. Macrovesicular hepatic steatosis in living liver donors: use of CT for quantitative and qualitative assessment. Radiology 2006;239:105–112.[Abstract/Free Full Text]
2. Valentin-Gamazo C, Malago M, Karliova M, et al. Experience after the evaluation of 700 potential donors for living donor liver transplantation in a single center. Liver Transpl 2004;10:1087–1096.[CrossRef][Medline]
3. Brown RS Jr, Russo MW, Lai M, et al. A survey of liver transplantation from living adult donors in the United States. N Engl J Med 2003;348:818–825.[Abstract/Free Full Text]

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