Case Report: Cerebellopontine angle metastatic adenocarcinoma with high-grade fetal morphology and no identifiable pulmonary primary on initial CT

Background Adenocarcinoma with high-grade fetal morphology can create diagnostic uncertainty when it presents intracranially without an identifiable systemic primary. Case presentation A 61-year-old man presented with gait instability for more than 1 month, dizziness, and left-sided hearing loss. CT and MRI showed a left cerebellopontine angle (CPA)-region mass with heterogeneous avid enhancement, fourth-ventricle compression, and suspected brainstem invasion, without internal auditory canal enlargement. On contrast-enhanced T1-weighted MRI, the lesion measured 37.6 × 24.7 × 35.9 mm (left-to-right [transverse] × anteroposterior × craniocaudal). Initial thoracic CT identified no pulmonary lesion. Subtotal resection was performed because of dense adherence to cranial nerves and the brainstem. Histology showed complex fetal-type glandular and papillary architecture, supranuclear and subnuclear vacuoles, high-grade atypia, and necrosis. The integrated diagnosis was metastatic adenocarcinoma with high-grade fetal morphology, favoring pulmonary origin; however, no pulmonary primary was identifiable on initial thoracic CT or confirmed pathologically. The diagnosis of pulmonary high-grade fetal adenocarcinoma (HG-FLAC) was therefore not established. Postoperative dysphagia and facial nerve dysfunction developed. Systemic staging remained incomplete; molecular testing and PD-L1 assessment were not performed. Remaining paraffin-embedded tissue was insufficient for additional immunohistochemical studies. No systemic anticancer therapy or radiotherapy was initiated after tertiary-center consultation, and detailed treatment-decision records were unavailable. The patient died 3 months after surgery; the immediate cause was undocumented. Conclusion This case illustrates an atypical CPA presentation of adenocarcinoma with high-grade fetal morphology. Pulmonary origin was favored but unconfirmed. Diagnostic terminology must reflect this uncertainty, and the short survival cannot establish HG-FLAC-specific prognosis.