Background- Liquid biopsies are a minimally invasive approach to obtain biomarkers such as circulating tumor DNA (ctDNA) in peripheral blood. ctDNA is validated as a powerful tool in lung and breast cancer1-3. To date, there is limited and conflicting studies evaluating its role in thyroid cancer4. Our study aimed to evaluate the utility of ctDNA in advanced thyroid cancers.
Methods: Eight patients >18 yo with metastatic medullary thyroid cancer (MTC) had 10 mL of blood collected in Streck cell-free DNA blood tubes. Plasma was isolated by two rounds of centrifugation at 2,000 g for 10 mins. The ctDNA was extracted using Qiagen’s QIAamp Circulating Nucleic Acid Kit. Purified ctDNA was quantified using a Qubit fluorometer, and 3.2-114ng ng analysed using Oncomine Pan-Cancer Cell-Free Assay run on an Ion Torrent Genexus system. Genetic testing of germline and tumoral DNA had been performed as per standard care; patients with germline mutations were excluded. Clinical data was obtained from medical records.
Results: Tumor DNA sequencing found 7 patients had a RET p.M918T and 1 a RET p.E632_L633del mutation. Detectable circulating levels of these mutations were found in 4/8 patients. Of the 4 patients negative by ctDNA sequencing, all samples were collected post-commencement of Selpercatinib treatment, except for one patient with a low (2 mL) plasma volume. In all patients positive by ctDNA, samples were collected prior to Selpercatinib treatment; in these patients sequential samples, 3 showed greatly reduced or undetectable levels following Selpercatinib. In contrast, the patient with a RET p.E632_L633del mutation showed increasing plasma concentrations of the mutation in the setting of progressive disease. Positive correlation was seen between mutant copies/ml and calcitonin (r=0.25) and CEA (r=0.64).
Conclusion: ctDNA is a novel biomarker, with potential to monitor disease progression in patients with driver mutations and advanced thyroid cancers.