Definition:

• Visually assessed temporal reduction in enhancement of the periphery of an observation relative to liver from an earlier to a later phase resulting in peripheral portal venous phase hypoenhancement or delayed phase hypoenhancement of the observation periphery.

Usage

• Applies to enhancing observations
• Feature that favors intrahepatic cholangiocarcinoma (ICC) or other non-HCC malignancy over HCC 
• How it should be assessed or characterized:
  • Portal venous or delayed phase
    • Typically manifests as a peripheral hypoenhancing zone within, not surrounding or around, an observation.  The apparent washout of contrast material from tumor periphery in ICC and other non-HCC malignancies has been attributed to hypercellularity with small extracellular volume in this portion of these tumors.
    • The center of the observation typically shows progressive enhancement from earlier to later phases, resulting in portal venous phase or delayed phase hyperenhancement which is more intense than the observation periphery and surrounding liver. The progressive central enhancement in ICC and other non-HCC malignancies has been attributed to gradual accumulation of extracellular contrast agents within the extracellular spaces of the fibrotic stroma in the center of these tumors.
  • Hepatobiliary phase
    • Appearance in the hepatobiliary phase is similar to that with extracellular agents in the portal venous or delayed phase:  the observation periphery is hypointense relative to surrounding liver and the observation center is hyperintense relative to the observation periphery. Unlike with extracellular agents, however, the observation center is usually not hyperintense relative to the observation periphery.  The hypointensity of the observation periphery has been attributed to the lack of hepatocytes in these tumors.  The relative hyperintensity of the observation center has been attributed to gradual accumulation or retention of hepatobiliary contrast agents within the extracellular spaces of the fibrotic stroma in the center of these tumors.  Since the fibrotic strom lacks hepatocytes, the enhancement of the central fibrotic stroma typically is not as intense as the enhancement of the surrounding liver.

If unsure about feature:

• If unsure of peripheral washout appearance vs washout appearance, characterize as peripheral washout appearance
  • Rationale: Peripheral washout appearance favors categorization of an observation as LR-M (possible non-HCC malignancy) over categorization as LR-3, LR-4, or LR-5. By comparison, washout appearance is a major feature that favors categorization as LR-3, LR-4, or LR-5.  As the presence of a non-HCC malignancy can substantially alter management, a tie-break between a feature favoring LR-M (peripheral washout appearance) and a feature favoring LR-3/LR-4/ LR-5 (washout appearance) goes to the feature favoring LR-M (peripheral washout appearance). 

• If unsure about peripheral washout appearance vs hepatobiliary phase hypointense rim, characterize as peripheral washout appearance.
  • Rationale: Peripheral washout appearance favors categorization of an observation as LR-M (possible non-HCC malignancy) over categorization as LR-3, LR-4, or LR-5. By comparison, hepatobiliary phase hypointense rim is an ancillary feature that favors HCC over non-HCC malignancy and over benign entities. As the presence of a non-HCC malignancy can substantially alter management, a tie-break between a feature favoring LR-M (peripheral washout appearance) and a feature favoring HCC (hepatobiliary phase hypointense rim) goes to the feature favoring LR-M (peripheral washout appearance).
• If unsure about peripheral washout appearance vs no peripheral washout appearance, do not characterize as peripheral washout appearance.

Potential pitfalls and challenges:

• HCCs may have a hepatobiliary phase hypointense rim which may be mistaken for the peripheral hypointensity (peripheral washout appearance) of ICC and other non-HCC malignancies. Typically, however, the hypointense rim of HCC is thinner, more regular, and more sharply defined than the peripheral hypointensity of ICC and other non-HCC malignancies. If the differentiation is difficult, evaluation of additional imaging features may be required for diagnosis. In some cases, biopsy may be necessary.
• Although progressive central enhancement is typical for non-HCC malignancies such as ICCs with central fibrotic stroma, necrotic portions of the tumor center will not show progressive enhancement.  Thus, radiologists should be aware that progressive central enhancement may be incomplete.
• Peripheral washout appearance and/or progressive central enhancement typically are visualized more confidently at MRI than at CT due to the higher contrast sensitivity of MRI compared to CT.