MR Imaging criteria for HCC diagnosis according to multistep carcinogenesis and histology
When the process of dedifferentiation occurs in cirrhotic liver, neo angiogenesis leads to an increase of hepatic artery inflow and decrease of portal vein inflow, because of abnormal neo-formed arterial vessels. On MRI these changes lead to arterial phase enhancement of the lesion and washout on portal venous phase. Therefore, according to all the above mentioned guidelines/reporting systems, the typical pattern of HCC is defined by arterial phase hyperenhancement (APHE) and wash out on portal or delayed phases (Fig. 4). APHE followed by “washout” in the portal venous and/or delayed phase has a sensitivity of 64%–89%; specificity of 96%; and positive predictive value of 93% for diagnosis of hepatocellular carcinoma in cirrhotic liver11.
Fig. 5 summarizes different International criteria for imaging diagnosis of HCC.
In addition to APHE and washout, other important imaging features for the diagnosis of HCC according to LI-RADS/AASLD and OPTN are enhancing “capsule”, lesion size (10 < mm >20), and threshold growth of the lesion12. Of these imaging features, EASL guidelines consider only lesion size as criterion with the cut-off of 1 cm.
Enhancing “capsule” appears as a peripheral rim of smooth hyperenhancement surrounding background nodules in the portal venous and delayed phases. It corresponds at histology to a tumor fibrous capsule or pseudocapsule, but its originating mechanism is not fully understood.
OPTN and LI-RADS/AASLD criteria for the diagnosis of definitive HCC are identical except for:
-10-19 mm observations with nonrim APHE + nonperipheral “washout” but without enhancing “capsule” or threshold growth, because these lesions are not assigned any OPTN class, but are diagnosed as LR-5 (i.e. HCC by LI-RADS/AASLD);
LI-RADS is the only diagnostic system that stratifies the probability of lesions to be HCCs13:
· LR-1 0% definitive benign;
· LR-2 13% HCC, 14% malignancy;
· LR-3 38% HCC, 40% malignancy;
· LR-4 74% HCC, 80% malignancy;
· LR-5 94% HCC, 97% malignancy;
· LR-M 36% HCC, 93% malignancy.
In addition, LI-RADS provide definitions of imaging features that suggest malignancy (not HCC specifically), thus accounting for potential cholangiocarcinoma, hepatocholangiocarcinoma, or atypical HCC that may occur in a cirrhotic liver.
According to 2017 update of the Asia-Pacific clinical practice guidelines on the management of HCC, typical HCC can be diagnosed by imaging, regardless of its size, if a typical vascular pattern (arterial enhancement with portal venous washout) is obtained on dynamic CT, dynamic MRI or CEUS. In addition, hypervascular lesions without washout but with hepatobiliary phase hypointensity are also diagnosed as HCC by APASL, which may lead to potential false positive diagnosis (Fig. 6).