Definition:

• A smooth border surrounding most or all of an observation that differs in attenuation or signal intensity from that of the observation and background liver parenchyma and sets the observation apart from other cirrhosis-associated nodules. 

Usage

• Applies to peripheral border surrounding an observation which does not qualify as a capsule appearance
  • Capsule appearance specifically refers to a peripheral rim of smooth hyperenhancement in the portal venous phase or delayed phase and is considered a major feature (not ancillary feature) for HCC.
  • When it does not qualify as capsule appearance, distinctive rim is considered an ancillary feature that favors malignancy
    • Distinctive rim specifically favors HCC as opposed to malignancy in general
    • Radiologists at their discretion may apply restricted diffusion to upgrade category (up to LR-4)
    • Cannot be used to upgrade category to LR-5
    • Absence of a discrete perilesional rim should not be used to downgrade the category.
    • How it should be assessed or characterized:
      • A discrete perilesional rim may be seen on all imaging sequences other than the portal venous phase and delayed phase, including:
        • T2 weighted images
        • Pre-contrast CT
        • Hepatobiliary phase images
          • Hepatobiliary phase hypointense rim

 If unsure about feature:

• Do not characterize as distinctive rim.

 Synonyms:

• Synonyms: rim, peripheral rim, border, peritumoral border, peritumoral rim, distinct rim
• Preferred terms: Discrete perilesional rim
• Rationale for preferred terms: To draw distinction from fibrosis present throughout the liver or surrounding perfusional alteration.  

Potential pitfalls and challenges

• Distinctive rim and capsule appearance can co-exist and are not exclusive to each other.  Category should be initially assigned based on capsule appearance, if present. 
• Peripheral washout appearance of an observation is not considered a distinctive rim. Peripheral "washout" is peripheral hypoenhancement corresponding to cellular tumor tissue often seen in ICC.  This feature is present within the observation, not around the observation or in the adjacent hepatic parenchyma.  This peripheral hypoenhancement is also typically thicker and more irregular than a distinctive rim.