Leiomyosarcoma is a malignant neoplasm showing pure smooth muscle differentiation ([12], Fourth revision). Extrauterine leiomyosarcomata typically occur in adults [8].

There are four main subtypes, including cutaneous, subcutaneous, deep soft tissue, and vascular leiomyosarcoma. Cutaneous leiomyosarcoma has a far better prognosis than the subcutaneous type.

Deep soft tissue leiomyosarcomata mostly occur in the retroperitoneum (usually in the perirenal space) and only in 12–41 % in the extremities (particularly in the thigh) [7, 32, 38]. Of note, bone leiomyosarcoma is a distinct and rare entity, which is not addressed in the WHO classification of soft tissue tumors and bone ([12], Fourth revision). Deep soft tissue leiomyosarcomata in the retroperitoneum present at a more advanced stage than vascular leiomyosarcomata, which is likely because first mentioned have more space to grow before causing symptoms [7].

Vascular leiomyosarcomata arise in large vessels, most commonly in the inferior vena cava or lower extremity veins. The clinical findings depend on the involved part of the vein and the extent of the lesion. In the inferior vena cava, upper-segment involvement may manifest as Budd-Chiari syndrome, middle-segment involvement as nephrotic syndrome, and infrarenal involvement as edema in the lower extremity [34]. The lesions may cause vascular obstruction and thrombosis [4, 23].

Cutaneous and subcutaneous leiomyosarcomata usually present as well-defined, relatively small, homogeneously enhancing tumors, often associated with fascial edema [16] (Fig. 15.2). Deep soft tissue and vascular leiomyosarcomata show similar imaging features [7, 16] and mostly present as large, heterogeneous masses with variable signal intensities, central necrosis, and marked peripheral and septal enhancement [44] (Figs. 15.3, 15.4, 15.5, and 15.6). Intralesional calcifications may be detected in a minority of leiomyosarcomata [5]. Secondary bone involvement is seen in up to 10 % [41, 44] (Fig. 15.6). Vascular leiomyosarcomata exhibit three main growth patterns: extraluminal (62 %), intraluminal (5 %), and combined extra- and intraluminal (33 %) [19]. Leiomyosarcoma of the inferior vena cava can be classified into three groups according to level affected: segment I, infrarenal; segment II, inter- and supra-renal up to but not including the main suprahepatic veins; and segment III, suprahepatic with possible intracardiac extension [22]. Intravascular leiomyosarcoma may be confused with deep vein thrombosis at US [8]. Both MR imaging and CT are useful for differentiating intravascular leiomyosarcoma of the inferior vena cava from a simple thrombus and for staging [34].