Fatty tissue gives rise to a range of lesions that can be divided into non-neoplastic lipomatous masses, lipomas and liposarcomas.

Non-lipomatous masses: These masses appear in the spermatic cord, between muscle fibers and sometimes in tendon sheaths and nerve trunks. When they occur in large areas of the trunk, and extremities, the term, lipomatosis is applied. These lesions consist of fat cells and are difficult to differentiate them from lipomas.

Lipomas: These tumors are the most common soft tissue tumors. The peak incidence of lipomas is in the 5^th and 6^th decades of life. They most frequently occur in the subcutaneous tissues of the back, shoulder and neck and less frequently in the mediastinum, retroperitoneum, within the wall of the GI tract and in the soft tissues of extremities. Most commonly occur as a single lesion, however, they may occur as multiple tumors.

Grossly, these tumors are 1-4 cm, show a delicate capsule and rarely may exhibit areas of hemorrhage and necrosis.
Microscopically, the tumor consists of fat cells which are indistinguishable from normal adult fat. Cytologic atypia is absent, however, such atypia may be seen in some subcutaneous lipomas. The tumor may have other constituents such as fibro-connective tissue (fibrolipoma), proliferating vasculature (angiolipoma) and bone marrow elements (myelolipoma). These latter tumors are commonly found in the adrenal gland and retrioperitoneum.

Liposarcomas: These tumors are uncommon and they may arise anywhere in the body. The peak incidence of the tumor is in the 5^th and 7^th decades of life. Most liposarcomas arise from the deep soft tissues such as those in the retroperitoneum and thigh and less frequently occur in the mediatinum, omentum, breast and axilla.

Grossly, they often appear as lobulated masses with permeations into the surrounding soft tissu. However, these tumors may appear as well circumscribed masses which seem to have a capsule. They range in size from 4-10 cm but may reach a massive size and weigh up to 40 kg. Their cut surface is yellow to gray-white and sometimes shows myxoid or gelatinous aeas, and areas of hemorrhage and necrosis.
Microscopically, four main different types of tumors are recognized : They include well differentiated, myxoid, round cell and pleomorphic variants. The features of these variants is shown in the table below: