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 131I (Figure 1). One day later, the patient was treated with 3700 MBq (100 mCi) of 131I. A post-therapy scan was performed 7 days later (Figure 2).
Quiz contributed by
Hung Q. Dam, MD
Charles M. Intenzo, MD
, from the Department of Radiology, Division of Nuclear Medicine,
Thomas Jefferson University Hospital, Philadelphia, PA.
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
I (Figure 1). One day later, the patient was treated with 3700 MBq
(100 mCi) of
I. A post-therapy scan was performed 7 days later (Figure 2).
The diagnosis and discussion are presented beginning on page
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
I therapy shows complete resolution of the previously identified
increased activity in the thyroid bed (Figure 2).
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)
I therapy, and 3) post-therapy scan. Given the wide availability
and relatively low cost, most institutions use
I for the diagnostic radiotracer, as opposed to
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
Once the diagnostic scan reveals functioning thyroid remnant or
metastases, the administered activity of
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
A post-therapy scan is then obtained several days after
I therapy. With the much higher dose of
I therapy compared with the lower diagnostic
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
I dose impairs the thyroid cells such that the subsequent
I therapy is less effective.
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.
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
I irradiation of cultured porcine thyroid epithelial cells.
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
I dose but at the expense of sensitivity.
Due to the lack of reported stunning with
I, some clinicians have exclusively used the more expensive
I but again compromising sensitivity compared with
As another alternative, some physicians have abandoned the
diagnostic scan altogether.
The two main drawbacks of this approach are: 1) the thyroid
carcinoma may not be
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.
Those authors who believe in thyroid stunning are still divided
regarding the clinical effects of stunning. Some researchers have
shown no significant difference in
I therapy success rates between stunned and nonstunned patients.
In addition, when comparing patients with
I diagnostic scans with those without diagnostic scans, no
significant difference in therapeutic efficacy was found.
Contrary to these other reports, one group of investigators
actually demonstrated higher ablation success rates in the stunned
versus the nonstunned population.
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.