Lipofibromatosis: report of a rare paediatric soft tissue tumour
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- Deepti, A.N., Madhuri, V., Walter, N.M. et al. Skeletal Radiol (2008) 37: 555. doi:10.1007/s00256-008-0458-z
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The clinical, radiological and pathological features of a case of lipofibromatosis, a rare paediatric soft tissue neoplasm, are described. The tumour involved the foot of a male infant and was present at birth. Magnetic resonance imaging showed a lipomatous mass, with splaying of muscles of the sole by lobules of fat. Histopathological examination revealed typical findings of an admixture of mature adipose tissue and fibroblastic elements. The radiological and pathological features helpful in differentiating this entity from other fibro-fatty paediatric soft tissue tumours is discussed, and the relevant literature is briefly reviewed.
Lipofibromatosis (LF), a rare paediatric soft tissue neoplasm, was recently described by Fetsch et al. . It presents as a poorly demarcated mass involving the subcutis or deep soft tissues and has a predilection for the distal extremities [1, 2]. Histologically LF, as its name suggests, is composed of an admixture of adipose tissue and fibroblastic elements [1, 2]. Although completely benign, it has a high rate of local recurrence after surgical removal, probably because of its poorly defined and infiltrative margins. Although the original clinical and pathological descriptions are comprehensive, documentation of the imaging findings in this entity is sparse. These findings are addressed in this case report of a lesion in the foot of an infant.
A 10-month-old boy presented with progressive swelling and deformity of the left foot. This had been present since birth, but, apart from this, there was no significant pre- or peri-natal history. The child had been born at term by caesarean section.
Marginal excision biopsy of the plantar aspect of the mass was performed. At surgery, there was diffuse fatty overgrowth of the sole, without capsulation. The lipomatous tissue collapsed during biopsy.
Lipofibromatosis is a fibro-fatty tumour of childhood that has been recently delineated as a distinct entity from other forms of fibromatosis and fatty tumours in children [1, 2]. There is a 2:1 male predominance . Typically, it forms an ill-defined, slow growing, painless mass in the hand or foot, but it may rarely involve the thigh, trunk or head . The lesion has been documented to occur from birth to early in the second decade of life. LF can also be congenital, as in the present case. In the series reported by Fetsch et al., eight tumours were noted at birth . The size varies from 1–3 cm, rarely exceeding 5 cm , but an unusually large tumour with a diffuse pattern of involvement from palm to axilla has been documented .
The MR features noted in this case were a TI and T2 hyperintense lipomatous mass, with thinning and splaying of muscle fibres by fat lobules but without hypertrophy or erosion of underlying bone. In contrast, in the paper by Teo et al., a largely T1 isointense and moderately T2 hyperintense mass, with heterogeneous enhancement and extensive bony erosion, was seen . Fat was the major component of the lesion in our patient, unlike the case described by Teo et al., where there were only occasional intra-lesional signal changes consistent with a fatty component . In the case of LF involving the forearm reported by Sasaki et al., MRI demonstrated a multi-lobular mass with high signal intensities on both T1-weighted and T2-weighted images, leading to consideration of intramuscular lipoma, lipoblastoma, and other types of lipomatous tumours in the radiological differential diagnosis .
On gross examination LF is yellowish or tan-white in colour, with a rubbery, firm consistency and irregular, poorly defined margins . The microscopic features in our case were in line with those previously described, i.e. abundant adipose tissue traversed by streaks of fibroblasts . The occasional small collections of univacuolated cells seen juxtaposed between the fibroblastic component and mature adipocytes may represent degenerating adipocytes or lipid-rich fibroblasts . Mitotic activity was not seen in this instance but was documented in approximately one-third of the cases reported by Fetsch et al. . Immunohistochemically, the cells may exhibit positivity for CD34, bcl-2, S-100 protein, CD99 and actins [1, 2]. However this is variable and none is specific.
LF should be differentiated from other adipocytic and fibroblastic/myofibroblastic tumours in the paediatric age group. These include lipomatosis of nerve, infantile fibromatosis, fibrous hamartoma of infancy (FHI), lipoblastoma and calcifying aponeurotic fibroma (CAF). Despite the confusingly similar terminology, LF and lipomatosis of nerve are different entities. Like LF, the latter may be congenital and also occurs in the distal extremities of infants and young children . However, it can be differentiated from LF by its characteristic presentation as fusiform enlargement of a nerve, typical appearance on MRI (serpiginous low-intensity structures on all sequences, surrounded by fat resembling a co-axial cable), and the microscopic demonstration of splayed-out nerve bundles in the lesion [5, 6]. In addition, associated macrodactyly, seen in a third of cases of lipomatosis of nerve, is not a feature of LF .
Infantile fibromatosis usually involves the muscles of the head and neck, shoulder, upper arm and thigh , whereas LF tends to involve the hands and feet. According to Ahn et al., the MR image of infantile fibromatosis is that of an enhancing mass that is largely hyperintense on T2-weighted images, with areas of high signal intensity that correspond to variable degrees of cellularity, collagenisation or myxoid change . Microscopically, infantile fibromatosis can be classified into diffuse (mesenchymal) and desmoid types, with the former characterised by small round or oval cells in a myxoid background associated with lipocytes, residual muscle fibres, and fascicles of fibroblasts. The lipocytes are probably the result of fatty proliferation secondary to atrophy of infiltrated muscle  and are not an integral component of infantile fibromatosis, unlike in LF . This fatty component can cause hyperintense areas on T1- and T2 weighted MR images . The desmoid type is composed of purely fibrocellular areas  and, hence, is radiologically and histologically easier to distinguish from LF.
Fibrous hamartoma of Infancy is a benign, poorly circumscribed, soft tissue growth presenting during the first 2 years of life and characteristically affecting the axilla, inguinal region and proximal extremities . Only exceptionally does it involve the hands and feet. Microscopically, apart from a fibro-fatty component, it also shows foci of primitive mesenchymal cells and neurofibroma—like fibrosis both of which are not features of LF . On MRI, the fibrous component of FHI has a relatively consistent low signal intensity, whereas the fat has a high signal intensity on T1- weighted sequences that slightly decreases on T2- weighted sequences, the overall appearance varying depending on the relative proportions of the different tissue components . The imaging features can overlap with LF in cases of FHI with predominance of fat.
Lipoblastoma is another soft tissue tumour of infancy and early childhood involving the extremities . Under the microscope lipoblastoma differs from LF by the presence of lobulation and immature adipose tissue, and the absence of a fibroblastic component [1, 11]. Variants of lipoblastoma lacking in lobularity and circumscription (lipoblastomatosis), or with increased septal cellularity, can be mistaken for LF histologically . An overwhelming preponderance of fat and areas of myxoid stroma in lipoblastoma can be discriminatory under these circumstances [1, 11]. The imaging appearance of lipoblastoma varies, depending on the extent of fat versus myxoid stroma. In older children fat predominates , and this results in MR findings similar to those of LF. In infants the prominence of myxoid matrix gives a low signal intensity on T1-weighted sequences and high signal intensity on T2-weighted sequences .
Calcifying aponeurotic fibroma, in its fully developed form, can be differentiated from LF by its foci of mineralisation. Early lesions of CAF lack this feature but can still be distinguished from LF by the absence of a fatty component .
LF has a high rate of non-destructive local recurrence, when incompletely excised, but no metastatic potential. As a result, complete surgical removal is the preferred therapeutic option if it can be accomplished with minimal morbidity . Congenital onset, male gender, mitotic activity in the fibroblastic component and incomplete excision may be risk factors for recurrence .
In summary, our experience with this case has led us to conclude that lipofibromatosis should be considered in the clinical and radiologic differential diagnosis of benign fibro-fatty tumours of children, particularly those of the hands and feet. The radiological features of LF, while suggestive, are not entirely specific, but histological examination should confirm the diagnosis in most cases.