Medullary thyroid carcinoma (MTC) accounts for 1.4% of all thyroid malignancies in Japan. Here, we studied the validity of a staging system evaluated preoperatively (Stage), intraoperatively (intra-Stage), and pathologically (pStage) based on the 6
th and 7
th UICC TNM classifications. One hundred and nineteen MTC patients who did not show distant metastasis at diagnosis and underwent locally curative surgery were enrolled in this study (average follow-up period: 173.4 months). Twenty-year clinical (not biochemical) disease-free survival (DFS) rates of Stage I, II, III, and IVA patients based on the 6
th edition were 100, 88.2, 66.8, and 38.9%, respectively. DFS of Stage IVA patients was significantly poorer than that of Stage III patients (
p = 0.03137). However, using the 7
th edition, only 1 patient was classified with Stage III. Intra-Stage III patients based on the 6th edition showed a significantly poorer DFS (20-year DFS 50.0%) than intra-Stage II patients (92.9%) (
p = 0.02668), and DFS of intra-Stage IVA patients (38.9%) tended to be poorer than that of intra-Stage III patients (
p = 0.05439). Only one patient was classified with intra-Stage III using the 7
th edition. In pStage, as many as 56 patients (47.1%) were classified with pStage IVA employing both editions. Taken together, Stage and intra-Stage were more useful to accurately discriminate high-risk patients than pStage, and their 6
th editions were better than 7
th editions. Although the number of patients was small, our data showed the possibility that intra-Stage in the 6
th edition was the best staging system for MTC patients.
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