Incidence of metastatic disease in low risk differentiated thyroid carcinoma patients

131I-NaI use for remnant ablation in patients who have undergone thyroidectomy for differentiated thyroid carcinoma (DTC) is long established with 85% of DTC patients cured by ¹³¹I-NaI therapy surgery in combination with surgery and TSH suppression. The presence of distant metastatic disease at initial presentation is low with disease being confined to the thyroid and local lymph glands. Low risk DTC Patient preparation for initial ¹³¹I-NaI treatment can use administration of recombinant human thyrotropin- (rTSH) instead of thyroxine cessation to elevate TSH levels. The use of rTSH (Thyrogen®) stimulation followed by 131INaI ablation dose for low risk DTC patients with low clinical suspicion of metastatic disease is standard practice at our institution. To determine the incidence of previously undiagnosed metastatic disease in low risk DTC patients undergoing initial Thyrogen® stimulation before ¹³¹I-NaI ablation. A retrospective review was performed on consecutive low risk DTC patients presenting for initial post thyroidectomy Thyrogen® stimulation and ¹³¹I-NaI ablation from 2014-2018. Patient demographics, surgery type, histopathology, post ablation imaging results - including thyroid bed uptake and presence of 131I avid metastatic diseases were collected. Statistical analysis was performed. A total of 108 patients with DTC reviewed with a follow up range of 12-61 months. The ¹³¹I post ablation imaging results were analysed and the incidence of ¹³¹I avid metastatic disease calculated, characterised and possible causal factors investigated. The incidence of undiagnosed ¹³¹I avid metastatic disease in the Thyrogen® low risk patient population was 5.5% (n=6) and 0.9% for distant metastases. Of this metastatic disease group 83% (n=5) of patients had regional lateral lymph node involvement and 1 patient with distant ¹³¹I avid bone disease. On followup (median = 32.5 month) 50% of these patients had undergone further 131I-NaI Therapy. Comparison of the metastatic spread patient group to the remaining 94.5 % of patients without disease spread identified no specific causal factors for the presence of ¹³¹I avid metastatic disease. ¹³¹I-NaI avid metastatic disease in low risk patients is rare and does not contraindicate the use of Thyrogen® stimulation in low risk DTC. (author)