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Adrenocortical adenoma manifesting as false-positive iodine accumulation in a patient with history of thyroid carcinoma

A 47-year-old female diagnosed with well-differentiated papillary thyroid carcinoma was referred to our center for a 131Iodine whole body scintigraphy as follow-up. The patient had been previously treated with total thyroidectomy and ablative dose of 175mCi 131I three years ago. Diagnostic 131I scan showed a zone of radioiodine uptake in posterior aspect of the left upper quadrant of the abdomen. Spiral abdominal and pelvic CT scan showed an enhancing solid mass in superior aspect of the left adrenal gland, which was in favor of metastasis to the lymph node or an adrenal tumor. A biopsy was performed from the lesion. Histological examination of the surgical specimen was consistent with adrenocortical adenoma. Even though rare, adrenocortical adenoma should be included in the potential causes of false-positive results of radioiodine scans.


Introduction
Thyroid carcinoma is treated by near-total or total thyroidectomy followed by 131 Iodine ( 131 I) ablation of any remnant thyroid tissue [1] . 131 Iodine whole body scan has been used for the follow-up of differentiated thyroid carcinoma for E-mail address: afsharm@sums.ac.ir (M. Haghighatafshar). several decades [2] . Various physiological and pathological processes and anatomical variants can arise misleading artifacts or interpretation in nuclear scintigraphy [3][4][5] . We herein describe the case of a female who had been treated with near-total thyroidectomy and radioiodine ablative therapy for papillary thyroid cancer with a false positive uptake whole body scan. Considering potential false-positive 131 I scintigraphy is critical to avoid the dispensable surgery or exposure to further radiation.

Case report
A 47-year-old female diagnosed with well-differentiated papillary thyroid carcinoma was referred to our center for a 131 Iodine whole body scintigraphy as follow-up. The patient had been previously treated with total thyroidectomy and ablative dose of 175mCi I-131 three years ago. Forty-eight hours after the oral administration of 5 mCi 131 I, whole body scan showed a zone of radioiodine uptake in posterior aspect of the left upper quadrant (LUQ) of the abdomen ( Fig. 1 a). Further evaluation was done and spiral abdominal and pelvic CT scan showed an enhancing solid mass about 40 × 35 mm in superior aspect of the left adrenal gland which was infavor of metastatic lymph node or adrenal tumor ( Fig. 1 b and c). This finding prompted the surgical resection of the lesion. Histological examination of the surgical specimen was consistent with adrenocortical adenoma. Because the scans of the patient were anonymous and no experimental intervention was done for the patients, ethical approval was not necessary.

Discussion
In patients diagnosed with differentiated thyroid carcinoma , the whole body 131 Iodine scintigraphy is considered as an essential part of their follow-up [3,4] . The specificity of 131 I whole body scan for detecting residual or recurrent differentiated thyroid carcinoma is reported to be greater than 90% [6] . However, there are many potential causes of false-positive 131 I whole body scintigraphy findings [6][7][8] . Radioiodine uptake is not specific for thyroid tissue and could also be observed in healthy tissues, including thymus [4] , liver, gastrointestinal tract, nasal activity, lactating breasts, Zenker's diverticulum, and Meckel's diverticulum or in benign diseases, such as hiatal hernia [3] , inflammatory lung disease, sialoadenitis, cysts, and inflammation, or in a variety of benign and malignant nonthyroidal tumors, which could be mistaken for thyroid carcinoma [6][7][8][9][10][11] .
The suggested mechanisms of iodine accumulation in variety of inflammatory and infectious disease are: increased blood flow that delivers increased levels of iodine to the site, and enhanced permeability of the capillary that increases diffusion of the iodine to the extracellular space [7,12] .
The suggested mechanisms of iodine accumulation in nonthyroidal neoplasms are: tumor expression of the NIS, that actively accumulates the iodine and increased vascularity, and enhanced capillary permeability that may be secondary to the inflammatory process due to neoplasm [12,13] .
Recognition of potential false-positive 131 Iodine scintigraphy is important to avoid additional costly investigations and unnecessary surgery or exposure to further radiation from repeated therapeutic doses of 131 Iodine. Also fused singlephoton emission computed tomography/computed tomography images could be very useful in these situations. To the best of our knowledge, this is the first report in the literature of an adrenocortical adenoma as potential causes of false-positive results of radioiodine scans. Even though rare, adrenocortical adenoma should be included in the potential causes of falsepositive results of radioiodine scans.

Conclusion
In this case, adrenocortical adenoma uptake of 131 I, misled to unnecessary surgical resection of the lesion. Hence, it is highly important to consider adrenocortical adenoma as a differential diagnosis of upper abdominal uptake of 131 I. Although rare, adrenocortical adenoma should be included among the potential causes of false-positive radioiodine scans.