Radiology of Soft Tissue Tumors Including Melanoma

Soft tissue tumors are defined as mesenchymal proliferations that occur in the extraskeletal, non-epithelial tissues of the body, excluding the viscera, coverings of the brain and lymphoreticular system [1]. The true frequency of soft tissue tumors is difficult to estimate because most benign lesions are not removed. A conservative estimate is that benign tumors outnumber their malignant counterparts by a ratio of at least 100:1. In the United States only 7,200 sarcomas are diagnosed annually (0.8 percent of invasive malignancies), yet they are responsible for 2 percent of all cancer deaths, reflecting their lethal nature [1]. Classification is based on the tissue from which the lesions arise (Table 16.1). The cause of most soft tissue tumors is unknown. There are documented associations between radiation therapy [2], and rare instances in which chemical burns, heat burns or trauma were associated with subsequent development of sarcoma [1]. Soft tissue tumors may arise in any location, with approximately 50 percent in the extremities (two-thirds of these in the lower extremities), 14 percent in the retroperitoneum, 15 percent in the viscera, 10 percent in the trunk and 11 percent in other sites [3]. Regarding sarcomas, males are affected more frequently than females (ratio 1.4:1), and the incidence generally increases with age. Fifteen percent arise in children and constitute the fourth most common malignancy in this age group [1]. Specific sarcomas tend to appear in certain age groups (e.g., rhabdomyosarcoma in children, synovial sarcoma in young adults and liposarcoma and malignant fibrous histiocytoma in mid- to late adult life) [1].