Teaching Case

A 1 year old boy presented with a soft mass, which had been growing since birth, on his left foot.

An ill-defined, soft, deeply fixed, and homogenous mass located on the dorsal and plantar surface of the foot was palpated during physical examination. His health was excellent, except for this condition.
On the antero-posterior and lateral X-ray, a mass with soft tissue density was evident under the foot (Fig 1). A sonogram showed that the mass was homogenously hyperechogenic, like fatty tissue, and deeply located.
MR showed a hyperintense mass on T1-W (Fig 2) and T2-W images and hypointense on the T2-W frequency selective fat suppressed (SPIR) images. The mass infiltrated the interphalangeal muscles but did not show invasion (Fig 3,4). The bones were normal. The diagnosis seemed to be diffuse subcutaneous lipoma, but to differentiate the mass from liposarcoma, a surgical exploration was planned.
Excision was possible where the mass was encapsulated, but the deeper side had infiltrated the muscles, where total excision was not possible.
Histologically the tumour was composed of mature adipose tissue without nuclear atypia. On the routine controls the rest of the lipoma was observed without any change in size.

Infiltrating lipomas have congenital (idiopathic) and acquired forms. The congenital form is typically seen before two years of age [1]. Exogenous and endogenous glucocorticoids, i.e., steroid therapy or Cushing’s disease, cause acquired infiltrating lipomas.

Infiltrating lipoma is excessive formation of fatty tissue that has an infiltrating growth pattern with a tendency to involve local muscles. The tumour is rarely or poorly encapsulated [2], and the muscle fibres are atrophic and dissociated. Fascia, bones, and nerves may also be infiltrated. The most common sites of involvement are the limbs and thoracic and abdominal walls. It can involve the extrapleural and extraperitoneal regions.
Congenital infiltrating lipoma of the limbs has been described in four cases in the literature [3-6]. In this report we describe a unique case of infiltrating lipoma of the foot in a paediatric case. Lipoblastoma should be included in the differential diagnosis of fatty tissue tumours in childhood as a benign soft tissue tumour. Lipoblastoma of the foot has been reported in literature [7,8].

MRI is the best imaging technique to diagnose soft tissue tumours. We were able to easily observe the homogenous fatty tissue invading the adjacent structures as a common feature of the infiltrating lipomas in all of our patients.

Just clinical examination or MRI is not reliable to differentiate lipoma, liposarcoma, and lipoblastoma. To diagnose the lipoma, histolopathological examination is usually required. Lipoblastomatosis and well-differentiated liposarcomas also have a fat tissue density [1]. Infiltrating lipomas are like normal fat. In our patients, besides histopathological confirmation, the widespread pattern of involvement with multiple sites was consistent with infiltrating lipomas.

A surgical approach is the best treatment for this tumour. But complete removal is usually difficult because these tumours infiltrate muscles or are near nerves or vessels. Subtotal excision often results in recurrence [9]. The reported recurrence rate varies from 5 to 62.5%. The recurrence rate depends on the period of follow-up and tumour location. All conclusions are from adult cases because of the limited number of paediatric cases reported.

Finally, as this case illustrates, infiltrating lipoma should be included in the differential diagnosis of fatty soft tissue tumours in childhood.