Fig. 1
Infantile hemangioma. 10-month old boy with an enlarging mass on the proximal left arm. (a) Coronal T1-weighted image shows a lobulated, relatively well-defined mass in the superficial soft tissues of the proximal arm. There is a vessel with a flow void inferior to the mass indicating a fast-flowing vessel. (b) Gadolinium-enhanced T1-weighted image shows diffuse enhancement of the mass. There is a vessel with flow void seen within the substance of the lesion. (c) Image from a time-resolved MR angiogram shows that there is a major feeding arterial vessel and that there is very early enhancement of the mass
Kaposiform hemangioendothelioma is the other important tumor, often presenting clinically with Kassabach-Merritt phenomenon, and characterized by a large soft tissue mass which is infiltrative, has fast-flowing vessels, although less than infantile hemangiomas, and shows no involution. It can be congenital and on imaging show poorly defined margins. Suggestive findings include subcutaneous stranding and hemosiderin deposition, but the diagnosis requires biopsy.
Vascular Malformations
Vascular malformations are disorders of endothelial morphogenesis. They are congenital lesions and do not proliferate or have a soft tissue mass. The most common, venous and lymphatic, do not have fast flowing vessels.
Venous malformations are present at birth but may not become apparent until childhood. They contain endothelial-lined large vascular spaces or varicosities, containing thrombi. Unlike hemangiomas, they are as frequent in the limbs as in the head and neck [6] (Fig. 2). They do not contain fast flowing vessels, but often the vascular spaces are interspersed with fat.
Fig. 2
Synovial venous malformation in a 13-year old girl with episodes of joint swelling. (a) Sagittal 3D gradient echo (DESS) image shows a multilobular mass high SI mass occupying a significant portion of Hoffa’s fat pad and abutting the articular cartilage. Small round low SI structures correspond to phleboliths or small thrombi. (b) Coronal gadolinium-enhanced T1-weighted image shows diffuse enhancement of the mass indicating that it is a venous malformation
Lymphatic malformations may contain large cystic spaces or be microcystic. Seventy to 80% occur in the head and neck [7] and usually are detected in the first years of life. The macrocystic variety is most commonly imaged and shows multiple fluid filled spaces. The contents vary in echogenicity and signal characteristics, and can often be highly proteinaceous and hemorrhagic, thus showing increased echogenicity and high SI on T1-weighted images and large fluid-levels. The lesions can involve multiple anatomic spaces, and, in particular, cervical lesions can have a significant mediastinal extension.
Imaging Approach to Vascular Lesions
The first step is to evaluate for fast-flowing vessels, seen as spaces with arterial waveforms on US and flow voids on MRI. If there are fast flowing vessels, is there are soft tissue mass? If the answer is yes, this constitutes a tumor, likely a congenital hemangioma. The tumoral component is typically echogenic and enhancing. If there is no mass, it is likely to be a malformation with an arterial component such as an arterio-venous malformation or fistula. If there are no fast-flowing vessels, it is paramount to determine whether the vascular spaces contain blood, indicated by a venous waveform on sonography and enhancement on contrast-enhanced MRI. Enhancing lesions are usually venous malformations; non-enhancing lesions are typically lymphatic malformations. Other characteristics suggesting venous malformation include thrombi or phleboliths within the vascular spaces, seen as low SI on water sensitive sequences, and multiple fluid levels.
Myofibroblastic Tumors
Fibroblastic and myofibroblastic tumors account for slightly more than 10% of soft tissue tumors in children [8]. Although they represent a wide variety of lesions, they all share a proliferation of fibrous tissue with extension along the fascial planes. They are usually echogenic, of low or intermediate SI on T2-weighted images. They present in different anatomic locations and age groups and can have peculiar characteristics such as a large mass with deformity of the adjacent bone (infantile fibrosarcoma), multiple subcutaneous or intramuscular lesions (infantile myofibromatosis), plaquelike pattern of growth pattern (Gardner fibroma), or presence of fat (lipofibromatosis). I will focus on those types that are unique to children.
Cranial Fasciitis
Although nodular fasciitis is the most common fibroblastic tumor, it is more often seen in adults than children. In newborns and young infants, cranial fasciitis, a form of nodular fasciitis, presents aslow SI proliferations of fibrous tissue in the scalp. In cranial fasciitis, imaging shows lesions of 1–2 cm in diameter with well-defined margins and a fascial tail.
Infantile Myofibromatosis
Infantile myofibromatosis is the most common benign fibrous tumor in infancy. It often presents in the neonatal period and almost always in the first two years of life. It can be unifocal, more often in the head and neck, or multicentric, involving the skin, subcutaneous tissues, muscle and bone. One distinguishing characteristic is central necrosis, which results in a hypoechoic center which has high SI on water sensitive images and peripheral enhancement after contrast administration. When there is associated bony involvement, the lesions are metaphyseal or metadiaphyseal, lytic, well-circumscribed.
Fibromatosis Colli
Fibromatosis colli is a selective fibrous proliferation of the sternocleidomastoid muscle presenting in the first two months of life. It is more common in the right side, may be associated with birth trauma, and results in torticollis and a palpable neck mass. The US appearance is characteristic, with the sternocleidomastoid being replaced by a fusiform echogenic mass.
Desmoid Fibromatosis
Desmoid Fibromatosis is the most common pediatric fibrous tumor [8]. There are two peaks, one in childhood around 5 years of age, and the other in early adulthood. Lesions are locally aggressive and often recurrent, but not metastasizing [8, 9]. Localized forms are more common than multifocal, and the lesions occur most often in the extremities and trunk. Although there is abundant fibrous tissue, the signal characteristics are complex, with areas of low SI intermixed with high SI on T2-weighted images. Desmoid can present in isolation or be part of Gardner’s syndrome.
Myositis Ossificans
Myositis ossificans, a post-traumatic lesion occurring often in the second decade of life, has three phases: an early proliferative phase, a phase of early calcification (seen 4–6 weeks after trauma) and a phase of late calcification after. Unlike soft tissue sarcomas, the calcification in myositis ossificans is always better defined in the outside. Imaging, particularly MRI during the early phase can be confusing as the lesion is very heterogeneous [8, 9].
Congenital Infantile Fibrosarcoma
Congenital fibrosarcoma (Fig. 3) presents as a large mass in the extremity or trunk either at birth or during the first months of life. Radiographs show elongation, bowing or attenuation of the bone adjacent to the tumor without focal destruction. The masses are heterogeneous and very vascular and are often confused with a hemangioma or vascular malformation [10]. Congenital fibrosarcoma has a more favorable prognosis that its adult counterpart.
