Tumor Grade Predicts for Calcitonin Doubling Times and Disease-Specific Outcomes After Resection of Medullary Thyroid Carcinoma

Background: Tumor grade is a new validated prognostic factor for medullary thyroid cancer (MTC). Calcitonin doubling time can predict MTC recurrence. We aimed to describe the association of tumor grade with calcitonin doubling and its effect on disease-specific outcomes times after resection. Methods: A retrospective analysis of MTC patients who underwent resection at a single tertiary-care cancer center between 1986 and 2017 were evaluated. Tumors were designated as high-grade MTC if two head and neck pathologists identified mitotic index ≥5 per 2 mm2, tumor necrosis, or a Ki67 proliferative index ≥5% within the tumor. Calcitonin doubling time was calculated using a validated calculator with at least three consecutive levels. Using Cox proportional hazards models, outcomes evaluated included locoregional relapse-free survival (LRFS), distant metastasis-free survival (DMFS), and overall survival (OS). Results: Among 117 patients, 95 were low grade and 22 high grade. Median follow-up was 70.2 months. High-grade patients demonstrated significantly faster calcitonin doubling times when compared with low-grade patients (8.51 ± 3.22 months vs. 38.42 ± 11.19 months; p < 0.001). In addition, most high-grade patients (66.7%) had calcitonin doubling times less than 1 year compared with fewer low-grade patients (1.0%; p < 0.001). High- and low-grade patients were further stratified by those who had calcitonin doubling times less than or greater than 2 years-a previously validated prognostic cutoff point. For patients with calcitonin doubling times less than 2 years, 70% were high grade, while 30% were low grade (p < 0.001). On multivariate analysis comparing grade and calcitonin doubling times, high-grade patients had significantly worse LRFS (hazards ratio [HR] 4.77 [confidence interval; CI 1.19-8.81]), DMFS (HR 7.25 [CI 2.36-22.28]), and OS (HR 6.04 [CI 1.85-19.72]; p < 0.05 for all), while calcitonin doubling times less than 2 years had worse DMFS (HR 7.22 [CI 1.05-49.75]). High-grade patients with calcitonin doubling times less than 2 years had associated worse LRFS and OS (both p < 0.05) compared with low-grade patients. Conclusions: The majority of high-grade MTC patients have calcitonin doubling times less than 2years. Close monitoring should be advocated for patients assessed to have high-grade tumors as they are at risk for poor disease-specific outcomes and structural recurrence.