Periosteal chondroma of the proximal phalanx of the thumb

A 52-year-old man presented with a periosteal chondroma in the base of the proximal phalanx of the right thumb. Magnetic resonance imaging (MRI) showed invasion of the bone marrow, which suggested malignancy. Although the clinical and MRI findings indicated a malignant tumour, the histopathological examination showed a benign periosteal chondroma.