Objectives <p>To determine whether routine prostate MRI can quantify atrophy and fatty infiltration of the obturator internus (OI) muscle in prostate cancer (PCa) patients undergoing androgen deprivation therapy (ADT), to explore concordance between MRI-derived OI metrics and CT-based body composition, and evaluate their impact on progression-free survival (PFS).</p> Materials and methods <p>In this retrospective study, ADT-treated PCa patients were compared with benign prostatic hyperplasia controls. Baseline and follow-up prostate MRI and chest/abdominal CT scans were performed within a 2-year interval. The cross-sectional area (CSA) of the OI and inter-muscular adipose tissue of obturator internus muscle (OI-IMAT) was measured using MRI. Abdominal skeletal muscle (SM) and inter-muscular adipose tissue (SM-IMAT) were assessed via CT. PFS was analyzed using Kaplan–Meier methods.</p> Results <p>Among 104 ADT-treated PCa patients, significant reductions in OI area, OI index (OII) and increases in OI-IMAT, OI-IMAT index (OI-IMATI) were observed between baseline and follow-up (all <i>p</i> &lt; 0.001). No changes were found in the 84 controls (all <i>p</i> &gt; 0.05). In 45 PCa patients with paired CT, significant decreases in SM, SM index (SMI) and increases in SM-IMAT, IMAT index (SM-IMATI) were observed between baseline and follow-up (all <i>p</i> &lt; 0.001). Loss of SM (CT) correlated with loss of OI (MRI) (r = 0.530, <i>p</i> &lt; 0.001), and an increase in SM-IMAT (CT) correlated with OI-IMAT (MRI) (r = 0.515, <i>p</i> &lt; 0.001). Furthermore, elevated OI-IMAT levels were found to adversely affect PFS (<i>p</i> = 0.039).</p> Conclusion <p>A greater increase in OI-IMAT was linked to shorter PFS in ADT-treated PCa patients. Routine MRI-based OI assessment may serve as an indirect marker of global skeletal muscle deterioration and help flag patients at heightened risk of treatment-related sarcopenia.</p> Key Points <p><Emphasis Type="BoldItalic">Question</Emphasis> <i>Routine prostate MRI can quantify obturator internus atrophy and fatty infiltration in PCa undergoing androgen deprivation therapy (ADT) to identify individuals at higher risk of muscle deterioration.</i></p> <p><Emphasis Type="BoldItalic">Findings</Emphasis> <i>MRI-detected obturator internus (OI) changes mirror CT muscle loss/fat gain, indicating sarcopenia. An inter-muscular adipose tissue of obturator internus muscle (OI-IMAT) increase &gt; 10% independently predicts shorter progression-free survival.</i></p> <p><Emphasis Type="BoldItalic">Clinical relevance</Emphasis> <i>Prostate MRI may provide an opportunity for opportunistic assessment of OI muscle atrophy and fatty infiltration. Monitoring changes in the OI may offer a cost-effective way to identify individuals at higher risk of muscle deterioration in PCa patients undergoing ADT.</i></p> Graphical Abstract <p></p>