Prostate cancer presenting as brain metastasis with concurrent elevation of carcinoembryonic antigen and carbohydrate antigen 19-9 levels: illustrative case.

BACKGROUND: Prostate cancer is a common malignancy in men and usually presents with elevated prostate-specific antigen (PSA) levels and metastases to the bones or lymph nodes. Brain metastases are rare, occurring in less than 1% of cases. Concurrent elevation of carcinoembryonic antigen (CEA) and carbohydrate antigen 19-9 (CA19-9) is also uncommon. OBSERVATIONS: A 61-year-old man presented with left hemiparesis and cognitive decline. Laboratory tests revealed elevated PSA, CEA, and CA19-9 levels. Imaging revealed a cystic frontal lobe mass, lung nodules, pelvic lymphadenopathy, and a prostatic lesion. MRI confirmed a 5-cm right frontal mass with heterogeneous enhancement and edema. A prostate biopsy revealed adenocarcinoma positive for CEA and CA19-9. Endoscopy revealed no gastrointestinal malignancies. Craniotomy and histopathological examination confirmed the diagnosis of adenocarcinoma. Immunohistochemical analysis revealed CEA positivity, partial CA19-9 positivity, strong NKX3.1 positivity, and PSA negativity. Postoperatively, the patient received androgen deprivation therapy with leuprorelin acetate and darolutamide, along with 5 cycles of docetaxel chemotherapy. At the 24-month follow-up, he remains neurologically stable without recurrence. LESSONS: Brain metastases from prostate cancer with elevated CEA and CA19-9 are rare. Although these markers often indicate a poor prognosis, this patient achieved a favorable outcome with multimodal treatment. https://thejns.org/doi/10.3171/CASE25735.