**4.5 I-124 PET/CT**

I-124 is a positron emitter with a half-life of 4.18 days, produced by a cyclotron. It can be used to directly image thyroid cancer using PET scanner. Van Nostrand et al. compared I-124 PET with I-131 DxWBS in detecting residual thyroid tissue and/or metastatic lesion. They reported that I-124 PET identified as many as 50% more foci of radioiodine uptake suggestive of additional residual thyroid tissue and/or metastasis (Van Nostrand et al., 2010). They suggest that I-124 PET produced superior results because the PET scanner provides images with reduced background noise and enhanced spatial and contrast resolution compared with I-131 DxWBS. The longer half-life of I-124 is useful for dose monitoring over an extended time period (Surti et al., 2009). It can be also used for lesionspecific dosimetry. I-131 mean absorbed dose distributions can be calculated from serial I-124 PET image (Erdi et al., 1999; Larson & Robbins, 2002).
