## Why Radionuclide CSF flow study (Indium-111 DTPA) is right The structure marked **A** — leptomeningeal carcinomatosis — causes CSF dissemination throughout the neuraxis and may result in focal CSF flow obstruction (CSF flow blocks). According to Harrison 21e and NCCN guidelines, a radionuclide CSF flow study using Indium-111 DTPA is MANDATORY before intrathecal chemotherapy to detect these blocks. Focal obstruction causes drug pooling in the subarachnoid space, leading to severe neurotoxicity and treatment failure. Identifying and treating blocks (with focal radiation or shunt placement) is essential before safe intrathecal drug delivery. ## Why each distractor is wrong - **Lumbar puncture with CSF cytology and flow cytometry**: While essential for diagnosis of leptomeningeal carcinomatosis (CSF cytology has ~50% initial yield, rising to 80-90% after three high-volume taps), this does NOT assess CSF flow dynamics or detect flow blocks. It is diagnostic, not a safety prerequisite for intrathecal therapy. - **Contrast-enhanced CT chest and abdomen for staging**: This is relevant for systemic cancer staging but has no role in assessing CSF flow or preventing intrathecal drug toxicity. It does not address the anatomic CSF flow problem posed by leptomeningeal disease. - **Electroencephalography to assess seizure risk**: Although seizures can occur in carcinomatous meningitis (from cortical involvement), EEG does not detect CSF flow obstruction and is not mandatory before intrathecal therapy. It may be useful clinically but is not the safety-critical investigation. **High-Yield:** Before intrathecal chemotherapy in leptomeningeal carcinomatosis, always perform Indium-111 DTPA CSF flow imaging to rule out flow blocks — failure to do so risks severe neurotoxicity from drug pooling. [cite: Harrison 21e — Neoplastic Meningitis; NCCN CNS Cancers Guidelines]
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.