Medical image. Pain and swelling in a child's thumb

Clinical—A 10-year-old girl presented to the Emergency Department with a 2-week history of pain and swelling of the thumb. There was no history of trauma nor signs of sepsis. Plain radiographs revealed an aggressive lesion in the proximal phalanx of the thumb (Figure 1). MRI showed the lesion to have breached the cortex, but confirmed sparing of the adjacent epiphysis (Figure 2 A and B).What is the diagnosis?Answer and DiscussionFigure 1. Oblique view of the left thumb showing a mixed sclerotic and lytic lesion in the proximal phalanx, with associated periosteal new bone formation Figures 2A & 2B: Coronal MR images of the left thumb (A) STIR, and (B) T1-weighted(A) (B) Note: There is a destructive lesion in the proximal phalanx, which has breached the cortex and elicited periosteal new bone formation (respectively indicated by the opposing arrows on the STIR image. Cortical destruction is signalled by the uppermost arrow on the T1-weighted image). The lesion is heterogeneous and there is associated marrow oedema. The adjacent soft tissue involvement is minimal. The proximal phalangeal epiphysis remains normal (more inferiorly placed arrows on both STIR and T1-weighted images).