What is the role of dosimetry in patients with advanced thyroid cancer?

Pretreatment dosimetry prior to I-131 treatment for patients with advanced differentiated thyroid cancer will provide critical information for optimizing dosing regimen for effectiveness and safety. The review briefly summarizes relevant information and updates the current advances.

I-124 positron emission tomography-computed tomography (PET-CT) with anatomical cross-section imaging provides a better assessment of lesional dosimetry in contrast to traditional I-131 whole body scan with planar imaging. The introduction of Thyrogen (Genzyme, Cambridge, Massachusetts, USA) primed dosimetry also provides an easy way to stimulate the uptake of iodine without prolonged withdrawal of thyroid hormone, although the impact of reducing lesional dosimetry requires attention and further investigation. Lower radioactivities of I-131 could provide similar outcomes in treatment of intermediate risk patients with thyroid cancer compared with traditional high doses. Despite a wide range of lesional doses being noted using fixed 100 mCi radioactivities of I-131, no dose-effective relationship can be established because of an overwhelming success in ablation.

Pretreatment dosimetry remains important to optimize the I-131 treatment dosages for patients with advanced or metastatic thyroid cancer, elderly patients with renal insufficiency or patients with renal failure. Modern I-124 PET-CT and Thyrogen may provide further improvement of this assessment although these advances may require further validation.