Iodine Uptake in Retroperitoneal Cysts after Radioactive Iodine Treatment

A 62-year-old woman with a history of breast cancer and a pancreatic cyst was newly diagnosed with papillary thyroid carcinoma that was incidentally detected on F-18 fluorodeoxyglucose positron emission tomography/computed tomography (CT) performed for breast cancer staging. She underwent total thyroidectomy with bilateral neck lymph node dissection. During surgery, a gross extrathyroidal extension invading the strap muscles was observed, with metastasis in one out of six lymph nodes (T3b N1a). Considering the high risk of disease recurrence as per the American Thyroid Association 2015 Risk Stratification System,[1] she received radioactive iodine therapy with 3.7 GBq (100 mCi) of iodine-131 (I-131) for ablation of the remnant thyroid tissue 4 months later. Seven days later, a whole-body iodine scan was obtained [Figure 1]. Focal radioactivity was observed in the anterior neck area, suggesting remnant thyroid tissue. Abnormal focal uptake was seen in the left upper abdomen. Remaining iodine uptake was unremarkable. Laboratory examination revealed a free thyroxin level of 1.47 (normal range: 0.93–1.70) ng/dl, thyroid-stimulating hormone level of 104.9 (normal range: 0.27–4.20) IU/ml, thyroglobulin antigen level of 0.04 (normal range: 3.5–77.0) ng/mL, and thyroglobulin antibody titer of 10.0 (normal range: 0–115) IU/ml.

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Figure 1 Postoperative iodine-131 scintigraphy shows radiotracer uptake in the neck area and in the left upper abdomen (a: anterior; b: posterior)

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Single-photon emission CT/CT (SPECT/CT) was used to verify iodine uptake in the left abdomen. Iodine uptake was observed in the posterior aspect of the left kidney [Figure 2a]. This activity corresponded to round soft-tissue densities in the left retroperitoneum on nonenhanced CT images obtained during SPECT/CT [Figure 2b]. Contrast-enhanced abdominopelvic CT, which was performed for the evaluation of a pancreatic cyst 4 months back, revealed nonenhancing lesions with low attenuation [Figure 2c]. The retroperitoneal cysts showed no enhancement or interval change in size; this suggested the presence of benign lesions. High signal intensity on T2-weighted abdominal magnetic resonance imaging performed for follow-up evaluation of the pancreatic cyst revealed that the multiloculated cystic lesion at the left posterior pararenal space was a cystic lymphangioma [Figure 2d]. False-positive iodine uptake can occur in many aberrant locations, including cystic lesions. Previous reports have shown iodine uptake in cystic structures, including bronchogenic, hepatic, renal, thymic, ovarian, and ganglion structures.[23456789101112] The mechanism may involve passive diffusion and retention of iodine in cysts. Our report demonstrates rare SPECT/CT images of iodine uptake in retroperitoneal cysts. Additional radiologic examinations helped in preventing invasive biopsy in this case.

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Figure 2 The transaxial single-photon emission computed tomography image reveals that iodine uptake occurred in the posterior aspect of the left kidney (a and b). This lesion is observed as nonenhancing low-attenuated soft-tissue density on contrast-enhanced abdominopelvic computed tomography scan (c) and high signal intensity on T2-weighted abdominal magnetic resonance imaging (d), suggesting a benign lesion

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