Endocrine Abstracts

Germinal mutations of the oncogene RET originate the development of medullary thyroid carcinoma (MTC) in carriers with phenotypic expression depending on the mutated codon and other unknown factors. Simultaneous detection of differentiated thyroid carcinomas is considered a casual phenomenon (collision tumours). We communicate two brothers with double heterozygous mutation of maternal inheritance at codon 611 of RET, TGC to TTT, changing to encode phenylalanine (C611F), with a simultaneous papillary thyroid carcinoma (PTC).

Case 1: A 42-year-old male who underwent through total thyroidectomy and bilateral central dissection (TT+BCD) after genetic diagnosis in 2006. Basal preoperative calcitonin was 6.0 pg/ml and stimulated, 34 pg/ml. Histology showed a bifocal MTC, the bigger of 3 mm, and a classic infiltrative PTC of 3 mm with intense desmoplastic stromal reaction and four metastatic lymph nodes out of five in central compartment, two with extranodal extension. BRAFV600E mutation was positive. He received 100 mCi of I¹³¹ with stimulated thyroglobulin of 1.9 ng/ml. Ten years later, he has undetectable calcitonin and excellent therapeutic response.

Case 2: A 48-year-old male received TT+BCD after genetic diagnosis in 2016, with basal calcitonin of 25.2 pg/ml and stimulated of 124.5 pg/ml. Histology showed a bifocal MTC, 6 and 5 mm, and two foci (5+2 mm) of well limited not encapsulated follicular variant of PTC, BRAFV600E negative. There was no involvement by any of the two tumours in six resected nodes. After 3 months, basal calcitonin is 0.8 pg/ml, and basal thyroglobulin, without radioiodo, reaches 0.31 ng/dl. We are remarking simultaneous occurrence of MTC and PTC, originated from different thyroid cell types, in siblings bearing the same RET mutation. This coincidence raises the possibility of an underlying common genetic drive in both PTC, although with different mutational events added that explain distinct differentiation, among them BRAF V600E mutation.