2141 - VMAT Craniospinal Irradiation for Leptomeningeal Disease - Coverage and OAR Tradeoffs with Bone Marrow Sparing Maximization

Purpose/Objective(s): Leptomeningeal disease (LMD) is a devastating manifestation of metastatic cancer with limited treatment options. Proton craniospinal irradiation (CSI) shows promising results, but access is limited. VMAT CSI is widely available; however, concern for hematologic toxicity hinders adoption. Here we examine trade-offs in target coverage and dose to other organs at risk (OAR) when optimizing VMAT CSI for bone marrow (BM) sparing.

Materials/Methods: CT datasets from ten previously treated CSI patients were retrospectively replanned using VMAT. Prescription dose was 30 Gy in 10 fractions. BM evaluation structure (BM_Eval) included vertebral bodies and sternum, excluding PTV. Three planning strategies were compared. For Plan A, coverage goal for Spine PTV was V100%= 98% without specific BM constraints. For Plan B, BM was maximally spared without compromising coverage. For plan C, coverage compromise for Spine PTV of V100%= 80% was permitted for additional BM sparing. Dose to targets, BM, and other OARs were assessed.

Results: Key dosimetric parameters for each plan are shown in Table 1. Transitioning from Plan A to Plan B and from Plan B to Plan C significantly reduced BM V20Gy, V10Gy, and V5Gy. While PTV Spine for Plan C had significantly lower V100% than Plan B, CTV Spine V100% decreased only slightly. Heart V5Gy decreased significantly from plan B to Plan C, while other OARs associated with lymphopenia changed modestly. Esophagus dose was not adversely affected by bone marrow sparing. Mean kidney dose and bowel V5Gy increased modestly.

Conclusion: Permitting modest reduction in coverage to the spine PTV allowed for a significant reduction in BM dose beyond that achieved with standard VMAT and BM sparing plans. Tradeoffs to OARs associated with lymphopenia and acute GI toxicity were modest. This strategy may mitigate hematologic toxicity and improve photon CSI tolerability in patients with leptomeningeal disease, particularly where proton therapy is unavailable.