Single-photon emission computed tomography/computed tomography iodine-131 uptake of bronchiectasis masquerading as metastatic thryroid disease

A 37-year-old female status post total thyroidectomy for multifocal papillary thyroid cancer was subjected to radioiodine-131 treatment. The largest tumor measured was 1.6 cm in the left thyroid lobe and 3/8 positive regional lymph nodes at the time of surgery. The patient had been on a low iodine diet for 2 weeks prior to this examination (September 2011). She was clinically hypothyroid, with a thyroid stimulating hormone being 82.1 μIU/ml prior to radioiodine administration. Anterior and posterior whole body iodine-131 (I-131) planar scintigrams were obtained 8 days postadministration of 105.9 mCi of I-131. Trace activity was detected in the neck. A focal region of activity was also noted in the right lung base suspicious for pulmonary metastasis [Figure 1].

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Figure 1 Anterior and posterior whole body iodine-131 (I-131) planar scintigrams obtained 8 days postadministration of 105.9 mCi of I-131 showing residual thyroid issue and uptake in lung region

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Single-photon emission computed tomography/computed tomography (SPECT/CT) performed subsequently for further evaluation localized the radioactive I-131 uptake to a focal area of bronchiectasis in the right middle lobe [Figure 2a–c], well demonstrated on chest CT [Figure 2d]. No metastatic pulmonary mass was appreciated. The patient was further evaluated in pulmonology clinic and underwent bronchiolar lavage. The bronchial washings were sent for acid fast bacilli culture, which returned positive for mycobacterium avium complex. Repeat whole body I-131 scan and SPECT/CT was again performed in July 2014 to look for papillary thyroid cancer recurrence. The patient's mycobacterial infection had progressed to involve the lingula, with a new area of bronchiectasis seen on CT [Figure 2e] that demonstrated corresponding activity on SPECT/CT [Figure 2f].

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Figure 2 Single-photon emission computed tomography/computed tomography (SPECT/CT) localized tracer activity to a focal area of bronchiectasis in the right middle lobe (a-c), well demonstrated on chest CT (d). No metastatic pulmonary mass was appreciated. Follow-up iodine-131 SPECT/CT showing progression of infection with a new area of bronchiectasis seen on CT (e) that demonstrated corresponding activity on SPECT/CT (f)

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This case demonstrates an important pitfall in the interpretation of post thyroidectomy scintigraphy. I-131 uptake has been reported in various infectious and inflammatory processes, but infrequently with bronchiectasis, particularly with SPECT/CT confirmation.[123456789] False positive uptake is often artifactual in etiology, and usually related to urinary or salivary artifacts.[10] Radioiodine uptake in well differentiated metastatic thyroid cancer depends on sodium-iodide symporter (NIS) by tumor tissue. Pitfalls in the interpretation of radioiodine uptake can result from the expression of NIS by normal tissues, concentration of radioiodine in intravascular spaces, gastrointestinal/genitourinary excretion, hepatic metabolism, and physiologic secretions.[11] SPECT/CT helps to minimize false positive interpretation by localizing the areas of radioactivity to anatomic structures and provides more precise localization of iodine uptake.[1] In this case, radioiodine uptake in the right middle lobe was localized to a site of bronchiectasis on SPECT/CT, avoiding a misdiagnosis of metastatic thyroid carcinoma.