Letter to the Editor: “Does quantitative assessment of arterial phase hyperenhancement and washout improve LI-RADS v2018–based classification of liver lesions?”

by Sébastien Mulé, Alain Luciani (sebastien.mule@gmail.com)

Does quantitative assessment of arterial phase hyperenhancement and washout improve LI-RADS v2018–based classification of liver lesions?

Dear Editor,

We read with great interest the article by Stocker and colleagues in the recent issue of European Radiology about diagnostic accuracy of LI-RADS v2018-based categorization of focal liver lesions using quantitative MRI assessment of arterial phase hyperenhancement (APHE) and washout [1]. The authors found that quantitative analysis of APHE and washout did not improve LI-RADS categorization of focal liver lesions, but rather decreased sensitivity of HCC diagnosis regarding washout. While authors should be congratulated for underlying the need for robust and thus quantitative measurement of both APHE and washout in liver lesions, some additional questions should be raised.

First, the terms used differ from LI-RADS v2018 lexicon terms [2]. Indeed, LI-RADS v2018 major features included nonrim APHE and nonperipheral washout. Beyond the inaccurate terminology, the features analyzed and thus the stratification of the probability of HCC may be inaccurate as well. Notably, analysis of the spatial distribution of both APHE and washout within the lesion is of paramount importance, as rim and nonrim APHE features – and peripheral and nonperipheral washout – have fully different meanings. Although rim APHE – i.e. APHE most pronounced in lesion periphery – may be seen in HCC, especially in sarcomatoid subtype of HCC, and in combined HCC [3-4], this feature is highly suggestive of non-HCC malignancy and is therefore recognized as a LR-M feature. In the same way, peripheral washout – i.e. washout most pronounced in lesion periphery – is also suggestive of non-HCC malignancy and thus should lead to lesion categorization as LR-M. Although the authors excluded LR-M lesions from their study, the inaccurate terminology still remains an issue which is also reflected in the definition and placement of regions of interest (ROIs) for the quantitative analysis. Authors mentioned that ROIs were placed at the same position and on the same image slice in all phases, which is inconsistent with LI-RADS v2018 that mention that APHE and washout appearance do not need to coincide in the same part. In the same way, ROIs were placed on the image with the largest cross-sectional diameter of the lesion if APHE was absent. They should at least have been placed in the image with the visually strongest area of washout. These points raise the question of the lesion segmentation and underline the requirement of a 3D analysis of the lesion to accurately detect and quantify both APHE and washout, and thus LI-RADS categorization.

LI-RADS v 2018 defined four major features for the diagnosis of HCC – nonrim APHE, nonperipheral washout, enhancing capsule, and threshold growth. If threshold growth (i.e. size increase of a mass by ≥ 50% in ≤ 6 months) cannot be determined from a single MRI exam, presence of an enhancing capsule should be systematically evaluated in the same way as nonrim APHE and nonperipheral washout. Therefore, quantitative LI-RADS should also include quantitative measurement of that major feature, especially as its interreader reliability may be moderate [5].

Quantitative analysis of enhancement features implies definition of threshold values. However, the choice of such threshold values may not be arbitrary but justified and optimized (with ROC curve analyses for example). The definition of AHPE as a minimum increase of 10% from pre-contrast to the arterial phase is not supported by any statistical analysis but seems to be individual arbitrary choice; this may lead to inaccuracies in APHE detection. Regarding washout, LI-RADS v2018 definition includes a temporal reduction in lesion enhancement (partly of fully) from arterial phase in addition to hypoenhancement relative to surrounding liver in the portal or delayed phase. The dynamic part of that definition has not been taken into account by the authors in their quantitative analysis of washout.

In summary, introduction of quantitative evaluation in LI-RADS algorithm may ultimately allow improvement of lesion classification and reduction in interreader variability and the authors should be acknowledged for that. However, accurate analysis of all enhancement patterns of the lesion – including APHE, washout and capsule – and also of their spatial distribution are required, making whole-volume lesion analysis possibly mandatory.