Quantitative evidence of thyroid stunning in 131I cancer treatment.
BACKGROUND
An obvious paradox remains in the diagnosis, treatment and follow-up of differentiated thyroid cancer by (131)I. The higher the activity for raising diagnostic precision, the greater is the potential reduction in therapeutic effects. This is due to a phenomenon called thyroid stunning.


METHODS
(131)I tracer and therapeutic parameters: uptake, effective half-life and thyroid remnants mass were compared in two different groups of patients. Pre-treatment planning of radioiodine (RI) ablation was performed in 30 patients after administration of 0,185-74 microBq. The same parameters comparison was performed in the second group of four cases. They received two or three single fractions of 600-2400 MBq 3-15 days apart (a long-term abandoned regime).


RESULTS
1. Comparative data collected by pre-treatment planning and subsequent RI administration supported the thesis that low range tracer activity (0,7-74 microBq) does not cause thyroid stunning. We have registered higher or similar uptake in thyroid bed after 1100-3700 microbq in 90% of cases. Only in 10% of cases was therapeutic uptake lower than the tracer one. In 18% of patients the higher rate of uptake was associated with additional thyroid tissue visualized on the post-treatment scan. Half-life reduction only could be interpreted in the direction of stunning, but such changes characterizes every RI treatment, if it takes more than one administration. Data elucidate why our pre-treatment planning failed in one-third of the patients. 2. We have clearly observed thyroid stunning after 600-2400 MBq (131)I. In one case only, even after 1200 MBq, stunning did not take place. Individual RI kinetics appear highly unpredictable. The author advocates avoiding high activity first pre-treatment scan in advanced cases with elevated thyroglobuline. It remains an unanswered question what time is necessary for stunning to recover.
Centre of Nuclear Medicine and Radiation Oncology, Faculty of Medicine, Sofia, Bulgaria. rad@cserv.mgu.bg
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Tatiana