A large desmoids-type fibromatosis in left deep buttock and thigh

Desmoid-type fibromatosis (DF) is a rare intermediately and locally aggressive tumor that occurs predominantly between puberty and 40 years, with female having higher incidence than male. This report describes a 48-year-old man with biopsy-proven DF in left intermuscular spatium of buttock and thigh. The mass had a wide longitudinal distribution from femoral neck level to popliteal fossa and measured about 40 cm. Plain CT showed a partially ill-defined mass with an irregular contour, with a density similar or slightly higher than skeletal muscle. On MR images, the tumor showed uneven signal in both T1WI and T2WI, mainly hyperintense to skeletal muscle and with stripe or patch-like markedly low signal. After contrast injection, the mass showed heterogeneous enhancement.