LOW PREVALENCE OF THE SOMATIC M918T RET MUTATION IN MICRO-MEDULLARY THYROID CANCER.

Background: The prevalence of RET somatic mutations in sporadic medullary thyroid cancer (MTCs) is approximately 40-50%, and the most frequent somatic mutation is M918T. RET-positive MTCs have been demonstrated to have a more advanced stage at diagnosis and a worse outcome. Aims: The aim of the present work was to compare the prevalence of RET somatic mutations in sporadic microMTCs ( 1 and 2 and 3 cm. Results: The overall prevalence of the somatic M918T RET mutation was 19.4% (31/160). RET mutations were distributed differently among the four groups. The prevalence was 11.3% (6/53) in group A, 11.8% (8/60) in group B, 31.8% (7/22) in group C and 58.8% (10/17) in group D, exhibiting an increase with increasing size of the tumor. When comparing the prevalence of mutations in the four groups, we found a lower prevalence in microMTCs (p< 0.0001). Conclusions: The overall prevalence of RET somatic mutations was lower than expected, and the prevalence of the somatic M918T RET mutation was significantly lower in microMTCs than in larger tumors. To explain this finding we can hypothesize either that other oncogene(s) might be responsible for the majority of microMTC, thus identifying a tumor subset, or that RET mutation might, or not, occur later during the tumor progression