1.
Previous studies that have evaluated the additional role of precontrast phase in the detection of HCC
Some investigators found no merit of adding precontrast phase [12,
13].
However,
Oliver et al.
[14] reported that the additional review of precontrast phase with portal venous phase images led to detection of 16 – 18 % additional lesions,
which were not identified on the portal venous images.
Kim et al.
[15] have also demonstrated that the addition of precontrast phase to the biphasic CT increased the detection and diagnostic confidence in assessment of viable tumors in patients with HCC treated with TACE.
In our study,
eleven of 28 (39%) HCCs demonstrated hypoattenuation on the precontrast phase,
whereas no AP shunts showed hypoattenuation. Visual hypoattenuation of the lesion on precontrast phase was an independent predictor for HCC detection with arterial enhancing feature,
which is comparable result with two previous studies [14,
15].
The discrepancy among these studies could be explained by the differences in the study design and study populations.
In other words,
previous two studies [12,
13] only enrolled the cases with washout on portal venous phase or delayed phase and with or without arterial enhancement.
Although the role of precontrast image still remains controversial,
we believe the precontrast phase is useful tool for the detection of small HCCs with atypical enhancement.
2. Reason that HCCs frequently show hypoattenuation on precontrast phase
Although the reason is unclear,
Takayasu et al.
reported that the amount of fat in tumor cells is closely related with the attenuation of tumor on precontrast phase [16].
In addition,
presumably it is because HCCs have heterogeneous organization and can contain hemorrhage,
necrosis and fibrous or cystic degeneration according to cellular differentiation.
In contrast,
AP shunts show a preservation of hepatic structures and cellular compartments because it is only a perfusion gradient between the artery and portal vein that results in a higher arterial inflow [6,
17].
3. HCCs with atypical vascular pattern
Several studies reported that small (< 2cm) or well-differentiated HCC frequently shows atypical vascular pattern including arterial enhancement with isoattenuation on the portal venous or delayed phases,
as like HCCs included in this study [18-20].
These results also suggest that we cannot easily exclude the diagnosis of HCC when a small arterial enhancing nodule is identified in clinical practice.
Therefore, precontrast phase can be helpful to distinguish small arterial enhancing lesions between AP shunts and HCCs.
Conclusion
Visual hypoattenuation of the lesion on precontrast phase and round or oval shape are independent predictors for differentiating small arterial enhancing HCCs from atypical AP shunts.
Therefore,
the precontrast phase on quadriphasic CT scans can be helpful in differentiating small HCCs from AP shunts with increased diagnostic accuracy,
sensitivity,
and improved interobserver agreement.
Careful evaluation of the precontrast phase may reduce unnecessary imaging study or procedure,
and possibly can achieve earlier diagnosis of small HCCs.