Imaging Pitfalls in Surface Bone Lesions: Florid Reactive Periostitis Mimicking Parosteal Osteosarcoma – A Multimodality Case Series

Purpose: To evaluate imaging features and clinicopathologic correlation in florid reactive periostitis (FP) and parosteal osteosarcoma (PO), emphasizing diagnostic pitfalls in surface bone lesions. Patients and Methods: Four patients (two FP, two PO) underwent radiography, CT, and MRI, including dynamic contrast-enhanced sequences in selected cases. Histopathology with immunohistochemical analysis of MDM2 and CDK4, including gene amplification testing, was performed in all cases. Results: FP demonstrated solid or layered periosteal reaction, soft-tissue swelling, and bone marrow edema without atypia or MDM2/CDK4 amplification. PO presented as lobulated ossified masses with cortical attachment or cleavage plane and positive MDM2/CDK4 expression. DCE-MRI showed overlapping enhancement patterns and did not reliably differentiate the entities. Conclusion: Differentiation between FP and PO requires careful evaluation of conventional imaging features combined with histopathological confirmation. Multidisciplinary assessment remains essential to prevent diagnostic overtreatment or undertreatment. Key Points: · FP may mimic PO on MRI. · Cleavage plane favors parosteal osteosarcoma. · DCE-MRI shows overlapping enhancement patterns. · MDM2/CDK4 are essential in equivocal cases. Citation Format: · Igrec J, Stranger N, Puseljic M et al. Imaging Pitfalls in Surface Bone Lesions: Florid Reactive Periostitis Mimicking Parosteal Osteosarcoma - A Multimodality Case Series. Rofo 2026; DOI 10.1055/a-2864-1949.