Atypical “Sclerosing” Osteoblastic Neoplasm: A Tumor of Intermediate Biological Potential Between Usual Osteoblastoma and Conventional Osteosarcoma

The existence of "aggressive" osteoblastoma (OB) or malignant transformation of OB is controversial. Over a few decades, we have encountered a group of "borderline" sclerosing osteoblastic lesions that are difficult to classify, tending toward local recurrence, especially following curettage. A search of the consultative and institutional files from 3 co-authors for atypical OB, malignant transformation of OB, well-differentiated osteosarcoma (OS), and OB-like OS diagnoses revealed 8 similar cases. There were 6 males and 2 females, ages 11 to 55 years (mean, 26 y). Three arose in metatarsals, 2 in the fibula, and 1 each in the humerus, tibia, and femur. Radiologically, most were expansile, lytic to sclerotic, with circumscribed and at least partially sclerotic borders. Pathologically, all displayed a predominant, sclerosing sheet-like neoplastic bone growth pattern, associated with minor components of conventional OB. No solid sheets of osteoblasts or permeation of surrounding bone were identified. Six cases were reviewed by >1 expert orthopedic pathologist, often with divergent opinions. Four were initially diagnosed as OB, 2 as low-grade OS, 1 as high-grade OS, and 1 as atypical sclerosing osteoblastic neoplasm. Clinical follow-up for 7 patients ranged from 12 to 138 months (mean, 71 mo). Four underwent curettage only; 2, curettage and en bloc resection with negative margins; 1, en bloc intralesional resection, and 1 amputation. 5 locally recurred, with 3 "reclassified" as OSs. One local recurrence was considered dedifferentiation. Whether these tumors represent low-grade OSs or aggressive forms of OB remains unclear. We recommend classifying these neoplasms as "atypical sclerosing osteoblastic neoplasm" and performing complete resection with negative margins. This study was presented in part at the 107th Annual Meeting of the United States and Canadian Academy of Pathology, March 20, 2018, Vancouver, Canada. Conflicts of Interest and Source of Funding: The authors have disclosed that they have no significant relationships with, or financial interest in, any commercial companies pertaining to this article. Correspondence: Scott E. Kilpatrick, MD, Department of Anatomic Pathology, Cleveland Clinic, 9500 Euclid Avenue/L25, Cleveland, OH 44195 (e-mail: kilpats@ccf.org). Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.

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