Safety and Efficacy of Whole Brain LDRT+ICI+Intrathecal Chemotherapy in Refractory Meningeal Metastasis of Lung Cancer

Whole brain LDRT will be administered at 3 cohorts with increasing dose fractions: Group 1: 4 Gy/2f of one cycle; Group 2: 4 Gy/2f of two cycles (Q3w); Group 3: 4 Gy/2f of three cycles (Q3w). WB-LDRT will be administered in a 4 Gy of 2 fractions over two days, starting from Day 1 in the first cycle (a daily dose of 2 Gy, 4 Gy/2f for one cycle, once per three weeks, at minmum in one cycle and maximum in four cycles in total).

Pemetrexed, 30 mg, intrathecal injection, once per three weeks, 4 cycles in total

PD-1 inhibitor (Sintilimab, dose as recommended in the instruction manual), intravenous infusion, once per three weeks, 4 cycles in total

Pemetrexed at a dose of 500 mg/m\^2, intravenous infusion, once per three weeks, 4 cycles in total