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Although malignant neoplasms of soft tissue are relatively uncommon, accounting for about 0.6% of all cancer deaths in the United States in 2010, soft tissue pathology is often considered as an overwhelming and intimidating area. Soft tissue pathology includes a wide spectrum of tumors from several supportive nonepithelial tissue types mostly of mesodermal origin (adipose tissue, smooth muscle, skeletal muscle, etc.) and the peripheral nervous tissue of neuroectodermal origin. Additionally, there are a significant proportion of tumors that cannot be classified beyond the generic term “sarcoma” despite the use of immunohistochemical tools, cytogenetic techniques, and molecular studies. The simplest way to approach soft tissue pathology is to divide the tumors into groups based on the line of differentiation of the neoplastic population. Therefore, this chapter will systematically explore the different types of soft tissue neoplasms based on lineage. Within each subset of tumors (lipomatous, smooth muscle, nerve sheath, etc.), there is a certain set of rules or principles that provide an approach to diagnostic workup. An understanding of these rules helps the pathologist to navigate through each group of tumors. The overall approach to the analysis of soft tissue masses will start with imaging studies most often using CT or MRI technology to define the location and extent of tumor and assay for the presence of potential metastatic lesions. Early biopsy using fine-needle aspiration, core needle, or open incisional biopsy will provide tissue to allow classification, histological grading, and often cytogenetic or molecular analysis. The study of chromosomal abnormalities is of particular importance in the classification of certain soft tissue tumors. For example, about 95% of Ewing sarcomas (ES) (see later) have a translocation involving the EWS gene on 22q12.


Normal Adipose Tissue Histology

Adipose tissue can be divided in to white fat and brown fat. White fat consists of spherical adipocytes containing a single lipid vacuole that compresses a crescent-shaped nucleus at the periphery of the cell (Figure 19-1A). White fat provides several key functions such as thermal insulation, mechanical protection, and storage and release of lipid/free fatty acids in response to physiologic stimuli. Brown fat has the primary function of heat production and is typically found during the neonatal period in areas such as the axilla, perirenal region, and posterior neck. The abundant mitochondria in brown fat cells along with tissue vascularity are responsible for the red-brown coloration of this tissue. Microscopically, brown fat consists of a mixture of multivaculoated, granular cells with centrally placed nuclei and univacuolated cells that appear similar to cells of white fat (Figure 19-1B).


(A) White fat. Note single lipid vesicles and peripheral nuclei. (B) Brown fat. Note the multivacuolated nature of the cells and the centrally placed nuclei. BV indicates a small blood vessel. Source: Modified from Mescher AL. Junqueira’s Basic Histology Text and Atlas. 12th ed. New York, NY: McGraw Hill ...

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