Abstract
Purpose: To determine the clinical significance of diffuse uptake of 68Ga-FAPI in the thyroid.Methods: From January 2020 to September 2021, all subjects with diffuse thyroid uptake in 68Ga-FAPI PET/CT were investigated in our hospital, and compared with the age and sex matched control group. The 68Ga-FAPI uptake in the thyroid gland was analyzed semi-quantitatively using the maximum standardized uptake value (SUVmax), and regression analysis was used to analyze the correlation between available serum thyroid stimulating hormone (TSH) and thyroid peroxidase antibody (TPOAb).Results: Among 815 subjects, 39 subjects were found diffuse FAPI uptake in thyroid gland; 11 subjects refused further examination; a total of 28 subjects were included in the analysis, and 27 subjects were diagnosed with chronic thyroiditis (including 20 subjects with Hashimoto's thyroiditis), 3 subjects with Grave's disease, 3 subjects with only serum TSH elevated, and 1 subject with malignant of thyroid and thyroiditis. The SUVmax of 27 subjects with thyroiditis was 5.75 ± 5.45. No significant correlation was found between the SUVmax and the level of serum TSH (P = 0.389) or TPOAb (P = 0.426).Conclusion: The incidentally discovered diffusely increased 68Ga-FAPI uptake in the thyroid gland is mostly related to chronic lymphocytic (Hashimoto's) thyroiditis. 68Ga-FAPI uptake level correlated neither with the degree of hypothyroidism nor with the titer of TPOAb. In addition, immune-related thyroiditis with immune checkpoint inhibitors may be accidentally found on 68Ga-FAPI, which may be helpful in facilitate timely intervention.
Highlights
68Ga-FAPI is a novel tumor-targeting agent, as fibroblast activation protein is overexpressed in cancer-associated fibroblasts [1,2,3]
Diffuse FAPI uptake of thyroid was found in 39 of 815 (4.8%) subjects between January 2020 and September 2021 [(man accounted for 2.6% (21/815) and woman accounted for 2.2% (18/815)], and their mean age was 53.3 ± 10.9 y
It was found that 3 subjects had thyroiditis caused by immunosuppressive therapy, and the thyroid uptake was relatively low, which may be related to the shorter PET/CT scan interval after treatment
Summary
Existing research has shown that FAPI revealed favorable pharmacokinetics and biodistribution in vivo and a clear delineation of primary tumors and their metastases [4,5,6,7]. Chronic thyroiditis is confirmed to be thyroid degeneration and fibrotic changes. We encountered a patient with diffuse FAPI uptake in thyroid, and the findings suggested a thyroid tumor. Histopathological examination revealed chronic thyroiditis (Hashimoto’s) in the nodule and surrounded by thyroid tissue [8]. We encountered a patient with diffuse FAPI uptake in the thyroid. At first, it was considered as chronic thyroiditis, but diagnosed as thyroid lymphoma (diffuse large B-cell lymphoma) [9]. When judging 68Ga-FAPI PET/CT images, diffuse FAPI uptake in the thyroid needs to be identified
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