Calcitonin doubling time in medullary thyroid carcinoma after the detection of distant metastases keenly predicts patients' carcinoma death

Therapy using tyrosine-kinase inhibitors (TKIs) is now available for recurring or advanced medullary thyroid carcinoma (MTC). Here we investigated the calcitonin doubling time (Ct-DT) of MTC patients with distant recurrence postoperatively and for those with distant metastasis at the initial surgery. Of the 13 patients, six died due to the MTC at 5-93 months after the detection of distant metastasis. Their Ct-DTs were ≤ 1.58 years. The remaining seven patients have been alive for 73-123 months after the detection of metastasis, and their Ct-DTs were low at -4, -2.25 years and 9.17-33.92 years. Similar results were obtained by analyzing the value of 1/Ct-DT to avoid discontinuity in the DT values among the patients with increasing serum Ct values over time and those with decreasing Ct values over time. These findings suggest that it is appropriate to use TKIs only for patients with a short Ct-DT and a large 1/Ct-DT with a cutoff at around 1.5 years and 0.67/year, respectively, even if they have distant metastases.