Regression of an enchondroma: a case report and proposed etiology.

Enchondromas are common benign bone lesions that are found in the medullary cavity of tubular bones, usually at the metaphysis. Regression is highly unusual, and loss of matrix mineralization in an existing enchondroma should prompt investigation for malignant transformation. We present the case of a 50-year-old woman with an enchondroma of the proximal humeral metadiaphysis, which underwent loss of matrix mineralization that corresponded to replacement with marrow fat on MRI. This transformation of the cartilage tumor matrix into normal bone marrow may occur in a process similar to that seen with endochondral ossification.