Abnormal body composition is associated with outcomes in patients receiving immune checkpoint inhibitors (ICIs), but its prognostic significance in advanced urothelial carcinoma (aUC) remains unclear. This study investigated the association between computed tomography (CT)-based body composition metrics and clinical outcomes in this population.

Patients with aUC treated with atezolizumab or pembrolizumab between January 2019 and December 2022 at our institution were retrospectively analysed. Body composition was assessed using abdomen–pelvis CT at the L3 vertebral level using artificial intelligence-based platform. Sarcopenia and myosteatosis were defined by T-scores below − 2.0. Visceral obesity (VO) was defined as visceral fat area ≥ 100 cm²; subcutaneous fat obesity (SFO) as subcutaneous fat area/height² ≥50 cm²/m² in men and 42 cm²/m² in women. Associations with overall survival (OS) and progression-free survival (PFS) were evaluated.

Among 212 patients (median age, 68 years; 75.0% male), 82.0% received atezolizumab and 18.0% pembrolizumab. Sarcopenia and myosteatosis were present in 11.3% and 51.9%; VO and SFO in 61.3% and 42.0%. Sarcopenia and myosteatosis at baseline were significantly associated with shorter OS (HR 2.08; 95% CI, 1.31–3.29; P = 0.002 and HR 1.54; 95% CI 1.14–2.07; P = 0.004, respectively). In multivariable analysis, age < 70, haemoglobin < 10 g/dL, liver metastasis, sarcopenia (HR 2.26; 95% CI, 1.39–3.71; P = 0.001) and myosteatosis (HR 1.49; 95% CI, 1.06–2.01; P = 0.015) were independently associated with OS. Risk stratification based on the presence of sarcopenia and myosteatosis identified the highest mortality risk among patients with sarcopenia alone (HR 3.58; P < 0.001), followed by those with both conditions (HR 2.15, P = 0.015) and myosteatosis alone (HR 1.60, P = 0.003), as compared with patients with neither condition. A model including sarcopenia and myosteatosis yielded modest discrimination (Harrell’s C-index, 0.601), which was improved with the addition of liver metastasis and hemoglobin level (Harrell’s C-index, 0.708). No body composition metric was associated with PFS.

Sarcopenia and myosteatosis were significant independent prognostic factors for OS in ICI-treated aUC patients. Routine CT-based body composition analysis may support risk stratification and guide early interventions.