Evaluation of Quality Metrics for Dose Assessment in Field Overlap Regions of VMAT-Based Craniospinal Axis Irradiation.

BACKGROUND/AIM: Patients with meningeosis carcinomatosa or primary brain tumors may require craniospinal irradiation. In craniospinal volumetric modulated arc therapy (VMAT-CSI), field junctions are sensitive regions prone to underdosage. This study investigated whether global, planning target volume (PTV)-based quality metrics can detect such localized deficits and how different field-overlap widths affect junction dosimetry. PATIENTS AND METHODS: Retrospective computed tomography datasets from six patients were analyzed. For each patient, VMAT-CSI plans with overlap lengths of 4, 6, 8, 10, and 12 cm were generated. Dose coverage (D98%) and homogeneity index (HI) were evaluated for the PTV and a junction-focused subvolume (PTV-VOI). Conformity indices (RTOG, Salt-Lomax, Lomax, and Van't Riet) were calculated for both structures. RESULTS: Increasing overlap improved D98% for both PTV and PTV-VOI. While the PTV often met D98% ≥95% at small overlaps, adequate PTV-VOI coverage was achieved only at ≥10 cm. VOI-based RTOG and Salt-Lomax indices increased with overlap, the Lomax index decreased slightly, and Van't Riet remained stable. Global PTV-based indices showed minimal variation. CONCLUSION: Adequate dose coverage at the field junctions in VMAT-CSI treatment is achieved with field overlaps of 10-12 cm. Since global PTV metrics mask local underdosages, clinicians should use VOI assessments at the field junctions rather than relying solely on global plan indices.