Assessment of pulmonary uptake in post-therapy I-131 whole body scan in well differentiated thyroid cancer patients

Objectives This study is to evaluate lung uptake in post-radioiodine therapy I-131 whole body scan (WBS) by comparing diagnostic performance with other modalities.

Methods From November 2006 to October 2008, 105 cases (M:F = 26:79. age 46.3 ±16.8 yrs) of I-131 WBS were reported as suspicious of lung metastasis. Their clinical information, serum Tg level with TSH stimulation, chest CT, chest X-ray, and F-18 FDG PET were reviewed to assess the radioiodine uptake in the lung. Concordant radiological findings of malignancy, diffuse nature, concurrent distant metastasis, and/or high Tg level, were regarded as findings preferring lung metastases. ROC curve to decide the cut-off level of serum Tg for highest positive likelihood ratio and McNemar test to compare diagnostic performance of each modality was analyzed.

Results 75 cases were regarded as true lung metastases, 23 as false positive, and 7 as equivocal. ROC curve showed 61% of sensitivity and 95% of specificity at 16.9 ng/ml of serum Tg level. Among lung metastases patients, 15 had normal chest X-ray and 9 had negative finding on chest CT. The detection rate of lung metastasis on I-131 WBS was significantly higher than chest CT (p=0.0156).

Conclusions Post I-131 therapy WBS is the most sensitive test to detect lung metastases in postoperative patients with thyroid cancer. However, benign lung lesions frequently show radioiodine uptake causing difficulty in deciding the need of continued radioiodine therapy. Concurrent measurement of serum Tg level and radiologic correlation are critical to evaluate lung metastases of thyroid cancer.