Spontaneous Deceleration and Acceleration of Growth Rate in Medullary Thyroid Carcinomas Suggested by Changes in Calcitonin Doubling Times Over Long‐Term Surveillance

Background Based on our long‐term observation of medullary thyroid carcinoma (MTC) patients, we hypothesized that some MTCs have spontaneous deceleration or regression of tumor growth over a long term and that a minority may acquire growth acceleration. We thus compared the calcitonin doubling time (Ct‐DT) in the earlier and later half‐periods of MTC patients’ postoperative course. Methods We followed 26 MTC patients (14 hereditary and 12 sporadic MTCs) with postoperative hypercalcitoninemia with periodic measurements of serum calcitonin (Ct) for >10 years without major interventions. The median period of Ct measurements was 18.3 years (range 10.6–30.2 years). We divided the individual patients’ study periods into the earlier and later halves and calculated the Ct‐DTs for both periods. Results In the hereditary group, the Ct‐DT in the later half‐period (Later‐Ct‐DT) was significantly longer than that in the earlier half‐period (Earlier‐Ct‐DT) (median 20.0 years vs. 7.1 years, p  = 0.013). These values in the sporadic group were 20.0 years versus 11.1 years, respectively ( p  =0.774). Twelve patients (seven hereditary and five sporadic) had Later‐Ct‐DTs significantly longer than their Earlier‐Ct‐DTs (median 27.4 years vs. 4.9 years) and good prognoses. Two patients (one hereditary, one sporadic) had Later‐Ct‐DTs significantly shorter than their Earlier‐Ct‐DTs, and both developed structural recurrence and died of the disease. Conclusion Many of the hereditary and some of the sporadic MTC patients had elongated Ct‐DTs over a long period, suggesting spontaneous deceleration and regression of tumor growth. A minority of the MTC patients showed Ct‐DT shortening, suggesting tumor growth acceleration.