Objectives: To evaluate the management and outcome of idiopathic retroperitoneal fibrosis (iRPF) in Japan, and identify its clinical biomarker. Methods: We retrospectively analyzed 129 patients with iRPF treated between January 2008 and May 2018 at 12 university and related hospitals. Patients treated with glucocorticoid were analyzed to identify a predictive biomarker. These patients were classified into three groups according to overall effectiveness (no change: NC, complete response: CR and partial response groups: PR), and each parameter was compared statistically.Results: Male-female ratio was 5: 1, and median age at diagnosis was 69 (33-86) years. Smoking history was reported in 59.6% of the patients. As treatment, 95 patients received glucocorticoid therapy with an overall response rate of 84%. As a result, serum concentration of IgG4 was significantly decreased in NC group compared with the other two groups (56.6mg/dL vs 255mg/dL, 206mg/dL, P=0.0059 and 0.0078). ROC analysis was performed between the non-responder (NC) and responder groups (CR+PR) to identify the cut-off value of serum IgG4 as a predictive marker. As a result, AUC was 0.793 and the values of sensitivity and specificity were 0.85 and 0.64, respectively, under the cut-off values of 67.6mg/dL. Conclusions: In the majority of iRPF patients, glucocorticoid therapy resulted in a favorable response. Pre-treatment serum IgG4 concentration may have potential as a predictive biomarker of steroid treatment.
