Abstract: Objective: To investigate the value of MRI-based measurement of calf soft-tissue thickness in assessing the clinical staging of primary lower extremity lymphedema (PLEL). Methods: The clinical and MR imaging data of 90 patients diagnosed with PLEL in our hospital were retrospectively collected, and all patients underwent bilateral lower limb MR examinations. Short Time Inversion Recovery (STIR) sequence was used to measure the total soft tissue thickness (T), musculoskeletal thickness (M) and subcutaneous soft tissue thickness (S) of bilateral lower legs, and the difference between T and S of bilateral lower legs (DT, DS) was calculated respectively. Patients were classified into stages Ⅰ, Ⅱ and Ⅲ with reference to the clinical staging criteria of the International Lymphatic Association 2020 and our lymphatic surgery department for lower limb lymphedema, excluding stage 0. One-way ANOVA was used to compare calf soft tissue thickness among different clinical stages, Spearman correlation was used to analyze the correlation between calf soft tissue thickness and clinical stage, and ROC curves were used to evaluate the efficacy of calf soft tissue thickness in discriminating clinical stage. Results: The differences among T, S, DT and DS of the three stages were statistically significant, while there was no statistical difference among M; when comparing two by two in each subperiod, T, S, DT and DS were statistically different between stage Ⅰ and Ⅱ and stage Ⅰ and Ⅲ, while there was no statistical difference between stage II and III. The correlation between DT (r=0.750) and DS (r=0.772) and clinical stage was significantly greater than that between T (r=0.669) and S (r=0.734), with DS showing the highest correlation with clinical stage; there was no significant correlation between M and clinical stage. ROC curves showed that the AUC values for each parameter to identify stage Ⅰ and Ⅱ were greater than those to identify stage Ⅱ and Ⅲ. The AUC value of DS (AUC=0.945) demonstrated the highest area under the curve (AUC) among all parameters to identify stage Ⅰ and stage Ⅱ. Conclusion: MRI soft-tissue thickness measurement of calf can be used as a quantitative adjunct in the clinical staging of unilateral PLEL, and for patients with unilateral PLEL, we recommend DS as the best thickness index to differentiate stage Ⅰ from Ⅱ lymphedema.