Case Report: Metabolic concordance and discordance in oligolesional disease—challenges in interpreting concurrent thyroid incidentaloma and vertebral lesions on 18F-FDG PET/CT

Background: F-FDG) positron emission tomography/computed tomography (PET/CT) plays a pivotal role in the evaluation of vertebral lesions and systemic disease assessment. When vertebral lesions and thyroid incidentaloma are detected simultaneously, determining their relationship can be challenging. Metabolic similarity between lesions is often considered suggestive of a common origin, such as a primary tumor with its metastasis, whereas marked metabolic disparity may suggest separate primary tumors. However, both assumptions may be misleading. Case: F-FDG uptake in both thyroid incidentaloma and vertebral lesion, initially suggesting a metastasis of thyroid carcinoma to the vertebrae. Surprisingly, the vertebral lesion was pathologically proven to be Langerhans cell histiocytosis (LCH) in a patient with a follicular thyroid neoplasm, representing a deceptive metabolic concordance. Conclusion: F-FDG uptake patterns on PET/CT may lead to diagnostic pitfalls when used as the sole criterion for determining the relationship between coexisting lesions. Careful integration of metabolic findings with morphologic imaging features and histopathological confirmation is essential. When interpreted in appropriate clinical context, PET/CT remains invaluable for whole-body assessment, therapeutic planning, and individualized management.