Primary tumour iodine avidity in relation to uptake in persistent metastatic disease in papillary and poorly differentiated thyroid cancer

Purpose: Patients with persistent or recurrent papillary and poorly differentiated thyroid cancer can be effectively treated with radioiodine, if the tumour tissue is iodine-avid. However, iodine-avidity status is often unknown at the time of initial radioiodine treatment, limiting any adaptive approach. This study aimed to clarify the relationship between pre-therapeutic iodine avidity in primary tumour tissue, initial lymph node metastases and iodine uptake in subsequent metastases. Methods: Iodine avidity was prospectively assessed pre-therapeutically in 35 patients by injection of tracer amounts of iodine-131 two days prior to surgery. Iodine concentrations in resected tissue samples were measured, enabling accurate and histologically verifiable iodine avidity data for both primary tumour and initial lymph node metastases. Iodine uptake in persistent metastatic disease was assessed by review of radiology, and treatment response was examined through journal studies. Results: Out of data from 35 patients, 10 had persistent disease at presentation or during follow-up (range 19–46 months). Four patients had non-avid persistent metastatic disease, all with low iodine avidity in their primary tumours and initial lymph node metastases. Patients with low pre-therapeutic iodine avidity did not appear to have greater risk of persistent disease. Conclusion: The results indicate a close link between pre-therapeutically measured iodine concentrations in primary tumours with iodine avidity of any subsequent metastases.