CT-Defined Osteosarcopenia Is Associated with Mortality in Metastatic Prostate Cancer: A Longitudinal Analysis of Opportunistic CT Imaging

Background/Objectives: In metastatic prostate cancer, loss of bone density (osteoporosis), muscle quantity and quality (sarcopenia) may contribute to fractures, falls, and mortality. Unfortunately, their longitudinal assessment is poorly characterized. In prostate cancer, ageing, advanced disease, and cancer treatments may accelerate bone and muscle loss. This study assessed longitudinal computed tomography-derived musculoskeletal biomarkers in metastatic prostate cancer and the association between baseline osteosarcopenia and mortality. Methods: This retrospective single-centre cohort included 437 patients, 3595 muscle computed tomography (CT) assessments and 2944 bone CT assessments. An automated deep-learning pipeline quantified lumbar trabecular bone mineral density (BMD), psoas muscle volume (PV), and psoas muscle density. Osteosarcopenia was defined as concurrent osteoporosis (BMD < 135 Hounsfield Units (HU)) and sarcopenia (volumetric psoas muscle index (vPMI) < 30). Patient-aware mixed-effects models characterized biomarker trends and reverse-time analysis. Kaplan–Meier analysis and Cox proportional-hazards regressions were utilized to assess mortality. Results: Baseline osteosarcopenia affected 75 patients (17.2%). Psoas density decreased by 1.569 HU/year (95% confidence interval (CI) −1.703 to −1.434 HU/yr). Nonlinear models estimated mean declines in psoas volume (−6.42 cm3/year) and bone density (−6.01 HU/year). Among deceased patients, modelled final-year declines showed −12.57% in psoas density and −10.28% in psoas volume. Baseline osteosarcopenia was associated with higher all-cause mortality (Hazard Ratio (HR) 1.611; 95% CI 1.134–2.288; p = 0.008). Conclusions: Osteosarcopenia is common and progressive in metastatic prostate cancer, and muscle loss accelerates near death. Baseline osteosarcopenia independently identifies patients at increased mortality risk. CT imaging provides a scalable approach for longitudinal assessment of musculoskeletal health and offers clinically important information without additional patient burden.