Periosteal chondroma presenting as a subcutaneous mass in the thumb.

We describe an unusual case of periosteal chondroma of the distal phalanx in the left thumb presenting as a subcutaneous mass in an 81-year-old woman. Magnetic resonance (MR) imaging demonstrated a subcutaneous lesion with hypointensity on T1-weighted images and marked hyperintensity on T2-weighted images, eroding the underlying cortical bone. Computed tomographic (CT) scans showed an erosive change in the dorsal cortex of the distal phalanx. Bone tumors arising from bone surface should be considered in the differential diagnosis of subcutaneous finger masses.