CALCITONIN SECRETORY INDEX AND UNSUSPECTED NODAL DISEASE IN MEDULLARY THYROID CARCINOMA.

OBJECTIVE: Medullary thyroid carcinoma (MTC) is a rare thyroid malignancy originating from parafollicular C-cells with the potential for aggressive behavior. The extent of lymph node (LN) dissection at the time of surgery is controversial, with different schools of thought prevailing. Some systematically perform LN dissections, whereas others base their decision on radiologic evidence of disease and some with the assistance of pre-operative calcitonin (CT) levels.

METHODS: We retrospectively assessed the correlation between pre-operative CT levels and clinico-pathologic factors among 42 patients with MTC between 1994 and 2015. Furthermore, we refined the use of pre-operative serum CT levels and explored for the first time a test called the Calcitonin Secretory Index (CSI, ng/mL/mm).

RESULTS: Pre-operative CT levels correlated independently with tumor size ( P<.0001), number of metastatic LNs ( P<.01), and increased rates of distant metastasis. The CSI better predicted early LN disease ( P<.045). Patients with early LN metastasis had a CSI >30 ng/mL/mm, a representative threshold above which the surgical cure declines considerably.

CONCLUSION: In our experience, pre-operative CT levels and now the CSI appear as sensitive and specific risk stratification markers for MTC. Despite negative findings on dedicated pre-operative neck imaging in addition to total thyroidectomy, a CSI >30 ng/mL/mm would prompt bilateral central node dissection. Due to the small sample size, our study provides preliminary evidence of the value of CSI in clinical practice.

ABBREVIATIONS: ANOVA = analysis of variance; ATA = American Thyroid Association; CSI = Calcitonin Secretory Index; CT = calcitonin; LN = lymph node; MTC = medullary thyroid carcinoma; ROC = receiver operating characteristic.