Monitoring the Response of a Pineal Parenchymal Tumor of Intermediate Differentiation With Cerebrospinal Fluid Dissemination Using a Circulating Tumor Cell Assay

Pineal parenchymal tumors of intermediate differentiation (PPTID) are aggressive, rare central nervous system tumors. The management of PPTID remains challenging, given the limited number of reported cases and the logistical difficulties in conducting studies on rare tumors such as these. Leptomeningeal disease (LMD) is a relatively frequent pattern of recurrence, and accurate monitoring is essential to optimize potentially efficacious therapies. We present the case of a 27-year-old woman who initially presented with positional headaches and mild blurry vision. Imaging revealed an enhancing third ventricular lesion with obstructive hydrocephalus. She underwent subtotal surgical resection of the tumor and was diagnosed with PPTID. Approximately 14 months later, imaging demonstrated local recurrence with leptomeningeal metastases. In addition to standard therapies, the Biocept IN/CNSide cerebrospinal fluid (CSF) liquid biopsy assay (Biocept, Inc., San Diego, California, US), including tumor cell protein expression biomarkers, ctDNA biomarkers, and tumor cell fluorescence in situ hybridization (FISH) biomarkers, was used to monitor treatment response at seven time points. During this period, CSF cytology and other conventional markers were unremarkable. This case demonstrates how novel CSF-based monitoring technology may improve disease surveillance in patients who develop leptomeningeal disease. Although leptomeningeal metastasis is not uncommon in patients with PPTID, the optimal monitoring approach remains unclear. While this assay remains investigational, it may represent a promising strategy to address an unmet need in the management of PPTID with LMD.