Given the clonal nature of these lesions, it is surprising that malignant transformation is so rare. Until now, there have been only 4 reports of an ABC undergoing malignant degeneration to osteosarcoma without prior radiation exposure.
In this article, we have presented a fifth case of a primary ABC degenerating into an osteosarcoma, which in this instance was the fibroblastic subtype. This diagnosis was strongly supported by radiologic and pathologic evidence. From a radiologic perspective, imaging at initial presentation showed absolutely no suspicious features, and the same was true when follow-up radiographs were obtained, 1 month later. Although 1 month is short for a follow-up, the complete lack of radiographic changes would be highly unusual if in fact there had been a coexisting, undetected lesion as aggressive as the one that ultimately developed. Furthermore, imaging at second presentation, almost 2 years later, showed an extremely rapid evolution of findings over 1 month. Extrapolating back in time, we think this time course indicates the malignancy developed not long before its aggressive features were detected.
Genetic evidence suggests that most conventional high-grade osteosarcomas arise de novo from a mesenchymal precursor driven by multiple genetic aberrations. Less often, low-grade osteosarcomas progress to high-grade osteosarcomas. Amplification of 12q13-15 with resulting overexpression of MDM2 and CDK4 proteins is found in low-grade osteosarcomas and persists in examples that progress to higher-grade forms.15 Not only did review of our patient’s initial biopsy sample reveal no evidence of malignant features or abnormal mitotic activity, but the complete absence of MDM2 suggests not even a low-grade osteosarcoma was present at the time. By contrast, the second incisional biopsy specimen, 2 years later, showed markedly different histology and pronounced expression of MDM2 throughout the specimen. This finding suggests the histologically high-grade osteosarcoma did not arise de novo but rather secondarily from a low-grade osteosarcoma that had arisen from an ABC. Results of the final heterogeneous histology of the very large mass, which contained benign ABC areas indistinguishable from the initial biopsy sample, as well as areas of high-grade osteosarcoma, further support a multistep process of de-differentiation. Together, these findings are compelling evidence of malignant transformation of a primary ABC.
We acknowledge that the initial surgery performed at the outside hospital might have properly included frozen-section analysis of the biopsy material and that sampling error may have occurred during the index procedure—possibilities in the absence of complete lesional resection. In this case, however, the radiographic findings and the dominant histologic immunophenotype from medullary canal bone were both consistent with ABC and not osteosarcoma, lending support to malignant degeneration.
We have presented a fifth case of primary ABC degenerating into an osteosarcoma, now with immunohistochemical evidence supporting traditional radiologic and histologic evidence. Despite the rarity of the diagnosis, this case yields considerable insight into the pathogenetic mechanisms underlying malignant degeneration. Despite the widely held view that ABCs are benign, physicians caring for these patients must be aware that malignant transformation can occur.
Am J Orthop. 2016;45(6):E367-E372. Copyright Frontline Medical Communications Inc. 2016. All rights reserved.