Endocrine Abstracts

Objective: The prevalence of incidentally discovered papillary thyroid microcarcinoma (iPTMC) has increased with the expanded use of diagnostic imaging, including nuclear medicine imaging techniques. While iPTMC appears to have a better prognosis than non-incidental PTMC, it was suggested that thyroid carcinomas that were incidentally discovered during 18F-fluorodeoxyglucose positron emission tomography/computer tomography (FDG PET/CT) or parathyroid scintigraphy may have poor prognostic features, with no particular emphasis on iPTMCs. The aim of this study was to assess and compare the histopathologic characteristics of iPTMCs discovered via various nuclear medicine imaging methods.

Methods: Patients with PTMC who had a confirmed histopathological diagnosis between 2014 and 2023 were evaluated retrospectively. Among a total of 957 subjects with PTMC, 59 were incidentally discovered during diagnostic imaging using nuclear medicine techniques including thyroid scintigraphy (n =27), parathyroid scintigraphy (n =11), or FDG PET/CT (n =21) that were performed for causes unrelated to thyroid carcinoma.

Results: iPTMCs discovered by either FDG PET/CT, parathyroid or thyroid scintigraphy represented 6% of all PTMC. The indications for using FDG PET/CT was to stage different cancers (n =19) or find the primary location of a metastatic cancer (n =2), while parathyroid and thyroid scintigraphies were to determine the origins of hypercalcemia and thyrotoxicosis, respectively. The mean patient age was 53.6±12.2 years, and 65% (n =39) were female. The cytological examinations were mostly reported as BETHESDA category III or V. Total thyroidectomy was performed in 93% (n =55) of the cases, with central lymph node dissection in twelve. In histopathological examination, most of the cases had follicular subtype (n =18) while seven patients had tall cell subtype. iPTMC size was greater than 5 mm in 68% of cases, and cervical lymph node metastasis was detected in 15%. iPTMCs discovered by FDG PET/CT showed higher rates of multifocality, capsular invasion, cervical lymph node involvement and extrathyroidal invasion than those discovered by parathyroid or thyroid scintigraphy; however, only extrathyroidal invasion reached statistical significance. HBME-1, CK-19, and Galectin-3 expressions were comparable between cases discovered by FDG PET/CT and other methods, whereas loss of CD-56 expression was prevalent in cases detected by FDG PET/CT. In two cases where FDG PET/CT was used to identify the primary origin of a metastatic cancer, scalp and thoracic vertebral lesions were biopsied to identify papillary thyroid cancer metastases.

Conclusions: Patients with PTMCs usually have a good prognosis. However, based on the results of this research, which looked at a subset of iPTMCs identified using nuclear medicine imaging methods, we concluded that iPTMCs found during FDG PET/CT may have poor prognostic characteristics, leading to more frequent extrathyroidal invasion and even distant metastases.