ESTRO 2025 - Abstract Book

S3650

Physics - Quality assurance and auditing

ESTRO 2025

high-attenuating materials like bones [1]. An in-depth verification of the DirectDensity TM consistency was performed in a multi-institutional setting.

Material/Methods: A consensus guide for Hounsfield look-up table (HLUT) generation [2] was adapted for DirectDensity TM . Eight institutions, routinely using DirectDensity TM , were audited by site-visits. The audit included ten CT scanners of four different models (SOMATOM go.Open Pro, go.Sim, Drive, Confidence). At each CT scanner, the Gammex Advanced Electron Density phantom (Sun Nuclear Corporation) was scanned applying a fixed CT acquisition protocol. This protocol included the full range of available tube voltages (70-140 kV), and applied relative electron density reconstruction (RED HU ). Following the consensus guide, fourteen insert configurations, divided over two phantom sizes (body and head) and two insert locations (centre and periphery), were acquired to evaluate the effect of phantom size and insert location on the RED HU numbers of bone inserts. Scanner-specific HLUTs were generated. Results: Size-dependent RED HU differences were within 28 HU RED on average for the three lowest dense bone inserts and comparable between the CT scanners (Figure 1A). The HE Cortical Bone insert showed scanner-type-specific differences (<10HU RED for Drive and Confidence, and 45HU RED on average for Go.Sim and Go.Open Pro). Location dependent RED HU differences for the bone inserts were within 66 HU RED (maximum of institutional averaged values over the different tube voltages for the HE Cortical Bone insert) (Figure 1B). Systematically larger differences were observed for Drive (mean: 42 HU RED ), while the lowest differences were seen for Confidence (mean: 17 HU RED ). Figure 1C shows an overall good RED HU consistency for the bone inserts over the different tube voltages, except for 70 kV and 110 kV (known software inconsistency, resolved in next version) for most scanners. Size-dependent RED HU absolute differences for lung, fat, soft tissues, and water are on average 5 HU RED . Maximum differences of these tissue groups were 32, 14, 5 and 5 HU RED , respectively. RED HU HLUTs were scanner-type-specific for bones (Figure 2).