Lipomatous tumors

Abstract

Histologically adipose tissue is classified into two main types: white fat (lipocytes), widely distributed through the body, and brown fat, which is more common in rodents and hibernating animals. Brown fat is considered an immature form of the white fat and is chiefly found in the interscapular region, neck, mediastinum, axilla, and retroperitoneum [46, 69]. Lipomatous tumors are common mesenchymal lesions that can be both benign or malignant. There is a wide range of histological subtypes, the lipoma being the most frequent of them. Some tumors, including lipoma and some types of liposarcoma,will display typical features which allow a specific diagnosis to be made [85, 86, 144]. In the following discussion, only the subdivisions of lipomatous tumors that arise in the soft tissues of the body are considered. The role of magnetic resonance imaging (MRI) for the study and evaluation of these common tumors is emphasized throughout this chapter. The superiority of this technique stems from its exquisite sensitivity in displaying minor differences in tissue composition. Many investigators have reported MRI to be diagnostically valuable in distinguishing fat-containing tumors from other tumors, because of its ability for detecting fatty components [21, 75, 89]. On plain radiographs, lipomatous tumors may not be apparent or can be detected as a mass of increased density or of fatty opacity. On ultrasound scans, these tumors can show different patterns of echogenicity: they are mostly isoechoic or hyperechoic, although a hypoechoic pattern may be also encountered [1, 54]. Using computed tomography (CT), diagnosis of a lipomatous tumor can be straightforward when typical low attenuation values are found (65 to 120 HU). On MR images, the signal intensity (SI) of fatty tumors tends to be equal to that of subcutaneous fat on all pulse sequences, including those obtained with fat suppression: short-tau inversion recovery (STIR) or chemical shift pulse sequences [40, 90, 89, 145, 144, 155] (Fig. 15.1).