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.
Tatiana
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HD
Centre of Nuclear Medicine and Radiation Oncology, Faculty of Medicine, Sofia, Bulgaria. rad@cserv.mgu.bg