Thyroid stunning after diagnostic dose of 185 MBq (5mCi)‎ iodine - 131 in patients with differentiated thyroidal cancer

Abstract EN

Thyroid stunning has been reported as a temporary impairment of thyroid tissue after diagnostic radioiodine (131I) dose that decrease the final absorbed dose in ablative 131I therapy.

The aim of this study was to determine whether a 185 MBq (5 mCi) diagnostic dose of 131I produce a stunning effect on thyroid remnant before 131I ablative therapy in patients with differentiated thyroid cancer.

Patients and methods: Forty-eight patients with differentiated thyroid cancer following total or near-total thyroi-dectomy with no evidence of distant metastases were included in this study.

Our patients were classified into 3 groups according to the time of ablation therapy.

The 3 groups were patient group 1 (PG1), patient group 2 (PG2) and patient group 3 (PG3) when ablation therapy was administered at 3, 7 and 14 days after diagnostic dose of 5 mCi 131I, respectively.

An ablative dose of 30-100 mCi 131I was administered 3-14 days after diagnostic dose.

Uptake over thyroid remnant was assessed 72 hours post-diagnostic and post-ablation using single head gamma.

Results : In PG1, the mean post-diagnostic and post-ablative uptake values were 8.87 % and 7.93 % respectively with no significant difference (p > 0.05).

However, in PG2 the post-ablation uptake was equal to 4.82 % and was significantly lower (p < 0.001) than the previous diagnostic measurement (9.1 %).

Also, uptake of the ablative dose (4.21 %) was significantly lower (p < 0.001) than the diagnostic uptake (8.23 %) in PG3.

The therapy / diagnostic uptake ratios in PG2 and PG3 (53 % and 51 % respectively) were significantly lower (p < 0.001) than that in the first group (89 %).

There was a good correlation between the visually apparent and quantitatively measured uptake when comparing post-ablation and post-diagnostic 131I studies.

Conclusion : our data suggested that qualitative and quantitative assessment of gamma camera studies showed no evidence of stunning when the ablative dose was given within 72 hours from the diagnostic study of 185 MBq (5 mCi) 131I.

However, stunning was evident with a delay of more than 3 days between the ablation and diagnostic activities of radioiodine.