Lipofibromatosis

Not clinically relevant

Identification of fusions involving ligands (EGF, HBEGF, TGF-α) to EGFR (HER1) or EGFR itself, or other receptor tyrosine kinases (ROS1, RET, PDGFRB), suggests that activation of the PI3K/AKT/mTOR pathway might be implicated in the pathogenesis of lipofibromatosis.

The firm, rubbery, poorly demarcated mass has a white or yellow cut surface; margins are difficult to discern. Most lesions are 1–7 cm in diameter, although larger tumors have been reported, with some involving the entire extremity.

Lipofibromatosis is composed of mature adipose tissue traversed by cellular fascicles of spindle cells with bland, uniform, elongated nuclei. Fat usually forms a major proportion of the lesion. In newborn patients, the fat lobules may have an immature appearance with myxoid matrix. Small collections of univacuolated cells may be present in the fibroblastic component, where melanin-laden cells have occasionally been documented. Lipofibromatosis entraps subcutaneous tissue and sometimes skeletal muscle. Mitotic activity is rare and necrosis absent. Occasional recurrent lesions are indistinguishable from calcifying aponeurotic fibroma. By immunohistochemistry, the fibroblasts show variable expression of CD34 and SMA. The spindle cells are negative for desmin, in contrast to the spindle cells in lipoblastoma.

FNA smears show bland-appearing fibroblasts and mature fat.

Lipofibromatosis has a 70% local recurrence rate but no metastatic potential. Congenital onset, male sex, acral location, mitotic activity in the fibroblastic component, and incomplete excision may be risk factors for recurrence.

Not clinically relevant

Essential: painless subcutaneous mass; most often in the hands and feet of young children; admixture of mature fat, short fascicles of bland spindle cells, and lipoblast-like cells; infiltrative margins.