Standardized reporting of sacroiliac joints in spondyloarthritis on CT: modified Delphi consensus on lesion definitions from the ESSR-Arthritis Subcommittee

Abstract Objectives To establish standardized definitions for structural sacroiliac joint (SIJ) lesions on computed tomography (CT) to enable consistent reporting. Materials and methods A modified Delphi process was conducted among members of the Arthritis Subcommittee of the European Society of Musculoskeletal Radiology (ESSR). Initial definitions were drafted by a core group of twelve experts from nine different countries, and iteratively refined across up to three rounds. In the first round, members indicated simple agreement or disagreement. Subsequent rounds employed a 10-point Likert scale (1 = strongly disagree, 10 = strongly agree), with a predefined threshold of > 75% agreement and a score of ≥ 8 required for a definition to advance. Explanatory texts and supporting references were introduced after the first round. Feedback was systematically reviewed, and consensus was sought on whether each definition should be retained or revised. Results Response rates were 66% (42/64), 69% (44/64), and 75% (48/64) across the three rounds. Seven structural SIJ lesion definitions were established: erosions, pseudo-widening, subchondral sclerosis, backfill, SIJ space narrowing, bone bud, and ankylosis. Fat metaplasia was excluded by agreement due to limited CT detectability, while backfill, primarily visible on MRI, was retained as a structural lesion. Agreement across definitions ranged from 76% to 96%, with all exceeding predefined consensus thresholds. Conclusions This study provides the first expert consensus definitions for structural SIJ lesions on CT. Standardized definitions may improve reporting consistency, reproducibility, and communication among radiologists, and form the basis for a future CT-based SIJ-RADS scoring system to support earlier recognition of sacroiliitis. Key Points Question Can standardized expert consensus definitions for structural sacroiliac-joint lesions on CT be established to enable consistent reporting globally? Findings Through a modified Delphi process, experts achieved consensus on seven structural sacroiliac joint lesion definitions for CT with high agreement . Clinical relevance Standardized CT definitions for sacroiliac joint lesions improve reporting consistency, interobserver reproducibility, and communication, enabling CT-based sacroiliac joint RADS and supporting earlier, confident recognition and stratification of sacroiliitis across institutions for patient care .