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Abstract Background Paget's disease is a metabolic disorder characterized by disorderly bone remodeling process of excessive osteoclastic and osteoblastic activities leading to a structurally disorganized appearance of the bone. Paget's disease is usually diagnosed based on the clinical presentation of bony pain, raised serum alkaline phosphatase, and typical radiological changes. Bone scintigraphy with 99mTc-labeled radiotracers is commonly used to assess disease extent, and one of the classical findings of Paget's disease on bone scintigraphy is diffuse radiotracer uptake in the mandible bone, widely described as Lincoln or Black Beard sign. On the other hand, FDG PET-CT is commonly used in cancer imaging and frequently for staging or assessing recurrence in various malignancies. However, its use in evaluating Paget's disease is not established, attributed mainly to the heterogeneity of FDG uptake in Paget's disease and the high false-positive and negative findings. Case presentation A 57-year-old female with metastatic mucinous rectosigmoid adenocarcinoma underwent multiple surgeries and completed 12 chemotherapy cycles with no evidence of local recurrence in the colon or distant metastasis. In the tenth year of active surveillance, her serum CEA levels were found to be elevated, yet there was no evidence of cancer spread from colonoscopy and contrast-enhanced CT. An 18F-FDG PET-CT was then ordered, to which a unique diffuse FDG uptake pattern in the mandible was seen, resembling the Lincoln or Black Beard sign classically described in bone scintigraphy. This appearance was then verified by congruent uptake during a 99mTc-MDP scan, thus leading to the diagnosis of Paget's disease. Conclusion The Lincoln sign is not limited to bone scintigraphy. Hence, we intend to add this FDG PET-CT finding to enrich the literature on the Lincoln sign and when to expect this pattern.