Intrathecal methotrexate in pediatric leukemia is used for what purpose, and what is key about dosing?

• A. CNS prophylaxis or therapy; dosing depends on age/weight and is given via lumbar puncture with precautions
• B. Systemic chemotherapy; dosing is fixed for all ages
• C. Pain management; dosing is randomized
• D. Skin-directed therapy; dosing based on surface area

Answer: (A)

Intrathecal methotrexate is used to treat or prevent leukemia in the central nervous system because the drug needs direct access to the CSF to kill leukemic cells that can hide in the CNS. Since methotrexate doesn’t penetrate the CSF well when given systemically, delivering it straight into the CSF via a lumbar puncture achieves therapeutic concentrations in this sanctuary site.

The dosing being based on age and weight matters because children vary in CSF volume and body size. Using a fixed dose for all ages could either under-treat a smaller child or raise the risk of toxicity in a larger one. Tailoring the dose to age/weight helps reach effective CSF exposure while minimizing adverse effects. Administration requires careful precautions—sterile technique, proper positioning for the lumbar puncture, and monitoring for neurotoxic or inflammatory reactions.

This approach isn’t for systemic chemotherapy, pain management, or skin-directed therapy, and the rationale for intrathecal dosing wouldn’t apply in those contexts.