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IntroductionThis study aimed to evaluate the independent associations of myopenia, myosteatosis, and frailty with survival outcomes in older patients with prostate cancer undergoing definitive radiotherapy.MethodsA total of 124 patients aged ≥65 years who received definitive radiotherapy for localized or locally advanced prostate cancer between 2014 and 2023 were retrospectively analyzed. Myopenia and myosteatosis were quantified using the psoas muscle index (PMI) and Hounsfield unit average calculation (HUAC), respectively. Comorbidity-based frailty was assessed using the modified frailty index-11 (mFI-11). The cut-off points for the PMI and HUAC were determined using receiver operating characteristic (ROC) curve analysis. Univariate and parsimonious multivariable Cox regression analyses were used to examine the prognostic value of these markers for progression-free survival (PFS) and overall survival (OS).ResultsThe median age of the patients was 72 years (range: 65-87). According to the mFI-11, 47.6% of the patients were classified as frail. The cut-off values for PMI and HUAC were 2.0 cm²/m² and 37.7 HU, respectively. In univariate analysis, only PMI was significantly associated with both PFS (HR: 2.23, p = 0.02) and OS (HR: 2.45, p = 0.03). When entered separately into parsimonious multivariable Cox models adjusted for age and NCCN risk group, PMI remained an independent prognostic factor for both PFS (HR: 2.36, p = 0.01) and OS (HR: 2.40, p = 0.04), whereas mFI-11 and HUAC were not significantly associated with either endpoint.ConclusionAmong the evaluated body composition and frailty-related markers, only PMI-defined myopenia remained independently associated with both PFS and OS in older patients receiving definitive radiotherapy for localized or locally advanced prostate cancer. As PMI can be obtained opportunistically from routine planning CT, it may have value for prognostic stratification, although confirmation in external prospective datasets is required.