Pulmonary sequestration: a 131I whole body scintigraphy false-positive result

Radioiodine whole body scan pitfalls in differentiated thyroid cancer

PURPOSE

whole body scan (WBS) performed following diagnostic or therapeutic administration of I-131 is useful in patients with differentiated thyroid carcinoma. However, it can be falsely positive in various circumstances. We aimed to report a series of pitfalls in a clinical perspective.

METHODS

A search in the database PubMed utilizing the following terms: "false radioiodine uptake" and "false positive iodine 131 scan" has been made in January 2023. Among the 346 studies screened, 230 were included in this review, with a total of 370 cases collected. Physiological uptakes were excluded. For each patient, sex, age, dose of I-131 administered, region and specific organ of uptake and cause of false uptake were evaluated.

RESULTS

370 cases of false radioiodine uptake were reported, 19.1% in the head-neck region, 34.2% in the chest, 14.8% in the abdomen, 20.8% in the pelvis, and 11.1% in the soft tissues and skeletal system. The origin of false radioiodine uptake was referred to non-tumoral diseases in 205/370 cases (55.1%), benign tumors in 108/370 cases (29.5%), malignant tumors in 25/370 cases (6.7%), and other causes in 32/370 cases (8.7%).

CONCLUSIONS

WBS is useful in the follow-up of patients with differentiated thyroid carcinoma, however it can be falsely positive in various circumstances. For this reason, it is critically important to correlate the scintigraphic result with patient's medical history, serum thyroglobulin levels, additional imaging studies and cytologic and/or histologic result.

Pulmonary sequestration: a rare cause of false-positive uptake in Iodine-131 whole-body scan

Background

High-dose radioactive iodine-131 (I-131) is generally used post-total or subtotal thyroidectomy in differentiated thyroid cancer (DTC) patients. I-131 whole-body scan (WBS) is performed to localize I-131 accumulation in the body and for restaging after therapy. Nonetheless, there are numerous causes of false-positive uptake in I-131 WBS in the absence of residual thyroid tissue or thyroid cancer metastasis.

Case presentation

We present a case of 51-year-old lady with false-positive uptake in I-131 WBS mimicking lung metastasis. Patient underwent total thyroidectomy, and histopathological examination showed well-differentiated papillary thyroid microcarcinoma. Diagnostic I-131 WBS followed by two subsequent high-dose radioiodine ablation showed persistent I-131 uptake in the left lower lung field. However, the serum thyroglobulin (Tg) is incessantly undetectable (< 0.04 μg/ml) with negative anti-thyroglobulin antibody (anti-TgAb). Three serial CT scans over the course of treatment coupled with MRI of the thoracolumbar region revealed stable lung lesion in the posterior segment of left lower lobe which signifies pulmonary sequestration. Based on the clinical assessment, imaging findings and blood investigations, patient was no longer subjected to I-131 therapy and is maintained on TSH suppression with L-thyroxine. After due consideration of the pros and cons of surgical intervention, the patient opted against surgical removal of the pulmonary sequestration.

Conclusions

Meticulous assessment of patient with suspected disease recurrence or metastasis in thyroid carcinoma is essential to avoid unnecessary I-131 ablation or surgical intervention. In this case, the positive I-131 imaging is associated with undetectable serum Tg (with negative anti-TgAb). Albeit uncommon, pulmonary sequestration should be taken into consideration as one of the differentials in false-positive findings of an I-131 WBS.

Diagnosis and Treatment of Acute Pleural Effusion following Radioiodine Remnant Ablation Post Lobectomy for Thyroid Cancer

Radioiodine remnant ablation (RRA) was previously demonstrated to be a safe and effective alternative to completion thyroidectomy for patients with differentiated thyroid cancer (DTC). However, its side effects have not been fully investigated, particularly in patients with lobectomy. We reported a young euthyroidal female who underwent RRA post lobectomy and lymph node dissection for papillary thyroid cancer, whose post-ablation ¹³¹I-whole-body scan accidentally showed diffuse radioiodine distribution on chest-mimicking pulmonary metastases. Immediately-added single-photon emission computed tomography/computed tomography (SPECT/CT), nevertheless, revealed a ¹³¹I-accumulating swollen left thyroid lobe and emerging pleural effusion, which relieved after short-term treatment with prednisone. In summary, acute pleural effusion ascribed to RRA-induced thoracic duct compression was reported for the first time. ¹³¹I-lobectomy-induced pleural effusion could be precisely diagnosed by SPECT/CT and efficiently manipulated via treating radiation thyroiditis with the short-term administration of corticosteroid.

Pictorial Review of False-Positive Results on Radioiodine Scintigrams of Patients with Differentiated Thyroid Cancer

Radioiodine has served an important role in the diagnostic workup and treatment of patients with differentiated thyroid cancer for more than 6 decades. The interpretation of radioiodine scintigraphic studies should be performed in conjunction with a comprehensive history, histopathologic correlation, and pertinent laboratory values, as well as correlation with available anatomic images and the findings from physical examination. A thorough understanding of the physiology and biodistribution of radioiodine is critical when interpreting radioiodine scintigraphic studies to avoid misinterpretation of physiologic and nonthyroid pathologic variants as thyroid cancer metastases. Differentiating a false-positive finding from a true metastasis on pretherapy radioiodine scintigrams is important to determine the appropriate radioiodine treatment dose. The correct interpretation of posttherapy radioiodine scintigraphic studies is also important to determine if repeat radioiodine treatment will be necessary and for the future clinical and imaging followup of the patient. A variety of different factors, such as the presence of the sodium-iodide symporter and the passive diffusion or retention of radioiodine in normal and pathologic structures, can result in false-positive results on radioiodine scintigrams. Numerous false-positive findings have been reported in the literature and are further demonstrated with the increasing availability of single photon emission computed tomography (SPECT) integrated with computed tomography (CT) as true dual-modality imaging (SPECT/CT). SPECT/CT has been documented to be of incremental value in the accurate anatomic localization and characterization of radioiodine uptake as false-positive findings, particularly in cases with discordant findings of a low serum thyroglobulin level but positive findings on radioiodine whole-body planar scintigrams. The objectives of this review are to describe the physiology and biodistribution of radioiodine and to provide examples of false-positive results on radioiodine scintigrams, with clinical and anatomic correlation, in the following categories of radioiodine uptake: functional uptake secondary to sodium-iodide symporter expression, radioiodine retention, nonthyroid neoplasms, inflammatory or infectious uptake, contamination, and other causes. ^©RSNA, 2017.