['To evaluate the effect of chest size on coronary calcium score (CCS) as assessed with new-generation CT systems from 4 major vendors.', 'An anthropomorphic, small-sized (300\xa0× 200\xa0mm) chest phantom containing 100 small calcifications (diameters, 0.5-2.0\xa0mm) was evaluated with and without an extension ring on state-of-the-art CT systems from 4 vendors. The extension ring was used to mimic a patient with a large chest size (400\xa0× 300\xa0mm). Image acquisition was repeated 5 times with small translations and/or rotations. Routine clinical acquisition and reconstruction protocols for small and large patients were used. CCS was quantified as Agatston and mass scores with vendor software.', 'The small-sized phantom resulted in median (interquartiles) Agatston scores of 10 (9-35), 136 (123-146), 34 (30-37), and 87 (85-89) for Philips, GE, Siemens, and Toshiba, respectively. Mass scores were 4 mg (3-9 mg), 23 mg (21-27 mg), 8 mg (8-9 mg), and 20 mg (20-20 mg), respectively. Adding the extension ring resulted in reduced Agatston scores for all vendors (17%-48%) and mass scores for 2 vendors (11%-49%). Median Agatston scores decreased to 9 (5-10), 79 (60-80), 27 (24-32), and 45 (29-53) units, and median mass scores remained similar for Philips at 4 mg (4-6 mg) and Siemens at 8 mg (7-8 mg) and decreased for the other vendors to 13 mg (11-14 mg) and 10 mg (8-13 mg), respectively.'] Are coronary calcium scores systematically underestimated at a large chest size : A multivendor phantom study?