Procalcitonin Doubling Time for Postoperative Monitoring of Medullary Thyroid Carcinoma: A Comparative Kinetic Analysis

BACKGROUND: Calcitonin doubling time (CT-DT) is an established prognostic tool in medullary thyroid carcinoma (MTC), yet CT measurement can be affected by analytical variability and assay interference. Procalcitonin (PCT) has shown promise as a complementary biomarker, but data on PCT doubling time (PCT-DT) remain scarce. METHODS: Longitudinal CT and PCT were measured using Elecsys immunoassays in 25 MTC patients followed for up to 24 months after thyroidectomy. Doubling times were estimated using log-linear regression and a rolling-window approach. Agreement was assessed using Spearman correlation, Bland-Altman analysis, and Cohen's κ across predefined DT categories. RESULTS: CT and PCT kinetics showed clear separation across response categories. In progressive disease, CT-DT and PCT-DT were strongly correlated with minimal systematic bias and substantial categorical agreement (κ = 0.76). Short DT values clustered in structural incomplete responders. CONCLUSIONS: PCT-DT closely mirrors CT-DT and may serve as a complementary kinetic marker in the postoperative monitoring of MTC, particularly when CT interpretation is uncertain; however, these findings should be considered exploratory and warrant confirmation in larger prospective cohorts before clinical implementation.