Radioiodine Remnant Ablation: A Critical Review

Abstract

Radioiodine remnant ablation is considered a safe and effective method for eliminating residual thyroid tissue, as well as microscopic disease, if at all present in thyroid bed following thyroidectomy. The rationale of radioiodine remnant ablation (RRA) is that in the absence of thyroid tissue, serum thyroglobulin measurement can be used as an excellent tumor marker. Serum Tg measurement is the most sensitive under TSH stimulation for the detection of persistent or recurrent disease. Other considerations are like the presence of significant remnant thyroid tissue makes detection and treatment of nodal or distant metastases difficult. Rarely, microscopic disease in thyroid bed if not ablated, in future, could be a source of anaplastic transformation. On the other hand, microscopic tumor emboli in distant sites could be the cause of distant metastasis too. The ablation of remnant tissue would in all probability eliminate these theoretical risks. It may be noted that all these are unproven contentious issues except post-ablation serum Tg estimation that could be a good tumor marker for detecting early biochemical recurrence in long-term follow-up strategy. Radioactive iodine is administered as a form of ‘adjuvant therapy’ for remnant ablation. There have been several reports with regard to the administered dose for remnant ablation. The first report of a prospective randomized clinical trial was published from India by a prospective randomized study conducted at the All India Institute of Medical Sciences, New Delhi in the year 1996. The study reported that increasing the empirical 131I initial dose to more than 50 mCi results in plateauing of the dose-response curve and thus, conventional high dose remnant ablation needs critical evaluation. Recently two more important studies were published at the time of writing this review: one was by a French group and the other was a HiLo trial from UK on a similar line. Interestingly all three studies conducted in three different geographical regions of the world showed exactly the same conclusion. The new era of low-dose remnant ablation has taken a firm scientific footing across the continents. The recent meta-analysis based on all randomized control trials on RRA (a class 1 category of evidence) published by Cheng et al has summarized the findings of several studies conducted during the past 15 years and concluded that 30 mCi of 131I is sufficient enough for ablation. This chapter critically analyzes the evolution of radioiodine remnant ablation over the years from very high dose empirical to a more logical and evidence-based low dose therapy.