['The prognostic value of serum calcitonin (CT) and carcinoembryonic antigen (CEA) doubling time has been recently demonstrated in medullary thyroid carcinoma (MTC) patients. No study has yet validated the surrogate role of these markers for survival during treatment. The aim of this study was to evaluate, in patients with advanced MTC treated with cytotoxic chemotherapy, the relationship between early changes of serum CT or CEA levels and progression-free survival (PFS).', 'The files of 28 consecutive metastatic MTC patients with progressive disease, treated with cytotoxic chemotherapy in a single tertiary referral center between 2000 and 2010, were retrospectively reviewed. Serum CT and CEA measurements and radiological Response Evaluation Criteria in Solid Tumors (RECIST) evaluations were collected every 3 months. The relationship between changes in serum CT and CEA levels at 3 months, defined by an increase or a decrease of at least 20%, and PFS according to RECIST 1.0, was estimated using Kaplan-Meier curves and log-rank test.', 'The median follow-up for the 28 patients was 68 months. According to RECIST, a partial response, a stabilization or a progression was observed in 14, 43, and 43% of cases respectively. Median PFS from the initiation of cytotoxic chemotherapy was 4.5 months. Median PFS among patients with and without significant CT increase at 3 months was 4.6 and 3.3 months respectively (P=0.75). Median PFS among patients with a significant CEA increase at 3 months was 2.7 months, whereas it was 19.1 months in patients in whom CEA did not increase (P=0.02).'] Are early changes in carcinoembryonic antigen but not in calcitonin levels correlated with the progression-free survival in medullary thyroid carcinoma patients treated with cytotoxic chemotherapy?