The additional value of non-enhanced CT acquired during 18F-FDG PET/CT in prediction of I-131 ablation success

Objectives The high content of iodine in thyroid parenchyma is the cause for high Hounsfield Unit (HU) in pre-contrast CT. A potential important factor in predicting I-131 ablation success is the innate iodine concentration ability of thyroid parenchyma. We theorize that the iodine content in the normal thyroid parenchyma measured by HU acquired during PET/CT can predict I-131 ablation success after surgery.

Methods A total of 131 patients who underwent PET/CT before surgery from January 2005 to December 2009 were selected for this study. These patients underwent low dose I-131 ablation (30mCi, 94 patients) or high dose I-131 ablation (150/200mCi, 24 patients) within one year of surgery, and then had a second diagnostic I-131 whole body scans (5mCi) after one year of initial I-131 ablation. Thyroid iodine content was measured by drawing ROI on the thyroid parenchyma and mean HU was recorded. 1st and 2nd I-131 whole body scan (WBS) images were evaluated for I-131 uptake in the neck. Increased I-131 uptake in the neck in the 2nd diagnostic I-131 WBS was considered to be Imaging Ablation failure.

Results 70 out of 131 patients had no I-131 uptake in the neck in the 2nd I-131 WBS. Pre-contrast CT HU was significantly higher in 2nd I-131 ablation success patients than in patients that had residual I-131 uptake in the 2nd I-131 WBS (108.1 ±17 vs 98.5±19.17, p=0.005). ROC analysis showed that a cut-off of 103.5 had the highest sensitivity in predicting I-131 ablation success (AUC=0.683, p<0.001). Using this cut-off value, the sensitivity, specificity and accuracy was 71%, 63% and 67%. High dosage of I-131 was also predictive of 2nd I-131 WBS success (x2, p=0.027).

Conclusions Pre-contrast CT acquired during PET/CT before surgery is helpful in predicting I-131 ablation success in thyroid cancer patients. Iodine content in the thyroid parenchyma may be an important factor in predicting I-131 ablation success.