**3.2 Types of radioiodine 3.2.1 I-131**

I-131 is produced in a nuclear reactor by neutron bombardment of natural tellurium (Te-127) and decays by beta emission with a half-life of 8.02 days to xenon-133 (Xe-133) and it emits gamma emission as well. It most often (89% of the time) expends its 971 keV of decay energy

Physiologic and False Positive Pathologic Uptakes on Radioiodine Whole Body Scan 7

electron capture and 25.6% positron emission. It emits gamma radiation with energies of 511

I-124 is administered intravenously with activities of 0.5–2.0 mCi for detection of metastatic

• Potential stunning

• Limited availablity

• Very limited availability

caregivers

• Very expensive

• Expensive

• Requirement of possible radiation safety precautions for family and

Types Advantages Disadvantages

Table 3. Advantages and disadvantages according to the types of radioiodine.(Nostrand,

Planar gamma camera imaging can be obtained with gamma emitting I-123 or I-131 for the detection of thyroid cancer tissue expressing the NIS gene which takes up iodine. The main emission energy peak of I-131 is approximately 364 keV, so it requires the use of a high-energy all-purpose collimator for imaging acquisition. The peak of the I-123 is 159 keV, which is close to the 140 keV from Tc-99m for which the gamma camera's design has traditionally been optimized. I-123 can be imaged with a low-energy high-resolution collimator, which is

With radioiodine's avidity for differentiated thyroid cancer tissues, planar radioiodine whole body image has been mainly used for the detection of metastatic thyroid cancer lesions. However, the limited resolution of planar imaging together with the background activity in the radioiodine images can give false-negative results for small lesions. Physiologic uptake of radioiodine is not always easily differentiable from pathologic uptake and it can give false-positive results. (Spanu, Solinas et al. 2009) Therefore, the sensitivity and specificity of planar images for the diagnosis of metastatic thyroid cancer may be

**3.3.2 SPECT (Single Photon Emission Computed Tomography) or SPECT/CT imaging**  Although a radioiodine whole body scan is one of the excellent imaging tools for the detection of thyroid cancer, false negative results may be observed in cases with small recurrent lesions in an area of rather high background activity or in cases with poorly differentiated cancer tissues, which have low uptake ability for radioiodine (due to dedifferentiation).(Geerlings, van Zuijlen et al.) SPECT, which can provide cross-sectional scintigraphic images, has been proposed as a way to overcome the limitations of planar

optimized for image acquisition with Tc-99m. (Rault, Vandenberghe et al. 2007)

and 602 keV.(Rault, Vandenberghe et al. 2007)

• Readily available

• Good image quality

• Superior image quality • Tomographic image

• Allows longer delayed image

• Allows intermediate delayed

• Fusion image with CT or MR

• Cheap

I-123 • No stunning

image

**3.3 Planar, SPECT and PET imaging** 

I-131

I-124

Bloom et al. 2004)

**3.3.1 Planar imaging** 

limited. (Oh, Byun et al. 2011)

lesions or assessment of the radiation dose related to I-131 therapy.

by transforming into the stable Xe-131 in two steps, with gamma decay following rapidly after beta decay. The primary emissions of I-131 decay are beta particles with a maximal energy of 606 keV (89% abundance, others 248–807 keV) and 364 keV gamma rays (81% abundance, others 723 keV).

I-131 is administered orally with activities of 1–5 mCi or less, with many preferring a range of 1–2 mCi because of the data suggesting that stunning (decreased uptake of the therapy dose of I-131 by the residual functioning thyroid tissue or tumour due to cell death or dysfunction caused by the activity administered for diagnostic imaging) is less likely at the lower activity range. However, detection of more iodine concentrating tissue has been reported with higher dosages.(Silberstein, Alavi et al. 2006)


Table 2. Recommended time intervals of withdrawal for drugs affecting radioiodine uptake. The time interval can be changed by the administered doses of the medications. The amount of iodine for the drug must also be considered.(Nostrand, Bloom et al. 2004; Silberstein, Alavi et al. 2005; Luster, Clarke et al. 2008)
