[Successful selpercatinib treatment of metastatic medullary thyroid cancer carrying rare RET variant].

aimMedullary thyroid carcinoma (MTC) is a rare neuroendocrine malignancy in which serum calcitonin plays a pivotal role in early diagnosis, surgical planning, guidance of systemic therapy, and monitoring of treatment response. This case highlights the importance of laboratory diagnostics during selpercatinib therapy in a patient with disseminated MTC harboring a rare RET pathogenic variant.

methodsA cervical vertebral fracture in a young, asymptomatic male revealed MTC metastasis with distant dissemination and markedly elevated serum CEA and calcitonin levels. Tumor tissue and circulating DNA analysis identified the rare p.Glu632_Leu633del RET mutation. No germline alteration was detected. First-line systemic therapy comprised the selective RET tyrosine kinase inhibitor selpercatinib combined with denosumab.

resultsRapid clinical improvement occurred without significant adverse effects. Serum calcitonin levels normalized within four weeks, and imaging revealed marked tumor regression.

conclusionThis case underscores the value of liquid biopsy detected mutations in guiding targeted therapy. Clinical, biochemical, and radiological findings support the potential use of selective RET tyrosine kinase inhibitors for this rare mutation. Serum calcitonin serves as a reliable quantitative biomarker of therapeutic efficacy.