2584 - Toxicity Comparison of Photon and Proton Craniospinal Irradiation for Leptomeningeal Carcinomatosis

08:00am - 09:00am PT

Hall F

Screen: 22

POSTER

Presenter(s)

Isabella Dreyfuss, DO - University of Miami, Miami, FL

I. Dreyfuss¹, D. J. Lee², J. B. Bell³, G. Azzam³, B. J. Rich⁴, G. J. Kubicek³, M. E. Freret⁵, and E. A. Mellon³; ¹University of Miami, Miami, FL, ²Sylvester Comprehensive Cancer Center, University of Miami Miller School of Medicine, Miami, FL, ³Department of Radiation Oncology, Sylvester Comprehensive Cancer Center, Miller School of Medicine, University of Miami, Miami, FL, ⁴Department of Radiation Oncology, University of Miami/Sylvester Comprehensive Cancer Center, Miami, FL, ⁵Department of Radiation Oncology, Sylvester Comprehensive Cancer Center, University of Miami Leonard M. Miller School of Medicine, Miami, FL

Purpose/Objective(s): Proton craniospinal irradiation (pCSI) improved overall survival (OS) for patients with leptomeningeal metastases (LM) from breast and lung cancer in a phase 2 trial compared to standard-of-care radiotherapy. Volumetric modulated arc therapy (VMAT) CSI is a widely available photon radiotherapy technique that may reduce CSI toxicity compared to conventional X-CSI approaches. We performed a retrospective analysis comparing outcomes among patients treated with intensity-modulated proton therapy (IMPT) versus VMAT CSI, hypothesizing pCSI superior toxicity outcomes.

Materials/Methods: We retrospectively analyzed 34 consecutively treated patients with LM (21 IMPT, 13 VMAT) treated with 30 Gy in 10 fractions of CSI and Karnofsky Performance Status =60. VMAT was used when IMPT was unavailable due to center capacity, logistical issues, or insurance denial. Changes in blood counts (paired t-test) and CTCAE v5.0 toxicities (Chi-Square) were assessed at baseline, 6 weeks, and 3 months post-CSI, as well as median follow-up. Concurrent cytotoxic chemotherapy was not permitted.

Results: Lab values and symptom major differences were not observed between IMPT and VMAT CSI groups. Hemoglobin declines at 6 weeks between IMPT (13.0 to 11.9 g/dL) and VMAT (12.4 to 10.45 g/dL) were not significantly different, but hemoglobin decline occurred at 6 weeks for all patients (delta of -2.5/dL, p<0.01). Platelets also declined significantly (223 to 147×10?/L, p=0.01), but were not statistically different between IMPT (195 to 145×10?/L, n = 21) and VMAT (267 to 232×10?/L, n = 13). White blood cells and absolute lymphocyte counts showed no significant changes between groups or timepoints. Blood counts remained stable at 3 months in surviving patients (n = 20).

CTCAE toxicities included hematologic, dysphagia, fatigue, alopecia, diarrhea, nausea, neurologic, cardiac, and lung dysfunction. No significant differences in grade 1+, 2+, or 3+ toxicities were observed between IMPT and VMAT. Most patients had alopecia, grade 1 fatigue, and at least one new/worsening hematologic toxicity at 6 weeks. New or worsening neurologic symptoms occurred in 38% of VMAT and 33% of IMPT patients, though treatment attribution was uncertain. Acute grade 1-2 nausea, vomiting, diarrhea, dysphagia, vision change, and lung dysfunction were uncommon (<10%).

Kaplan-Meier estimated OS for all patients was 4.4 months (IMPT 6.79, VMAT 2.89, log-rank p=0.192), with a 28% one-year OS at median follow-up for all surviving patients of 4.3 months (range 1-43 months).

Conclusion: Our single-institution analysis found no significant evidence that IMPT reduces toxicity for CSI of LMD compared to VMAT. Limited sample size prevents conclusions on subtle or long-term effects. Larger multi-institutional studies are needed to determine OS survival differences, toxicity differences, and the most cost-effective LM therapy between IMPT and VMAT.