Unveiling the ``vessel-inside-vessel'' phenomenon: A rare case of extensive portal vein tumor thrombosis in hepatocellular carcinoma.

Carcinomas often metastasize to distant organs by detaching from the primary tumor and entering the bloodstream. These circulating tumor cells are transported to sites such as lung capillaries and subsequently disseminated through systemic circulation. However, hepatocellular carcinoma (HCC) rarely metastasizes to distant organs, even in advanced stages. Instead, HCC frequently leads to intravascular and intrahepatic parenchymal metastases. Portal vein thrombosis (PVT), present in 10%-40% of HCC patients at diagnosis and 35%-44% at death or liver transplant, is a significant prognostic marker, associated with worse outcomes and reduced survival. Patients with PVT demonstrate markedly shorter overall survival compared to those without PVT, with main portal vein thrombosis indicating the poorest prognosis. Here, we report an unusual case of an enormous portal metastasis from liver HCC, characterized by a ``vessel-inside-vessel'' appearance.