Quiz contributed by Hung Q. Dam, MD and Charles M. Intenzo, MD , from the Department of Radiology, Division of Nuclear Medicine, Thomas Jefferson University Hospital, Philadelphia, PA.

CASE SUMMARY

A 41-year-old woman with a history of multifocal papillary thyroid carcinoma had a total thyroidectomy performed 2 months prior to presentation. A diagnostic whole-body scan was obtained 48 hours after the administration of 185 MBq (5 mCi) of ¹³¹ I (Figure 1). One day later, the patient was treated with 3700 MBq (100 mCi) of ¹³¹ I. A post-therapy scan was performed 7 days later (Figure 2).

The diagnosis and discussion are presented beginning on page 38.

DIAGNOSIS

Thyroid stunning

IMAGING FINDINGS

Anterior and posterior whole-body diagnostic scan reveals increased activity in the thyroid bed (Figure 1). However, a whole-body scan obtained 1 week after ¹³¹ I therapy shows complete resolution of the previously identified increased activity in the thyroid bed (Figure 2).

DISCUSSION

Radioiodine plays an important role in the management of well-differentiated thyroid cancer. There are three principle nuclear medicine procedures performed on patients with thyroid cancer: 1) diagnostic scan, 2) ¹³¹ I therapy, and 3) post-therapy scan. Given the wide availability and relatively low cost, most institutions use ¹³¹ I for the diagnostic radiotracer, as opposed to ¹²³ I. Several days after the administration of the diagnostic dose, a diagnostic scan is obtained, which serves several purposes: 1) to show any postsurgical thyroid remnant, 2) to reveal any regional or distant metastases for staging, and 3) to demonstrate that the thyroid tissue takes up radioactive iodine and is therefore amenable to ¹³¹ I therapy.

Once the diagnostic scan reveals functioning thyroid remnant or metastases, the administered activity of ¹³¹ I therapy can then be prescribed based on histologic classification and stage of disease. There are several treatment strategies for thyroid cancer therapy, ranging from standardized fixed doses to individual dosimetry. ¹ A post-therapy scan is then obtained several days after ¹³¹ I therapy. With the much higher dose of ¹³¹ I therapy compared with the lower diagnostic ¹³¹ I dose, it is not uncommon to detect functioning thyroid tissue that was not seen on the diagnostic scan. Therefore, the post-therapy scan can upstage patients' disease.

Several authors have hypothesized that the diagnostic ¹³¹ I dose impairs the thyroid cells such that the subsequent ¹³¹ I therapy is less effective. ^2-6 This phenomenon has been termed thyroid "stunning." The stunning effect may be observed when activity on a diagnostic scan appears less intense on the post-therapy scan. The existence of thyroid stunning remains controversial, since other authors have not been able to produce it. ^7,8 These authors argue that stunning may be merely a product of differences in protocol between the diagnostic and post-therapy scans. However, at a cellular level, researchers have shown a dose-dependent decrease in iodide transport 3 days following a 48-hour ¹³¹ I irradiation of cultured porcine thyroid epithelial cells. ⁹

Due to the potential for stunning, many physicians have altered their protocols for treating thyroid carcinoma. Since stunning may be dose-dependent, many have lowered the diagnostic ¹³¹ I dose but at the expense of sensitivity. ^2,9-11 Due to the lack of reported stunning with ¹²³ I, some clinicians have exclusively used the more expensive ¹²³ I but again compromising sensitivity compared with ¹³¹ I. ^2,4,5 As another alternative, some physicians have abandoned the diagnostic scan altogether. ^12,13 The two main drawbacks of this approach are: 1) the thyroid carcinoma may not be ¹³¹ I-avid so ¹³¹ I therapy may be inadvertently given to patients who would not benefit from it; and 2) the diagnostic scan may reveal unsuspected metastases requiring a larger therapy dose. ¹³

Those authors who believe in thyroid stunning are still divided regarding the clinical effects of stunning. Some researchers have shown no significant difference in ¹³¹ I therapy success rates between stunned and nonstunned patients. ^6,14 In addition, when comparing patients with ¹³¹ I diagnostic scans with those without diagnostic scans, no significant difference in therapeutic efficacy was found. ¹² Contrary to these other reports, one group of investigators actually demonstrated higher ablation success rates in the stunned versus the nonstunned population. ¹⁵

CONCLUSION

There has been no consensus achieved in the literature concerning 1) the existence of thyroid stunning; or 2) its effect on clinical outcome. Reports include myriad different protocols varying from institution to institution that are based on uncontrolled retrospective studies, making it difficult to reproduce results. Further investigation with prospective studies and large sample sizes should help clarify these issues.