Fig. 1

a: The cut surface of a resected specimen. A whitish solid tumor measuring 5.3 × 3.5 cm in maximal diameter was observed. The tumor showed infiltrative growth around the bile duct, and the border of the tumor was indistinct. The tumor directly invaded the omentum. b-d: Representative histological hematoxylin-eosin (HE) photographs of the tumor. b: The tumor showed invasive growth without capsule formation (HE × 40). c: The tumor partly showed squamous differentiation, including a sheet-like growth pattern, streaming nuclear arrangement, and mild keratinization. These tumor cells were considered squamoid cells. d: Although goblet-like cells were not apparent, a gland-like structure and intracytoplasmic and a small amount of extra-cytoplasmic mucin-producing cells were focally observed (HE × 200)