Post Therapy Imaging andFollowUp

The sensitivity of radioiodine imaging increases asymptotically toward a maximum with increasing administered dose up to the therapeutic range. Thus, the best possible opportunity for accurate staging is by whole body scanning 5-7 days following administration of a therapy dose. Several scenarios will emerge:

• If the scan reveals only a small remnant within the thyroid bed and no metastases and the Tg is normal, then the prognosis is excellent. These patients may be reassured and scheduled for long term follow up.

• If the scan reveals only thyroid bed remnants and the Tg is slightly elevated, the latter may have originated from the stimulated normal tissue. These patients may be guardedly reassured but should be rescanned in 6-12 months to ensure that ablation was complete and that the Tg has normalized.

• If the scan reveals metastatic disease, the patient will need a further scan and possible retreatment in 6-12 months.

• If the scan reveals no metastases and the Tg is high, then iodine-negative metastases must be suspected. These patients need supplementary imaging, including ultrasound of the neck and CT of the neck and chest. Positron

Figure 8. Recommended follow-up of patients after total thyroid ablation, on the basis of serum thyrotropin (TSH) and thyroglobulin (Tg) measurements and iodine-131 total-body scanning (TBS). The decision whether to perform iodine-131 scanning depends on the assay used to measure serum thyroglobulin; with a given assay, it depends on the tumor stage and the clinical likelihood of recurrent or persistent disease.

Figure 8. Recommended follow-up of patients after total thyroid ablation, on the basis of serum thyrotropin (TSH) and thyroglobulin (Tg) measurements and iodine-131 total-body scanning (TBS). The decision whether to perform iodine-131 scanning depends on the assay used to measure serum thyroglobulin; with a given assay, it depends on the tumor stage and the clinical likelihood of recurrent or persistent disease.

emission tomography (PET) with 18F-FDG is becoming a useful examination for iodine-negative metastases (Fig. 6). 111In-pentetreotide can also be useful to image disease that is not iodine-avid (Fig. 7).

Imaging with 99mTc-sestamibi is a useful screening tool that can be used while the patient is on thyroid hormone replacement. However, RAI imaging remains the reference standard and is necessary to predict response to RAI treatment.

All patients should be placed on TSH-suppressive doses of thyroxine within 2448 hours of receiving RAI and returned to normal diet. Systematic follow-up is necessary to detect evidence of recurrence in the higher risk patients and to ensure that TSH suppression is maintained (Fig. 8). A T4 level above normal is acceptable so long as the free T3 level remains within normal limits.

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