2639 - Proton vs. VMAT-Based Craniospinal Irradiation for Leptomeningeal Carcinomatosis: An Institutional Dosimetric and Hematologic Toxicity Analysis

Purpose/Objective(s): Leptomeningeal carcinomatosis (LMD) is defined by the spread of malignant cells in the meninges of the brain and spine. It is associated with a dismal prognosis. Craniospinal irradiation (CSI) may improve outcomes in some patients with LMD. Often, proton beam therapy (PBT) is preferred over volumetric modulated arc therapy (VMAT) for its perceived sparing of healthy tissues, including the anterior vertebral bodies, essential sites of active bone marrow. We retrospectively analyzed all patients treated with palliative CSI at our institution from 2020 onward to quantify dosimetric differences between bone marrow-sparing VMAT (bmsVMAT) and PBT in adult patients with LMD treated with CSI. We believe that bmsVMAT CSI can result in similar dosimetry to bone marrow structures compared to PBT.

Materials/Methods: Eleven patients treated with bmsVMAT and 18 patients treated with PBT CSI were eligible for the retrospective study. Median prescription dose was 30 Gy/10 fractions (range: 25-30 Gy). Bone marrow structures were contoured as follows: the anterior vertebral bodies from C1 through the coccyx make up the vertebral body (VB) structure while the bony pelvis and proximal femurs comprise the pelvic bone (PB) structure. Together, the VB and PB volumes combined compose the total bone marrow (BM) volume, which was optimized as an avoidance structure for bmsVMAT. Mean doses and volumes receiving 5 Gy, 15 Gy, and 25 Gy were analyzed.

Results: bmsVMAT had higher total marrow doses than PBT for mean (10.7 Gy vs 8.9 Gy, p=0.005) and V5 (54.0% vs 37.5%, p <0.001). However, there were no significant differences between bmsVMAT and PBT for V15 (33.1% vs 29.7%, p=0.092) or V25 (17.7% vs 19.5%, p=0.374). When assessing dose to the VB structure, V5 (91.6% vs 77.4%, p<0.001) and V15 (68.6% vs 61.7%, p=0.032) were statistically higher for bmsVMAT vs PBT. V25 for VB was similar between bmsVMAT and PBT (37.1% vs 40.5%, p=0.354). High-grade (grade =3) hematologic toxicities were similar between VMAT and PBT, at 91% and 94%, respectively (p=0.71).