Detectability of foci of radioiodine uptake in the thyroid bed and neck comparing pinhole with parallel-hole collimators

Radioiodine Imaging for Differentiated Thyroid Cancer: Not All Radioiodine Images Are Performed Equally

Background: Radioiodine scanning may help risk stratify patients with differentiated thyroid carcinoma (DTC) during initial and subsequent restaging. To maximize the information obtained from radioiodine scanning, image quality and interpretation should be optimized. However, not all radioiodine scans are performed equally. This illustrated article reviews seven techniques that may significantly improve the information obtained from a radioiodine scan in patients with DTC, which in turn may alter management such as showing regional or distant metastases that were otherwise unknown and/or help classify whether a metastasis is radioiodine avid. Summary: The first of the techniques is spot imaging of anatomical areas of interest using a gamma camera with a parallel-hole collimator. Spot images typically provide superior spatial resolution and enhanced lesion detection compared with whole-body scans using the same equipment. The second technique is spot imaging of the thyroid bed and neck with a pin-hole collimator, further improving spatial resolution. Two other techniques, delayed image acquisition and longer acquisition time, may clarify the nature of indeterminate foci of uptake or areas with negative initial findings. Delayed image acquisition may increase tumor-to-background ratio and thus improves lesion detectability. Longer acquisition times also increase contrast resolution between lesions and background activity, again increasing the detectability of malignant lesions. The fifth technique, adjustment of image brightness/contrast on film or on the computer screen, may reveal previously unobserved subtle differences in counts. The sixth technique, focus-specific history, comprises additional patient information that is specific to a focus of radioiodine uptake and elicited by the nuclear medicine physician or technologist. The goal is to help determine if the focus represents a metastasis or an artifact, which in turn decreases false positives and increases specificity. The seventh technique, single-photon emission computed tomography/computed tomography, improves contrast resolution and helps localize foci of uptake to anatomical structures. Conclusions: Technique is important to maximize information obtained from radioiodine scans in patients with DTC. With the greater usage and understanding of these seven techniques, physicians will significantly improve the information obtained from a radioiodine scan in patients with DTC, which in turn may alter management and potentially outcomes.

Number of Foci of Functioning Thyroid Tissue Remaining after Thyroidectomy for Differentiated Thyroid Cancer: Institutional Experience

Radioiodine imaging of the neck with a pinhole collimator (PinC) is frequently performed in differentiated thyroid cancer (DTC) patients for visualizing individual, and a total number of foci (NOF) of functioning residual thyroid tissue (FRTT) within the thyroid bed in postthyroidectomy patients. The objective of this study was to determine our experience regarding the NOF of FRTT visualized on pretherapy radioiodine PinC images. We performed a retrospective review of radioiodine PinC images of the neck of patients with very low-risk DTC and who had thyroidectomy performed by selected surgeons at MedStar Washington Hospital Center. For each patient's image, the NOF of FRTT was determined by two blind readers. Statistical analysis was performed. One hundred and twenty-six patients met the criteria. Surgeon (S1, S2, S3, and S4) performed 17, 10, 86, and 13 thyroidectomies, respectively. The analysis (mean, range, and standard deviation) of NOF of FRTT was: S1: (2.2, 0-5, 1.3); S2: (1.6, 1–3, 0.8); S3: (2.6, 0–7, 1.5); S4: (3.3, 1–5, 1.3). The percentages of < 2, ≤3, ≤4, and <5 foci remaining were 4.9%, 21.5%, 77.0%, and 91.3%, respectively. For the selected surgeons, the NOF of FRTT in the thyroid bed or neck in postthyroidectomy patients never exceeded 7, rarely exceeded 5 (2.4%), and infrequently exceeded 4 (8.7%). Based on these data, our thresholds of the NOF for which we perform further workup for possible locoregional disease are ≥5. Each facility is encouraged to establish their own criteria for their facility and preferably for each of their surgeons.