Fig. 25.9 Functionally significant thyroid carcinoma imaged three days after 131I therapy (I50 mCi, 5.55 GBq). In this case, imaging shows irregular increased uptake in the superior mediastinum, in the paratracheal and supraclavicular nodes, and diffuse high lung uptake. Such uptake may occur rarely when there is a normal chest X-ray, but is usually associated with miliary lung metastases and shows a good response to 131I therapy. High lung uptake may be followed by radiation fibrosis if the dose administered is too great. Indications for 131I therapy are biopsy proven papillary or follicular carcinoma and known or suspected incomplete surgical excision. Differentiated tumours concentrate 131I better than undifferentiated and H┼╕rthle cell cancers. Thyroid carcinoma tissue, which at first appears to take up 131I poorly, may show significant uptake of tracer when all higher avidity normal gland has been ablated.
