Prognostic Significance of Circulating RET M918T Mutated Tumor DNA in Patients With Advanced Medullary Thyroid Carcinoma

Abstract

Context: Interpretation of calcitonin measurement to predict the prognosis of medullary thyroid carcinoma (MTC) requires multiple measurements over an extended time period, making it an imperfect biomarker for evaluating prognosis or disease behavior. Single circulating cell-free DNA (cfDNA) values have been shown to be a valuable prognostic marker for several solid tumors.

Objective: We tested the hypothesis that cfDNA containing the RET M918T mutation could be detected in the blood of patients with advanced MTC whose tumor harbored an M918T mutation and would be able to predict overall survival more reliably than calcitonin.

Design: The level of cfDNA containing RET M918T mutation was measured in the plasma of patients with MTC via droplet digital polymerase chain reaction.

Patients: Patients had a confirmed sporadic MTC diagnosis, a serum calcitonin measurement >100 pg/mL, and tumor tissue biopsy results providing RET M918T mutation status. There were 75 patients included in this study, 50 of whom harbored an RET M918T mutation by tissue biopsy.

Results: RET M918T cfDNA was detected in 16 of 50 patients (32%) with a positive tissue biopsy. The detection of RET M918T cfDNA strongly correlated with worse overall survival and more accurately predicted a worse outcome than calcitonin doubling time.

Conclusions: Liquid biopsy is able to detect RET M918T mutations in patient plasma with high specificity but low sensitivity. In patients with established somatic RET M918T mutations, the allelic fraction of circulating tumor DNA is prognostic for overall survival and may play a role in monitoring response to treatment.

Figure 1.

Validation of RET M918T cfDNA detection by liquid biopsy. The study cohort included 75 patients with MTC with tumor tissue biopsy performed to determine the status of somatic RET M918T mutation. Somatic mutation was absent in tumor tissue of 25 patients (RET WT) and present in 50 patients (RET M918T). The detection of RET M918T in plasma cfDNA from these patients is displayed as either the proportion of mutated to total DNA (AF) or as number of mutant DNA copies per milliliter of plasma. WT, wild type.

Figure 2.

Overall patient survival was examined for the same 48 patients by calcitonin doubling time <24 months (top plot) or with an RET M918T cfDNA AF cutoff of 0.25% (bottom plot). For both plots follow-up was from time of first cfDNA sampling, with calcitonin doubling time based on retrospective data. Two RET M918T–positive patients were excluded because of the lack of three calcitonin measurements in the previous 2 years. Patients A and B are described further Fig. 3.

Figure 3.

Longitudinal examination of calcitonin and RET M918T cfDNA levels. The biomarker levels and interventions for (A) patient A and (B) patient B from Fig. 2 are shown. Time shown is relative to the first cfDNA sampling, which was 0 for patient A, who died 25.3 months later.