Prognostic Impact of Ectopic Fat Deposition Within the Psoas Muscle in Patients with Stage IV Pancreatic Cancer Receiving Systemic Chemotherapy

Background: Computed tomography-based body composition parameters are emerging prognostic markers in pancreatic cancer. While sarcopenia and myosteatosis have been widely studied, the prognostic significance of ectopic fat deposition within the psoas muscle remains unclear. We aimed to evaluate the prognostic impact of the fat ratio within the psoas muscle (FRPM) in patients with stage IV pancreatic cancer receiving first-line systemic chemotherapy. Methods: This retrospective cohort study included 99 patients with stage IV pancreatic cancer treated with first-line chemotherapy. Baseline CT images at the L3 level were analyzed, and FRPM was calculated as the proportion of intramuscular fat to total psoas muscle area. FRPM was analyzed primarily as a continuous variable. Exploratory low- and high-FRPM groups were defined using sex-specific medians for descriptive comparisons and Kaplan–Meier analyses. Overall survival (OS) and progression-free survival (PFS) were assessed using Kaplan–Meier and Cox regression analyses. Multivariable models were adjusted for age, sex, ECOG performance status, liver metastasis, and C-reactive protein (CRP). Results: Among 99 patients, 48 were categorized as having low FRPM and 51 as having high FRPM based on exploratory sex-specific median-based groups. Higher FRPM correlated with older age and higher BMI and inversely correlated with psoas muscle size and PMI. Median OS was 9.76 months in the low-FRPM group versus 5.78 months in the high-FRPM group, and median PFS was 5.29 versus 3.68 months. In the main multivariable Cox model, higher FRPM was associated with worse OS when analyzed as a continuous variable and reported per 1-standard deviation increase (adjusted HR: 1.43, 95% CI: 1.08–1.91, p = 0.014). After additional adjustment for first-line treatment-regimen category, the association remained directionally consistent but did not retain conventional statistical significance (adjusted HR: 1.32, 95% CI: 0.99–1.77, p = 0.060). Conclusions: Higher FRPM was associated with shorter OS and PFS in patients with stage IV pancreatic cancer receiving first-line systemic chemotherapy. These findings suggest that ectopic fat deposition within the psoas muscle may represent a potential CT-based muscle-quality marker associated with adverse prognosis. External validation and comparison with conventional adiposity parameters are required before clinical application.