Primary CNS lymphoma

• Standard CHOP chemotherapy is not effective because of poor blood–brain barrier penetration.

• S: Methotrexate (IV) 8 mg/m²/day twice a week until remission (as printed in STG 2026).

⚠ Dose check note: STG 2026 prints methotrexate for primary CNS lymphoma as "8mg/m2/day twice a week". Primary CNS lymphoma is treated with HIGH-dose methotrexate, usually 3.5–8 g/m² IV every 2 weeks with folinic acid rescue, hydration, urine alkalinisation and methotrexate-level monitoring; 8 mg/m² would be ineffective, and a gram-level dose must never be given without rescue. Cross-check this dose against the WHO recommendation before prescribing.