Small medullary thyroid carcinoma: post-operative calcitonin rather than tumour size predicts disease persistence and progression.

Objective: Recently, small medullary thyroid carcinomas (smallMTCs; ≤1.5 cm) are frequently diagnosed, occasionally as incidental findings in surgical specimens. Their clinical course varies. We examined tumour size as a predictor of clinical behaviour.

Design: A retrospective study.

Methods: A total of 128 smallMTC patients (35.2% males and 45% familial) were followed up for 0.9-30.9 years. According to tumour size (cm), patients were classified into four groups: group 1, 0.1-0.5 (n=33); group 2, 0.6-0.8 (n=33); group 3, 0.8-1.0 (n=29) and group 4, 1.1-1.5 (n=33).

Results: Pre- and post-operative calcitonin levels were positively associated with the tumour size (P<0.001). Capsular and lymph node invasion were more frequent in groups 3 and 4 (P<0.03); the stage was more advanced and the outcome was less favourable with an increasing tumour size (P<0.001). Groups 1 and 2 patients were more frequently cured (group 1, 87.8%; group 2, 72.7%; group 3, 68.9%; and group 4, 48.5%; P=0.002). The 10-year probability of lack of disease progression according to the tumour size differed between patients with tumour sizes of 0.1-1.0 and 1.1-1.5 cm (96.6%, 81.3%, x(2)=4.03, P=0.045 for log-rank test). Post-operative calcitonin was the only predictor significantly associated with the 10-year progression of disease. Post-operative calcitonin levels ≥4.65 pg/ml predicted disease persistence (sensitivity 93.8% and specificity 90%) and ≥14.5 pg/ml predicted disease progression (sensitivity 100%, specificity 82%, receiver operating characteristic curve analysis).

Conclusions: Tumour size may be of clinical importance only in patients with MTCs >1 cm in size. Post-operative calcitonin is a more important predictor than size for disease progression.