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Introduction Forced vital capacity (FVC) is a key indicator for monitoring and therapeutic decision-making in systemic sclerosis (SSc). We aimed to investigate the impact of the thoracic wall muscle mass on FVC and quality of FVC test.Material and Methods The study included adult patients with SSc who were admitted to the Hacettepe University Rheumatology Clinic between April-September 2023, underwent pulmonary function test (PFT), and had a thoracic computed tomography (CT) within 6 months. During the same period, control subjects from the chest diseases clinic who underwent PFT for any reason and had a thoracic CT within 6 months were enrolled. Based on the updated 2019 ATS guideline, participants were stratified into two subgroups: high-quality FVC (grades A/B/C) and low-quality FVC (grades D/E/F/U). The boundaries of all thoracic wall muscles included in the axial slices at the 4th (T4) and 10th (T10) thoracic vertebral levels, as well as the pectoral muscles specifically at the T4 level, were manually delineated on CT images for each patient. The volume of the segmented regions was automatically calculated. For calculating cross-sectional area (CSA), the following formula was applied: cm3 × (10/slice thickness in mm).Results The study included 54 SSc patients (F/M: 50/4) and 20 controls (F/M: 11/9). The median age was comparable between groups [57 (42-60) vs 56.5 (49.5-64) years, p=0.48], whereas BMI was lower in the SSc cohort [24.4 (21.8-26.9) vs 27.4 (26.5-32.5) kg/m2, p<0.001]. Predicted FVC (%) was significantly reduced in the SSc group compared with controls [82.5% (68-99) vs 101% (90.5-114), p=0.006], while the proportion of high-quality FVC tests was similar (53.7% vs. 45%, p=0.506). Thoracic wall muscle mass measurements were lower in SSc, including T4 CSA [121.6 (106.6-145.9) vs 147.9 (115.2-164.4) cm2, p=0.063], T4 pectoralis CSA [15.4 (12.5-19.7) vs 18.2 (10.2-25.7) cm2, p=0.404], and T10 CSA [59.2 (48.1-66.4) vs 108.7 (92.2-127.4) cm2, p<0.001], with the greatest difference observed at T10 level. In SSc patients, thoracic wall muscle mass measurements were significantly correlated with predicted FVC (%) with the exception of the T4 pectoralis CSA ( table 1). Thoracic muscle mass did not differ significantly between SSc patients with high- and low-quality FVC tests.Conclusions In this study, we observed that thoracic wall muscle mass was reduced in patients with SSc and significantly correlated with FVC. By contrast, no significant association was found between muscle mass and FVC quality, suggesting that FVC quality is influenced by multiple factors.Abstract P.397 Table 1Correlation between FVC (%) and respiratory muscle mass measurements