ESTRO 2025 - Abstract Book

S3583

Physics - Quality assurance and auditing

ESTRO 2025

Purpose/Objective: The Japanese computed tomography number (CTN) calibration audit (J-CTCA) phantom was developed to create CTN-to-mass density and CTN-to-relative electron density conversion curves using a stoichiometric method to assess the clinical CTN conversion curves used in radiotherapy treatment planning 1 . In contrast, the European Particle Therapy Network (EPTN) phantom utilizes 15 highly tissue-equivalent materials to create a CTN-to-stopping power ratio (SPR) conversion curve 2 . The purpose of this study is to compare the CTN-to-SPR conversion curve created by J-CTCA 3 with the EPTN-based CTN-to-SPR conversion curve to enhance the reliability of J-CTCA. Material/Methods: Tough lung and tough bone materials (Kyoto Kagaku Co. Ltd, Kyoto, Japan) used in J-CTCA were analyzed for elemental composition, from which plugs for the ETPN phantom were manufactured (See Figure 1). Two plugs were inserted into the EPTN phantom and scanned using a Siemens Somatom Definition AS scanner (Siemens Healthineers, Forchheim, Germany) under clinical conditions for head and body phantom configurations. The CTNs for the two plugs were measured, and three fitting parameters were obtained using the stoichiometric method. The theoretical CTNs for 10 representative tissues were calculated based on the three fitting parameters, and the densities and compositions of the body tissue model 4 . The theoretical SPR for each representative tissue was calculated for 100 MeV protons. A CTN-to-SPR conversion curve based on the EPTN phantom was created according to the EPTN consensus guide 5 . Differences in SPR between the two curves were evaluated and categorized based on density as lung (0.2–0.8 g/cm³), soft tissue (0.9–1.07 g/cm³), and bone (1.07–1.25 g/cm³).

Figure 1. Tough lung and tough bone plugs

Results: Figure 2 shows the CTN-to-SPR conversion curves under head and body scan conditions. For the head scan conditions, the mean ± standard deviation (SD) of the SPR differences were -0.4% ± 0.1% for the lung region, 0.7% ± 0.4% for the soft tissue region, and 0.5% ± 0.4% for the bone region. For the body scan conditions, the mean ± SD of the SPR differences were -1.0% ± 0.3% for the lung region, 0.9% ± 0.4% for the soft tissue region, and 0.5% ± 0.4% for the bone region.

Figure 2. Comparison of CTN-to-SPR conversion curves for J-CTCA and EPTN under head (a) and body (b) phantom configurations.