Radioiodine Uptake of a Benign Peritoneal Cyst on 131-I Whole-body Scan in a Patient Treated for Papillary Thyroid Cancer

Introduction

A 52-year-old man presented with newly diagnosed papillary thyroid cancer and rh-TSH-stimulated thyreoglobulin at 0.7 ng/ml during primary radioiodine therapy following total thyroidectomy. Aside from the anticipated intense uptake of ¹³¹I in the neck area, an unexpected, solitary, strong focal uptake was detected in projection to the caudo-lateral aspect of the liver on ¹³¹I whole-body scan ([Fig. 1]a, arrowhead). On the following SPECT/CT-scan a lesion with an intense accumulation of ¹³¹I was detected adjacent to liver segment VI ([Fig. 1]b, arrow).

Fig. 1 a Whole-body-131I scan with expected intense uptake in the neck area, physiological uptake in the GIT and urinary tract and suspicious focal uptake in projection onto to liver (arrowhead). In the SPECT/CT-scan (b, arrow) an intense radioiodine-avid lesion was detected adjacent to liver segment IV.

Subsequent abdominal MRI performed 8 weeks after the ¹³¹I whole-body-scan showed a small exophytic lesion adjacent to liver segment VI without contrast-enhancement on fat-saturated post-contrast T1-weighted images ([Fig. 2]a, arrowhead) and with high signal on fat-saturated T2-weighted images ([Fig. 2]b, arrowhead). Due to the intense ¹³¹I uptake of the lesion, the otherwise unspecific MRI finding was categorized as suspicious of metastasis, and a multidisciplinary tumour board decision was made to resect the lesion.

Fig. 2 Abdominal transaxial MRI scan showing a 11 mm exophytic lesion adjacent to liver segment VI on fat-saturated post-contrast T1-weighted images (a, arrowhead) with high signal on fat-saturated T2-weighted images (b, arrowhead).

During laparoscopic surgery, the lesion was detected in the peritoneum adjacent to liver segment VI. Histopathologic examination showed a benign cyst ([Fig. 3]) with partial squamous epithelium. Immunohistochemistry was conducted as well, revealing negativity for thyreoglobulin and Thyroid transcription factor-1 (TTF-1), thus excluding metastasis of the known thyroid carcinoma.

Fig. 3 Microscopic section of the cyst with immunohistochemical positive staining for CK7.

Solitary metastases to the liver from thyroid cancer are exceptionally rare with 0.5% or less [1], especially in a pT1b tumour, and can present as ¹³¹I-negative or positive.

131 I-positive liver metastases from differentiated thyroid cancer are likewise an uncommon occurrence [2]. Radioiodine is a sensitive marker for the detection of thyroid cancer and metastasis. Nevertheless it has a low specificity due to its accumulation in many healthy tissues like thymus, breast, liver, gastrointestinal tract [3]. One of the known mechanisms for radioiodine uptake is the expression of sodium-iodide symporter (NIS) in functioning thyroid tissues [4]. Nevertheless, few cases of unexpected radioiodine uptake in benign tumours have been reported, for example in renal hamartoma [5] or fibroadenoma of the breast [6]. Various cystic structures can also show uptake on radioiodine whole-body-scan, some cases include nabothian cysts [7], functional ovarian cysts [8] or renal cysts [9]. It is assumed that radioiodine enters cysts via passive diffusion or partially active transport and remains trapped due to slow exchange of water and chemicals between the cysts and the surrounding tissue [3]. Nevertheless, focal abdominal uptake of iodine in benign lesions is a rare occurrence and should be further investigated in order to definitively exclude metastasis of the primary.

Publication History

Received: 20 August 2021

Accepted: 15 November 2021

Article published online:
16 December 2021

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