Phantom benchmarking for quality assurance of synthetic computed tomography in cranial magnetic resonance-only radiotherapy

Background and purpose Synthetic computed tomography (sCT) from magnetic resonance imaging (MRI) enables computed tomography (CT)-free cranial radiotherapy planning, yet no consensus exists on which head phantoms are usable for sCT quality assurance (QA). This study benchmarked head phantoms to identify the most promising candidates for cranial sCT QA and the design characteristics associated with favorable sCT-to-conventional CT agreement.Materials and methods Ten head phantoms underwent clinical MRI (1.5 T) and CT imaging. Two vendor-provided sCT algorithms (2D slice-based, 3D volume-based) were applied to identical input. sCT and CT were compared using Dice similarity coefficient (DSC), 95th-percentile Hausdorff distance (HD95), CT number mean error (ME) within both a self-mask (sCT-derived) and a common-mask (CT-derived), and percentage dose deviations (%ΔD2%, %ΔD98%, %ΔDmean) across six beam configurations. Metrics were interpreted against a priori three-tier acceptance thresholds.Results Phantoms without internal skull-equivalent structures showed bone DSC ≤ 0.001 and common-mask bone ME≤ -1100 HU. Phantoms with skull-equivalent anatomy achieved bone DSC of 0.24-0.71, bone HD95 of 4.2-24.4 mm, and self-mask bone ME between -95 and +470 HU across both algorithms. Common-mask bone ME in skull-containing phantoms ranged from -198 to -1115 HU across both algorithms. Soft-tissue ME stayed within ±120 HU across skull-containing phantoms for both masks. Average absolute |%ΔDmean| was 3.2% (2D) and 3.7% (3D) without significant inter-algorithm difference (p = 0.49).Conclusions Internal skull-equivalent anatomy is a necessary but not sufficient design characteristic; low-susceptibility and MR-compatible materials proved essential. Joint evaluation of geometric, CT number, and dose agreement was required to rank phantom performance. No phantom met all criteria simultaneously.