124I PET identifies more foci suggestive of residual thyroid tissue or metastases than 131I planar whole-body imaging in patients with well-differentiated thyroid cancer.

Van Nostrand et al. studied 25 patients with well-differentiated thyroid cancer who were suspected of having metastases and who were referred for 131I whole-body dosimetry. The prescribed activity of imaging studies were 37–74 MBq for 131I and 63 MBq for 124I. A blinded reader categorized every focus highlighted by 131I or 124I radioiodine uptake on the images; a focus was categorized as positive if considered to be residual thyroid tissue, most probably metastasis, or definitely metastasis.

In eight patients (32%), a greater number of positive foci were observed on the 124I than the 131I images, and three of these had metastases confirmed in at least one of the additional positive foci detected on the 124I images. In 16 patients, 124I images and 131I images contained the same number of foci, although only two of these patients had positive foci. In one patient, an additional positive focus was observed on the 131I image that was not seen on the 124I image, but this additional focus is not yet confirmed as a metastasis. In total, 97 positive foci were identified on either 124I or 131I images, but 49 additional positive foci were observed on the 124I images, whereas only one additional positive focus was observed on 131I images.

Van Nostrand et al. suggest that 124I PET produced superior results because the PET scanner provides images with reduced background noise and enhanced spatial and contrast resolution compared with 131I planar imaging. They thus argue that the availability of 124I should be expanded.