Choroid Plexus Mimicry: Metastatic Papillary Lung Adenocarcinoma in the Fourth Ventricle.

Brain metastases are the most common type of brain tumors, typically presenting as intraparenchymal lesions. Intraventricular or choroid plexus involvement is very rare, posing a significant diagnostic challenge. This case report describes a rare instance of a solitary choroid plexus metastasis from a highly well-differentiated lung adenocarcinoma in a 41-year-old woman. The metastatic tumor's close morphological resemblance to a primary choroid plexus tumor added to the diagnostic difficulty, initially leading to a misdiagnosis of choroid plexus carcinoma. However, further investigation, including detailed immunohistochemistry and the detection of a BRAFV600E mutation, ultimately identified the tumor as a metastatic lung adenocarcinoma. Notably, this choroid plexus metastasis served as the index diagnosis, revealing the previously undiagnosed metastatic lung cancer. This case highlights the critical need to consider metastasis in the differential diagnosis of solitary choroid plexus tumors in adults. Recognizing the cytokeratin staining patterns in choroid plexus tumors, along with the use of organ-specific antibodies and molecular analysis, is essential for achieving an accurate diagnosis and ensuring that patients receive the most appropriate and effective treatment.