Persistent cerebrospinal fluid tumor cells during transient radiographic response in bilateral retinoblastoma with central nervous system involvement: a pediatric case report.
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BACKGROUND: Central nervous system (CNS) involvement in retinoblastoma carries a poor prognosis, and the frequency and clinical meaning of discordance between structural magnetic resonance imaging (MRI) and cerebrospinal fluid (CSF) cytology are not well defined. This report describes a compartment-specific mixed response and emphasizes the complementary roles of MRI and serial CSF assessment. CASE DESCRIPTION: A 6-year-11-month-old boy presented after 6 months of progressive left proptosis, tearing, and conjunctival injection, without seizures or somnolence. Examination showed marked left proptosis and corneal opacity. MRI demonstrated bilateral ocular abnormalities, direct extension along the left optic nerve and chiasm, and a 29.3 mm × 25.8 mm intracranial lesion; malignant cells were present in CSF. Four cycles of vincristine, carboplatin, and etoposide reduced the intracranial lesion to 8.7 mm × 7.5 mm, but the left intraocular lesion did not clearly regress, optic nerve thickness increased from 4.9 mm after two cycles to 5.4 mm after four cycles, and CSF remained cytologically positive. Treatment was intensified for cycles 5-6 with cyclophosphamide-containing systemic chemotherapy and intrathecal methotrexate plus cytarabine. After six cycles, the intracranial lesion enlarged to 11.4 mm × 8.6 mm, optic nerve thickness increased to 6.7 mm, and CSF tumor cells persisted. Severe myelosuppression, infection, transfusion requirements, and family-related barriers prevented further treatment. The child died in March 2026, approximately 10 months after diagnosis. Germline testing prompted by bilateral disease identified a pathogenic RB1 c.772_776del (p.Asn258GlufsTer11) variant that was absent from parental blood. CONCLUSIONS: The early reduction in measurable intracranial tumor burden did not represent CSF clearance and was followed by radiographic progression. In selected patients with CNS-involved retinoblastoma, MRI and CSF cytology provide complementary, non-interchangeable information; this single case supports integrated reassessment but does not establish a new response protocol.