To report outcomes with proton craniospinal irradiation (pCSI) for solid tumor leptomeningeal metastasis (LM).

Treatment options for LM are limited. A recent phase II study found pCSI was well-tolerated and improved survival. We report our experience with pCSI for solid tumor LM.

This is a retrospective review of patients treated with pCSI for solid tumor LM from 12/2020-1/2024 at our center. Characteristics were summarized using descriptive statistics. Median overall survival (mOS) and median central nervous system progression free survival (mPFS) from first day of radiation were estimated using Kaplan-Meier.

We identified 45 patients who completed pCSI. Median age was 54 years (range, 23-79); 73% were female and 52% lived >100 miles from our center. Breast (53.3%), lung (20.0%), and melanoma (8.9%) were the most common primary cancers; 51.1% had stable systemic disease at LM diagnosis. All had imaging evidence of LM and 64.4% were confirmed by CSF cytology. Eighty percent had symptomatic LM and the median KPS was 80. The median time from primary cancer to LM was 23.1 months (range, 0-221.3). Fifty-three percent had active brain metastasis at LM diagnosis; 33.3% of all patients received prior intracranial radiation. Median time from simulation to pCSI was 12 days. At first visit following pCSI, median KPS was 70. Seventy-six percent had new/worsening nausea, 51.1% headache, and 31.1% fatigue; predominantly grade 1 or 2. Following pCSI, 8 were dispositioned to surveillance, 2 to intrathecal chemotherapy, 30 to systemic therapy, and 4 to hospice. mOS was 13.7 months [95% confidence interval (CI), 11.2 to infinity] and mPFS was 6.5 months (95% CI, 4.9 to 12.8).

Our outcomes data are comparable to those recently reported in a phase II study. Further study is indicated to determine the optimal candidates for pCSI and sequential therapies.