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INTRODUCTION

Musculoskeletal oncology is a field of medicine that involves the diagnosis and management of neoplastic conditions affecting the musculoskeletal system. This not only entails neoplasia of mesenchymal origin (derived from embryonic mesoderm), but also metastatic carcinoma and a variety of pseudotumorous conditions. Mesenchymal tumors are an extremely heterogeneous group of neoplasms including over 200 benign conditions and 90 types of sarcoma. The majority of this chapter is dedicated to these conditions. The relative incidence of benign to malignant disease is 200:1. These tumors are classified histomorphologically based on features of differentiation, but there is considerable overlap. It is reasonable to consider these separate conditions as points on a continuum, rather than entirely distinct entities. Classification, nevertheless, is important because it may yield insight to the behavior, treatment response, and overall prognosis. Benign disease, by definition, exhibits little tendency to locally recur or metastasize, and is generally not aggressive in terms of local tissue destruction. However, some benign lesions are more likely to exhibit aggressive growth than others, and several lesions traditionally described as benign have relatively high rates of local recurrence and the potential to metastasize. Sarcomas (malignant tumors of mesenchymal origin) are often rapidly destructive, have metastatic potential, and have a tendency to locally recur.

Neoplastic processes arise in tissues of mesenchymal origin far less frequently than those of ectodermal or endodermal origin. According to the National Cancer Institute’s Surveillance, Epidemiology, and End Results (SEER) program, approximately 12,300 new soft-tissue sarcomas and 3300 new bone sarcomas developed in the United States in 2016, with over 4900 sarcoma-related deaths. When compared with the overall cancer mortality of 595,000 cases in 2016, sarcomas are a small fraction of the problem. While they are a relatively uncommon form of cancer, these tumors behave in an aggressive fashion, with currently reported mortality rates that may be greater than 50%. The associated morbidity is much higher. These tumors inflict a tremendous emotional and financial toll on individuals and society alike. Furthermore, sarcomas preferentially affect older patients, with only 15% occurring in patients younger than 15 years and 40% in patients older than 55 years. Accordingly, as the population ages, the incidence of these conditions will increase.

ETIOLOGY OF MUSCULOSKELETAL TUMORS

Tumorigenesis is a multifactorial process, which, despite considerable fiscal and intellectual interest, is still poorly understood. Generally, genetic mutations that cause cells to replicate in an unregulated fashion are the driving force behind tumorigenesis. Many such mutations have been identified, each often associated with multiple neoplastic processes. The development of a colony of abnormally proliferating cells out of normal tissue is referred to as transformation. This process involves acquired mutations in oncogenes, tumor suppressor genes, and other genes that directly or indirectly control proliferation, cell motility, and properties of invasiveness. Such a process may progress beyond the state of benign disease to an aggressive, dedifferentiated state with considerable genomic instability.

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