Teaching Case

Growing, soft, painless fatty mass at the left thoracic wall.

A 11-month-old male presented with a history of a growing mass at the left thoracic wall, incidentally discovered by the patient's parents two weeks previously. The mass was soft and painless on palpation. It was located above the left breast and extended from the sternum to the left axillary space. Physical examination was otherwise normal. A chest radiograph and an abdominal ultrasound were unremarkable. Ultrasonography showed a circumscribed, solid, relatively homogeneous mass of intermediate echogeneity, containing echogenic septations. CT revealed an ovoid space-occupying lesion, located between the major and minor thoracic muscles, extending from the sternum to the left axillary space. The lesion was of low attenuation (-80 to -100 HU) suggestive of fatty composition and contained soft tissue septations. The muscles adjacent to the lesion appeared displaced but not invaded or atrophic and there was no evidence of intrathoracic extension or rib involvement. A few small lymph nodes were depicted at the left axillary space. The patient was operated on, the lesion was totally excised and there were no signs of recurrence one year postoperatively.

Lipoblastoma is a rare tumor composed of fetal-embryonal fat that occurs almost exclusively in infants and children, most commonly less than 3 years of age [1]. The tumor presents in two forms: a localized well-circumscribed lesion (lipoblastoma) and an unencapsulated diffuse type (lipoblastomatosis)[2]. Prognosis is excellent, despite tendency of the mass to invade locally and to recur if incompletely excised [2,3]. These tumors do not metastasize, have been shown to grow rapidly and may eventually differentiate into a mature lipoma [3]. Lipoblastomas most commonly involve the extremities [4]. Few cases located at the chest wall have been previously reported [1,4-5], mostly being of the diffuse form and thus causing difficulty in resection [5]. In the two recently reported cases of thoracic location, the amount of fat was variable but generally less than in our case and the lesions appeared more infiltrative and aggressive with muscle and bone involvement [4, 5]. Differential diagnoses of adipocytic tumors include mainly lipomas and liposarcomas. Rarely, other lesions such as myxoid tumors or hemangiomas that invade adipose tissue and/or engulf a portion of it, may also simulate a fatty lesion. Lipomas tend to have well-defined borders, exclusively fatty density, superficial location and to displace rather than infiltrate adjacent structures. Liposarcoma is the second most common soft tissue sarcoma encountered in adults but is rare in infants and young children [2]. It usually appears as a non-specific inhomogeneous mass with both soft tissue and fatty components but it may also demonstrate imaging findings similar to those of lipoblastoma. Liposarcomas are exceedingly rare in infants and children younger than 10 years of age with the myxoid variant representing the most frequent histologic subtype [2]. Histologic differentiation of lipoblastoma from myxoid liposarcoma may be difficult [2] and cell cultures from fresh tissue in order to determine the tumor karyotype have been recommended [3]. Age is an important parameter when confronted with growing, ill-defined fatty tumors that contain soft tissue components, in order to preoperatively consider a benign lesion and to avoid large mutilating procedures.