Bizarre parosteal osteochondromatous proliferation

Case contributed by Mohamed Saber

Presentation

Left-arm mass felt for a few months, not increasing in size with occasional pain

Patient Data

Age: 45 years
Gender: Male

Left anterolateral mid-arm irregular bony mass lesion seen broad-based on the outer cortex of mid humeral diaphysis with no line of cleavage. The lesion is seen projecting within the deltoid muscle. No definite continuation with the humerus medullary cavity. No evidence of osseous or soft tissue aggressive changes. It elicits typical fatty signal in all pulse sequences (high signal in T1 & T2 and suppressed in STIR).

CT study shows a well-marginated wide-based bony growth on the outer cortex of mid humeral diaphysis, projecting into the soft tissues, appear well-formed bone with medullary cavity and outer cortex however, there is no evidence of continuation with the humeral medullary cavity. No associated aggressive or destructive changes.

Case Discussion

Bizarre parosteal osteochondromatous proliferations, also known as Nora lesions, are benign exophytic osteochondral lesions that have an appearance similar to an osteochondroma and are typically seen in the hands and feet. This case is atypically showing this lesion involving the humerus

On imaging, Nora lesions are shown to be continuous with the underlying cortex, but usually without continuation of the medulla, however, radiographic features alone cannot reliably diagnose the lesion. Differential diagnosis includes myositis ossificans and bone surface osteogenic or chondrogenic tumors.

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