Hepatocellular carcinoma with a "nodule-in-nodule" appearance reflecting an unusual dilated pseudoglandular structure.

We present a case of hepatocellular carcinoma (HCC), showing an atypical "nodule-in-nodule" appearance on ultrasonogram (US). In general, a "nodule-in-nodule" appearance is found as a hyperechoic tumor containing a hypoechoic nodule. In the present case, however, there was a hyperechoic subnodule in the center of the tumor, which was surrounded by a hypoechoic tumor area. Histologically, the subnodule consisted of moderately differentiated HCC with a markedly dilated pseudoglandular structure, and the outer tumor consisted of well-differentiated HCC with a thin-trabecular pattern. It should be noted that there is a rare HCC with dilated pseudoglandular structure showing the inversed "nodule-in-nodule" appearance.