[When the neuroendocrine phenotype is misleading: A hepatic tumor with a deceptive profile]

We report the case of a 45-year-old woman in whom a solitary 5.5cm hepatic tumor was discovered during oncologic surveillance for a papillary thyroid carcinoma diagnosed ten years earlier. Biopsy revealed a tumor cell proliferation with "endocrinoid" morphology and convincing immunohistochemical expression of neuroendocrine markers, initially suggesting a well-differentiated grade 3 neuroendocrine tumor. FDG-PET/CT demonstrated isolated hypermetabolic activity in the liver lesion, with no corresponding uptake on DOTATOC-PET. Following neoadjuvant chemotherapy, the patient underwent segmental liver resection. Histopathological examination of the resected specimen showed a proliferation of monomorphic cells with ovoid nuclei, arranged in a tubulo-solid architecture, with focal areas reminiscent of a "thyroid-like" pattern. Tumor cells exhibited heterogeneous expression of neuroendocrine markers and strong, diffuse positivity for alpha-inhibin. RNA sequencing identified a NIPBL::NACC1 fusion transcript, leading to a revised diagnosis of hepatic carcinoma with NIPBL::NACC1 fusion. This recently described and rare hepatic tumor is challenging to diagnose on biopsy. Histologically, it is characterized by a monomorphic ovoid cell proliferation with a tubulo-solid growth pattern and focal thyroid-like morphology. Neuroendocrine marker expression is variable, but strong and diffuse alpha-inhibin staining is a consistent feature.