In a minority of cases, part or all of the tumor may exhibit a radiodensity sufficiently different to that of water for the tumor to be visualized directly. Only fat and gas will give a radiodensity less than that of muscle. Lipomas, the commonest of all the soft tissue tumors, produce a low radiodensity between that of muscle and air. For this reason lipomas are well demarcated from the surrounding soft tissues and can be diagnosed with moderate confidence [21] (Fig. 2.2). It should be noted that low-grade liposarcomas may contain variable amounts of lipomatous tissue, which also appears relatively radiolucent on radiography (Fig. 2.3). Air in the soft tissues is said to be specific to infection [30]. While infection is certainly the commonest cause, it may also be seen in necrotic fungating tumors, albeit with secondary infection (Fig. 2.4), as well as being a normal feature following open biopsy or other surgical procedures. Air in the soft tissues of the thoracic wall and neck always suggests the possibility of surgical emphysema.

Increased radiodensity may be seen in the tissues due to hemosiderin, calcification, or ossification. Hemosiderin deposition typically occurs in synovial tissues exposed to repeated hemorrhage, such as (tenosynovial) giant cell tumor, pigmented villonodular synovitis (see Chap. 15), and hemophilic arthropathy. Radiographs can distinguish between calcification and ossification and the differing patterns [54]. Mineralization in the soft tissues is a feature of a large spectrum of disorders including congenital, metabolic, endocrine, traumatic, and parasitic infections [38]. Primary soft tissue tumors are one of the less common causes of calcification that the general radiologist can expect to see in his or her routine practice. Close attention to the clinical details and location will exclude many of the nonneoplastic causes. For example, soft tissue calcifications in the hands and feet are rarely associated with neoplasia, and many of the multifocal lesions will be either due to a collagen vascular disorder or the residuum of a parasitic infection. Again, the clinical details and country of origin of the patient should be pointers to the correct diagnosis. Occasionally certain normal variants, including companion shadows and the fascia lata, may simulate soft tissue calcification or periosteal new bone formation and should not be mistaken for a neoplastic process [23].

Analysis of the pattern of calcification within a soft tissue tumor can indicate the tissue type. Circular foci with a lucent center representing a phlebolith, when identified outside the pelvis, is diagnostic of a hemangioma (Fig. 2.5). Phleboliths are not usually apparent until adolescence, so that conditions such as Maffucci syndrome (Fig. 2.6) in the child may not be radiographically distinguishable from multiple enchondromatosis (Ollier disease).

Chondroid tissue reveals ring-and-arc calcification. While this does not distinguish between benign or malignant cartilage formation, the majority of soft tissue masses with this feature, in the vicinity of a joint, will arise from synovial chondromatosis (Fig. 2.7) [37] and in the hands and feet will be soft tissue chondromas. Calcification or ossification in the infrapatellar (Hoffa) fat pad is typical of a para-articular chondroma (Fig. 2.8) [17].

Osteoid mineralization may occur as “cloud-like” densities or mature trabecular bone. The latter suggests a slow-growing lesion such as a lipoma, low-grade liposarcoma, or hemangioma [30]. Poorly defined, amorphous calcification is found in up to 30 % of synovial sarcomas (Fig. 2.9) [6, 18, 31] and approximately 50 % of extraskeletal osteosarcomas (Fig. 2.10). This is an extremely useful distinguishing feature from the tumor-mimicking myositis ossificans, which exhibits marginal calcification (Fig. 2.11) [13, 34]. Another traumatic condition that presents with a peripherally mineralized mass and platelike configuration of calcification along the course of muscle, almost exclusively in the calf, many years after a major injury, is calcific myonecrosis (Fig. 2.12) [9, 22].