RAI-R DTC patients starting standard-of-care lenvatinib treatment will be included in this study. Prior to lenvatinib treatment, patients will undergo I-124 PET/CT to quantify RAI uptake and retention at baseline. The first half of the intended sample size (cohort 1) will be treated with lenvatinib for a total of 12 weeks. After 6- and 12-week treatment, patients will undergo I-124 PET/CT dosimetry to evaluate the redifferentiation effect, assess expected absorbed lesion doses and maximum tolerable activity. Results between 6- and 12-week lenvatinib treatment will be compared to select the lenvatinib treatment duration that leads to highest extent of redifferentiation. The next patients (cohort 2) will then receive lenvatinib for either 6 or 12 weeks.~Patients will undergo subsequent I-131 therapy if a clinically meaningful lesion dose is expected and toxicity is deemed acceptable. For all patients eligible for I-131 therapy, lenvatinib is discontinued prior to administration of I-131 and intra-therapeutic I-131 SPECT dosimetry will be performed for dose verification. Patients who are not eligible for I-131 therapy, will continue lenvatinib treatment at the discretion of the treating physician.~Biopsies are performed at baseline and after 6-week lenvatinib treatment to evaluate alterations at the transcriptional and translational level in biopted tumor lesions. Patients will be followed up according to current guidelines for a total of 9 months after initiating lenvatinib treatment. Metabolic and biochemical response will be assessed using F-18 FDG PET/CT and Tg levels, respectively.
This is a single-centre open label phase II study evaluating the effect of lenvatinib treatment for restoring radioiodine uptake and retention in radioiodine-refractory (RAI-R) thyroid cancer to warrant I-131 therapy.