Predictors of radioiodine (RAI)-avidity restoration for NTRK fusion-positive RAI resistant metastatic thyroid cancers.

Introduction Two-thirds of metastatic differentiated thyroid cancer (DTC) patients have radioiodine (RAI)-resistant disease, resulting in poor prognosis and high mortality. For rare NTRK and RET fusion-positive metastatic, RAI-resistant thyroid cancers, variable success of re-induction of RAI-avidity during treatment with NTRK or RET inhibitors has been reported. Case Presentation and Discussion: We report two cases with RAI-resistant lung metastases treated with larotrectinib: 83-year-old male presenting with an ETV6::NTRK3 fusion-positive tumor with the TERT promoter mutation c.-124C>T, and a 31-year-old female presenting with a TPR::NTRK1 fusion-positive tumor (and negative for TERT promoter mutation). Post-larotrectinib treatment, diagnostic I-123 whole body scan revealed unsuccessful RAI-uptake re-induction in the TERT-positive tumor, with a Thyroid Differentiation Score (TDS) of -0.287. In contrast, the TERT-negative tumor exhibited successful I-131 reuptake with a TDS of -0.060. Conclusion: As observed for RAI-resistance associated with concurrent TERT and BRAF mutations, the co-occurrence of TERT mutations and NTRK fusions may also contribute to re-sensitization failure.