Introduction: Due to a limited number of hospital beds dedicated to radioiodine therapy (RIT) in some countries, a fractionated dose of radioiodine may be considered as the ablation therapy of differentiated thyroid cancer (DTC). The aim of the study was to compare the late effects of ablation therapy with single and fractionated dose of radioiodine in patients with DTC. Patients and methods: Patients with low-risk DTC referred to our institution 5–16 weeks after thyroidectomy, treated with 2.2 GBq of 131I, either in a single dose (2.2 GBq, group 1) or in two fractions (1.1 GBq+1.1 GBq administered with a 24 h interval, group 2) were retrospectively included. Clinical outcome of the treatment and overall survival (OS) was evaluated. Results: 83 patients treated with single dose and 186 patients treated with fractionated dose of radioiodine were included. Mean duration of follow-up was 8.0 vs.7.8 years, respectively (p=ns). There were no significant differences between the groups in male to female ratio, age at the time of the first RIT, proportion of papillary thyroid cancers, volume of the thyroid tissue, thyroid-stimulating hormone and thyroglobulin levels before first RIT. RIT was repeated in 55.4% and 54.8% of patients from group 1 and 2 respectively (p=ns). There were no significant differences including the course and outcomes of the treatment between the groups, measured by: cumulative dose of 131I, mean number of 131I administrations and mean thyreoglobulin concentration at the follow-up. Also the overall survival did not differ significantly between the groups. Probability of 5-year OS was 98.6% for patients treated with single and 99.5% with fractionated dose of 131-I, 10 year OS – 98.6 and 97.1% respectively, 15 year OS – 95.5 and 92.9% respectively (p=ns). Conclusions: In the long-term follow-up, radioiodine ablation therapy with fractionated doses in low-risk DTC patients is equally effective as with single dose.

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