Dichotomous Uptake Pattern on [ 18 F]F-FDG and [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT in Lytic, Sclerotic and Marrow Metastatic Skeletal Lesions in Medullary Thyroid Carcinoma.

In medullary thyroid carcinoma (MTC), skeletal metastases are present in 19% of patients at initial diagnosis and increase up to 29% during follow-up, predominantly involving the spine and pelvis. We describe 2 patients of metastatic MTC illustrating divergent lesion morphologies: one exhibiting sclerotic bone metastases and the other displaying marrow and lytic lesions. The lytic and marrow-based metastases demonstrated more avidity for [ 18 F]F-FDG PET/CT, whereas [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT provided enhanced visualization of the sclerotic lesions. This dichotomous tracer behaviour underscores the importance of tailoring PET agent selection to lesion morphology, potentially optimizing diagnostic accuracy and guiding individualized management strategies.