[Clinically unclear painful swelling of the upper arm].

In contrast to the intramedullary (80%) and intracortical (14%) subtypes, the subperiostal subtype of aneurysmatic bone cysts (sABC) is relatively rare (6%). Females are affected more frequently than males, whereby the diaphysis is predominantly affected and less frequently the metaphysis of long bones as well as the vertebral column. Especially in conventional radiography sABCs can mimic aggressive lesions. Cross-sectional imaging can potentially reduce the wide range of differential diagnoses. Due to typical imaging features magnetic resonance imaging (MRI) is the most valuable modality to reduce the range of possible differential diagnoses. MRI usually presents a multicystic appearance with a hypointense rim of the lesion, contrast-enhancing cyst walls, fluid levels and edema-like changes in the adjacent tissue. In sABCs with solid components the diagnosis cannot be made with confidence and the suspicion must be confirmed by biopsy. The therapy of primary lesions consists of curettage or the complete excision of the sABC and the defects are subsequently filled with spongiosa or bone cement depending on the size of the lesion.