Tissue Type

Musculoskeletal tumors may originate from any mesenchymal tissue, including bone (osteoid), cartilage (chondroid), fat (lipoid), connective tissue (fibrous), or vessels (endothelium). The primary cell of origin for some musculoskeletal tumors is unknown; for example, the spectrum of Ewing sarcoma-primitive neuroectodermal tumor (PNET).8

Quite often, in cases of benign or low-grade malignant tumors where the principal architectural structure of the underlying cell type is preserved, the tumor origin can be determined with imaging because of the distinct imaging properties of each cell type. In contrast, for the moderate- and higher-grade malignancies, the imaging appearance is generally nonspecific because of alteration of the cells of origin. Plain films show only a mass or a destructive lesion of bone, computed tomography (CT) shows a density similar to muscle, and magnetic resonance imaging (MRI) shows a low to intermediate signal on T1-weighted images and a high signal on T2 images.9

Tumors that make an osteoid (e.g., osteosarcoma) or chondroid matrix (e.g., chondrosarcoma) are usually mineralized on plain film, and the calcification will almost always show on CT. Osteoid mineralization is amorphous and cloudlike, whereas chondroid mineralization is usually punctate, often in small circles and arcs (Figures 32.1, 32.2).

Osteoid

Calcified lesions can be recognized as such by the high density seen on plain film and CT. On MRI, calcification will be low signal on both T1- and T2-weighted images.10,11 Ossification, on the other hand, will show a peripheral rim of low signal similar to cortex, representing the calcified surface, and an internal signal characteristic of fat, similar to marrow.12,13

Chondroid

Cartilage lesions are usually lobulated in appearance, with typical chondroid mineralization. As a result of the high inherent water content of hyaline cartilage, the density on CT is greater than that of fat but less than that of muscle. On MRI, cartilage will show low signal on T1 images and very high signal with T2 imaging.3,14 Chondrosarcomas are often secondary tumors, arising from an underlying benign lesion, such as enchondroma or osteochondroma.15,16 Tissue sampling with percutaneous biopsy can be misleading as only a portion of the tumor may contain malignant cells. Thus, imaging plays an important role in evaluating for more aggressive behavior of the lesion, such as cortical thinning (endosteal scalloping), cortical destruction, and a soft tissue mass.17 The biopsy should then be directed toward these areas. If doubt exists, the entire lesion must be removed.18

Low-grade lesions composed of fat will appear as a lucent mass on plain film and show a density equal to subcutaneous fat on CT. The MR appearance will also follow that of subcutaneous fat, appearing high signal (white) on conventional T1 spin-echo images and low signal (black) with fat

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