Zhengyuan Hu ¹ Yan Wang ² Xiaojian Ji ¹ Bo Xu ³ Yan Li ¹ Jie Zhang ¹ Xingkang Liu ¹ Kunpeng Li ¹ Jianglin Zhang ¹ Jian Zhu ¹ Xin Lou ^2* Feng Huang ^1*

• ¹ Department of Rheumatology and Immunology, Chinese PLA General Hospital, Beijing, China
• ² Department of Radiology, Chinese PLA General Hospital, Beijing, China
• ³ Basic Research Center for Medical Science, Academy of Medical Science, Zhengzhou University, Zhengzhou, China

Objectives: Hip involvement is an important reason of disability in patients with ankylosing spondylitis (AS). Unveiling the potential phenotype of hip involvement in AS remains an unmet need to understand its biological mechanisms and improve clinical decision-making. Radiomics, a promising quantitative image analysis method that had been successfully used to describe the phenotype of a wide variety of diseases, while it was less reported in AS. The objective of this study was to investigate the feasibility of radiomics-based approach to profile hip involvement in AS.Methods: A total of 167 patients with AS was included. Radiomic features were extracted from pelvis MRI after image preprocessing and feature engineering. Then, we performed unsupervised machine learning method to derive radiomics-based phenotypes. The validation and interpretation of derived phenotypes were conducted from the perspectives of clinical backgrounds and MRI characteristics. The association between derived phenotypes and radiographic outcomes was evaluated by multivariable analysis.Results: 1321 robust radiomic features were extracted and four biologically distinct phenotypes were derived. According to patient clinical backgrounds, phenotype Ⅰ (38, 22.8%) and Ⅱ (34, 20.4%) were labelled as high-risk while phenotype Ⅲ (24, 14.4%) and Ⅳ (71, 42.5%) were at low risk for hip involvement. Consistently, the high-risk phenotypes were associated with higher prevalence of MRI-detected lesion than the low-risk. Moreover, phenotype I had significant acute inflammation signs than phenotype Ⅱ, while phenotype Ⅳ was enthesitis-predominant. Importantly, the derived phenotypes were highly predictive of radiographic outcomes of patients, as the highrisk phenotypes were 3 times more likely to have radiological hip lesion than the lowrisk [27 (58.7%) vs 16 (28.6%); adjusted odds ratio (OR) 2.95 (95% CI 1.10, 7.92)].We confirmed for the first time, the clinical actionability of profiling hip involvement in AS by radiomics method. Four distinct phenotypes of hip involvement in AS were identified and importantly, the high-risk phenotypes could predict structural damage of hip involvement in AS.