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Osteofibrous Dysplasia/Adamantinoma

Osteofibrous Dysplasia Clinical Features Osteofibrous dysplasia (OFD) is a benign fibro-osseous lesion characteristically occurring in children younger than 10 years, with an age range from newborn to 39 years. Most cases (95%) arise in patients younger than 15 years. Males and females are equally affected. Characteristically, these lesions occur in the diaphysis of the tibia, with a small proportion exhibiting synchronous fibular involvement. Involvement of the fibula alone is…

Notochordal Cell Tumors

Notorchodal Remnants/Ecchordosis Physaliphora Definition Notochordal remnants are small deposits of notochordal tissue that fail to regress after embryogenesis. General Features The notochord is an ectodermal-derived structure present in all organisms in the phylum Chordata and, as a major regulator of developmental signals in embryogenesis, plays a critical role in defining the primitive axial skeleton. Although it typically regresses by gestational week 10, it may be aberrantly…

Giant Cell Tumor of Bone

Giant Cell Tumor of Bone Giant cell tumor of bone is a relatively common bone tumor of intermediate malignancy that typically affects the epiphysis of long bones in skeletally mature individuals. The tumor is characterized by neoplastic mononuclear cells that harbor H3F3A G34 mutation and a large number of reactive osteoclast-like giant cells. Malignant transformation rarely occurs; primary malignant giant cell tumor is defined as a…

Hematolymphoid Neoplasms of Bone

Introduction Substantial advances in the diagnosis and classification of lymphoid, myeloid, and histiocytic neoplasms have been made in recent years. Dramatic improvements in genetic techniques have led to recognition of the underlying genetic abnormalities in many of these neoplasms, opening the door to identifying targeted therapy and improved prognosis. Lymphomas of Bone Primary lymphoma of bone (PLB) is a malignant neoplasm of lymphoid cells producing one…

Fibroblastic and Fibrohistiocytic Tumors

The fibroblastic and fibrohistiocytic tumors of bone represent a heterogeneous group of lesions ranging from purely benign to highly malignant neoplasms. Non-ossifying fibroma at one end of the spectrum is a common and self-limiting tumor developing during childhood and usually not requiring treatment. Desmoplastic fibroma is a locally aggressive neoplasm that can show morphologic overlap with central low-grade osteosarcoma. Sclerosing epithelioid fibrosarcoma rarely arises within bone…

Cartilage-Forming Tumors

Cartilage is an extracellular matrix rich in aggrecan, a proteoglycan that retains a large amount of water. The matrix consists of approximately 65% water, 15% proteoglycan, 15% collagen (predominantly type II), and less than 5% cells. Cartilage is unique in that it receives nutrients and oxygen by simple diffusion rather than a capillary network, thus limiting the extent of growth of native cartilaginous structures to, at…

Bone-Forming Tumors

Introduction Bone-forming tumors of the skeletal system represent a broad spectrum of neoplasms that vary significantly in their morphology and biologic potential. They are defined as neoplasms that arise within or on the surface of a bone in which the neoplastic cells synthesize and secrete the organic components of bone matrix, which may or may not undergo subsequent mineralization. Neoplastic cells have the phenotype of osteoblasts…

Mesenchymal Tumors of Miscellaneous Type or Uncertain Lineage

A significant number of soft tissue tumors either show no clear line of differentiation or appear to differentiate along the lines of a cell type with no normal counterpart. Some of these tumors, for example myxoma, appear to show principally fibroblastic differentiation, but have not traditionally been included among the fibroblastic tumors of soft tissue. Others, such as extraskeletal myxoid chondrosarcoma, have historically been considered to…

Undifferentiated Small Round Cell Sarcomas of Bone and Soft Tissue

Introduction Undifferentiated sarcomas – tumors for which the cell of origin and/or line of differentiation is unclear – are often subdivided based on their predominant cell shape (e.g., round cell, spindle cell, pleomorphic); while convenient, this is admittedly imprecise since many tumors contain overlapping morphologies. Small round cell sarcomas constitute a vast and heterogeneous subset of these undifferentiated neoplasms. This chapter focuses on Ewing sarcoma and…

Tumors of Synovial and Tenosynovial Tissue

Introduction The synovium is one of the more recent phylogenetic developments of the vertebrate locomotor system, with the first synovial joints appearing in the piscine jaw of ancestors of the modern lungfish. Embryologically, the synovium is derived from specialized mesoderm termed interzonal mesenchyme, which differentiates from the primitive mesenchyme that lies between and connects the developing mobile cartilaginous anlage of the skeleton ( Fig. 13.1 ). In…

Matrix-Forming Tumors of Soft Tissue

The somatic soft tissues do not contain mature osteoblasts or chondrocytes, and these tissues do not typically contain bone or cartilage. However, under certain circumstances, undifferentiated mesenchymal cells are stimulated to differentiate into these specialized cells. This results in a variety of bone and cartilage matrix-producing tumors. These span the spectrum from previously considered reactive “quasi-tumors” through benign to frankly malignant neoplasms. This chapter deals with those soft tissue lesions that frequently…

Nerve Sheath Tumors

Nerve sheath and neuroectodermal tumors are derived from the neuroectoderm or neural crest tissue of the developing embryo. A normal nerve consists centrally of axons, surrounded by Schwann cells ( Fig. 11.1 ). The connective tissue of the axons, including the Schwann cells, fibroblasts, mast cells, capillaries, and collagen, is called the endoneurium. Groups of these axons with their Schwann cells are surrounded by perineurium, comprising a…

Vascular Tumors of Soft Tissue and Bone

Reactive Vascular Proliferations Papillary Endothelial Hyperplasia Clinical Features Papillary endothelial hyperplasia (PEH) may occur in any location, but most commonly presents in the superficial soft tissues of the extremities, head, and neck. The thumb is a common location. A history of trauma is occasionally present. PEH may be superimposed on a pre-existing benign vascular lesion, such as a cavernous hemangioma. Pathologic Features Gross Findings PEH typically…

Gastrointestinal Stromal Tumor

Clinical Features Symptomatic gastrointestinal stromal tumors (GISTs) affect predominantly middle-aged to older adults, at a median age of 60 years. They are slightly more common in men. Approximately 60% arise in the stomach, and 30% arise in the small intestine; the other 10% arise in the esophagus, colon, rectum, gallbladder, appendix, and rarely in abdominal soft tissues, involving the omentum, mesentery, retroperitoneum, or pelvis, collectively known as…

Tumors of Perivascular Cells

Introduction This chapter will cover two groups of tumors thought to recapitulate features of normal perivascular myoid cells, the myopericytoma family and the glomus tumor family. Although thought at one time to be related, recent molecular genetic evidence suggests different pathogenesis for myopericytic and glomus tumors. Nonetheless, there is morphologic overlap between members of these families, and their discrimination may at times be subjective. Tumor of…

Skeletal Muscle Tumors

Mesenchymal tumors that show skeletal muscle differentiation are rare, and as opposed to other soft tissue tumors, the benign variants (rhabdomyoma) are much rarer than their malignant counterpart (rhabdomyosarcoma). Nevertheless, rhabdomyosarcomas are an important category because they represent the largest subgroup of sarcomas in children. In addition, thanks to more reliable use of immunohistochemistry, it has become clear that rhabdomyosarcomas are not so rare in adulthood…

Smooth Muscle Tumors

Smooth muscle neoplasms are categorized by their site of origin; smooth muscle tumors with similar histologic features in different sites vary in terms of their prevalence, clinical behavior, and diagnostic criteria for malignancy. For example, benign smooth muscle tumors (leiomyoma) are common in the uterus, but are rare in the deep soft tissue. Most smooth muscle tumors arise in association with pre-existing smooth muscle (e.g., pilar,…

Adipocytic Tumors

Adipocytic Tumors There exist a relatively large number of adipocytic tumors, spanning a clinical spectrum from very common benign lipomas, to relatively common atypical lipomatous tumors of borderline malignancy and finally a variety of aggressive and rare liposarcomas. Over the past several decades, the classification of fatty tumors has undergone considerable refinement, as it has become clear that specific genetic aberrations are “entity-defining” for most adipocytic…

Fibrohistiocytic Tumors

The group of so-called fibrohistiocytic tumors encompasses those tumors that were formerly believed to show evidence of hybrid fibroblastic and histiocytic differentiation. In truth, the neoplastic cells in these lesions likely represent either modified fibroblasts/myofibroblasts, or cells of histiocytic lineage, but not both. The hybrid appearance of these lesions originates from the variable tumor microenvironment, in which non-neoplastic monocytic cells infiltrate fibroblastic tumors or fibrosis that…

Fibroblastic and Myofibroblastic Tumors

Introduction Tumors with myofibroblastic and fibroblastic differentiation comprise an ever-expanding and diverse group of largely spindle cell neoplasms. Some are characterized by recurrent gene fusions, resulting in chimeric transcription factors that initiate proliferative programs driving tumorigenesis and fibrous phenotype, while others have gene rearrangements, resulting in the upregulation of growth factor signaling pathways. A subset of malignant fibroblastic tumors is characterized by complex genomic copy number…