Concurrent osteochondroma and osteoblastoma of the proximal humeral shaft

We present what we believe is the first reported case of synchronous osteochondroma and osteoblastoma, occurring in the proximal humerus of a young man. A 15-year-old boy presented with a painful left arm mass for 3 months. A firm mass was palpable in the proximal medial arm, and he had mild triceps weakness secondary to pain. Imaging showed an eccentric lesion involving the proximal one-third of the humerus, with central lysis, surrounding sclerosis and edema, endosteal scalloping, and cortical thinning and expansion. The humeral diaphysis contained a bony exostosis with corticomedullary continuity, consistent with an osteochondroma with a cartilage cap <5 mm thick. Within the proximal portion of the osteochondroma was a 2-cm, edematous, rim-enhancing cystic lesion, concerning for a secondary process such as malignant transformation. On open biopsy, the mass grossly appeared to be a sessile osteochondroma, and was removed with a rongeur. The cystic lesion was curetted out of the bone. Pathology confirmed that the raised lesion was an osteochondroma. The cystic lesion contained osteoblasts in a matrix of osteoid and immature bone, characteristic of an osteoblastoma. The multidisciplinary team agreed that this was an osteoblastoma within an exostosis. At follow-up, the patient's pain had fully resolved and radiographs showed good early healing. We wished to document the extraordinary, simultaneous existence of an osteochondroma and adjacent osteoblastoma in the proximal humerus of a young patient. Although similar in presentation, the tumors consist of cells of different origins, making the pathogenesis unclear.