Technical evaluation of a multipurpose phantom for selected CBCT QA endpoints across linac platforms.

Abstract

PurposeTo evaluate a multipurpose MZ CUBE-based workflow for selected CBCT QA endpoints, including translational IGRT-relevant geometric verification and core image quality (IQ) assessment, across three clinical Varian CBCT systems and multiple phantom units.MethodsCBCT was acquired on three Varian linacs under Head and Pelvis protocols. Unless otherwise specified, analyses used iterative CBCT (iCBCT). For HyperSight, datasets were additionally reconstructed using FDK, iCBCT, Acuros CTS, and Acuros CTS + MAR. Measurement accuracy was assessed using an imaging-guided couch-repositioning loop. IQ metrics included spatial linearity, spatial resolution, low-contrast detectability, HU constancy, HU uniformity, and noise. Cross-platform consistency was evaluated using a single phantom, while inter-phantom reproducibility was assessed using five independent units. Rotational verification was not included.ResultsTranslational IGRT-relevant geometric verification demonstrated sub-millimeter residual couch translation with tight dispersion. Spatial linearity remained near unity, and spatial resolution and low-contrast detectability exceeded protocol thresholds (Head up to 7th group; Pelvis up to 6th group). HU constancy across four materials met acceptance limits (±50 HU), while HU uniformity and noise were within institutional CBCT QA tolerances aligned with AAPM TG-142 and TG-179. Results were consistent for both cross-platform constancy and inter-phantom reproducibility. On HyperSight, Acuros CTS produced the lowest HU uniformity differences among reconstruction methods. Comparison with a clinically established phantom demonstrated comparable HU constancy and uniformity.ConclusionsThe MZ CUBE-based multipurpose phantom workflow yielded consistent performance for selected translational geometric and image quality QA endpoints across platforms and phantom units, supporting its use for constancy checks, benchmarking, and reconstruction-method QA.