Clinical outcomes following empiric radioiodine therapy in patients with structurally identifiable metastatic follicular cell-derived thyroid carcinoma with negative diagnostic but positive post-therapy 131I whole-body scans.

BACKGROUND

While radioiodine (RAI) therapy remains the most effective treatment modality for RAI-avid distant metastatic follicular cell-derived thyroid cancer, the therapeutic utility of empiric RAI therapy in patients with structurally identifiable distant metastases that demonstrate RAI avidity only on the post-therapy scan (negative diagnostic whole-body scan [DxWBS]) remains uncertain.

METHODS

We report a retrospective assessment of the structural response to RAI therapy in 27 patients (median age 54 years, 59% male) with metastatic thyroid cancer (45% classical papillary thyroid cancer, 21% poorly differentiated, 15% tall-cell variant, 15% follicular variant, and 4% Hurthle cell carcinoma) with structurally identifiable distant metastases (86% pulmonary metastases) in whom a properly conducted DxWBS was negative, and the post-therapy scan showed RAI-avid metastatic lesions at the time of RAI remnant ablation.

RESULTS

In response to the initial RAI ablation, none of the selected patients demonstrated structural disease regression, and no patient was rendered free of disease. However, 12 patients (44%) demonstrated stable lesions on serial structural imaging after an RAI ablation. Structural disease progression was seen in the remaining 56% (15/27), a median of 6 months after ablation. Unfortunately, additional RAI therapies given to 12/15 patients with progressive disease and 5/12 patients with stable lesions failed to cause structural disease regression, cure, or conversion from progressive to stable disease in any patient. All of the disease-specific deaths (7/27) were in patients who had structural disease progression (n=15) in response to RAI ablation. None of the patients with persistent but stable lesions on structural imaging (n=12) have died of thyroid cancer over a median follow-up period of 3.7 years.

CONCLUSIONS

While 44% of patients with the DxWBS-negative/post-therapy scan-positive macroscopic distant metastasis will have stable cross-sectional imaging after RAI remnant ablation, the other 56% will demonstrate structural disease progression that cannot be effectively treated with repeated empiric RAI activities. Furthermore, the high disease-specific mortality rate seen within the first few years of remnant ablation in this small subset of patients with persistent progressive disease despite a positive post-therapy RAI scan argues that treatments other than repeated empiric RAI dosing be strongly considered.