Aggressive fibromatosis in the popliteal fossa

A 34-year-old female patient presented with a 2-year history of a slowly growing left popliteal mass. She had noticed posterior left knee dull pain and decreased range of motion of the knee. He also had experienced pain which increased with movements of the knee.

Left popliteal fossa ultrasound showed a solid and predominantly hypoechoic mass with a macrolobulated shape and a well-circumscribed margin. Colour Doppler mode showed the mass with absent flow (Fig 1).
Axial and sagittal T1-weighted magnetic resonance images (MRI) through left knee showed low-signal-intensity and large, nodular, fusiform, infiltrating mass arising within popliteal fascia (Fig 2). Superiorly this lesion blurred fat plane and contacting with common fibular nerve. It showed heterogeneous signal intensity on T2-weighted (Fig 3a) and Spectral Attenuated Inversion Recovery (SPIR) images (Fig 3b), with high signal areas interspersed with low-signal-intensity bands. There is avid contrast-enhancement predominantly within peripheral of the lesion (Fig 4-5). On dynamic contrast-enhanced MRI it shows a type 2 pattern as a predictor of benign lesion histology (Fig 6) [1]. The mass showed mild restricted diffusion (Fig 7).
The surgical team decided to perform resection of the mass. Histopathological examination revealed an aggressive fibromatosis.

Fibromatosis is a rare mesenchymal tumour histologically characterised by proliferation of fibroblasts and myofibroblasts with marked production of intercellular collagen. It comprises a broad group of fibrous tissue proliferations of similar histologic appearance that has biologic behavior intermediate between that of benign fibrous lesions and fibrosarcoma. The term “desmoid tumour” is usually used by some authors and it is synonymous with this type of fibromatosis [2].
The aggressive fibromatoses are known to occur more frequent in female youth, mainly in the 15-to 40-year-old age group.
According to the literature, fibromatosis can be divided into two groups : superficial and deep. The deep category is usually more aggressive and relapses after surgery [3].
We as radiologists, must be familiar with this disease and consider its presence within soft tissue tumours. Although the definitive diagnosis is histopathological, the main purpose of cross sectional imaging modalities is in planning the surgery and postsurgical follow-up.
As with other types of fibromatosis, on ultrasound it shows hypoechoic nodular lesions, with well-defined borders and a fusiform morphology [4].
On MRI most fibromatoses appear homogeneously isointense or mildly hyperintense on T1-weighted image (WI). They appear as a high heterogeneous signal on T2-WI and STIR. These hyperintense areas represent more the cellular areas of the tumour, while the hypointense areas correspond more to fibrous areas with collagen [5].
Aggressive fibromatosis usually shows enhance and uptake contrast areas are typically areas with more collagen.
Diffusion coefficients of benign soft tissue masses were assessed in one study and fibromatosis showed very low true diffusion coefficients similar to diffusion values of malignant soft-tissue tumours, this may due to different proportions of spindle-shaped cells [6].
The managment of aggressive fibromatosis must be addressed by multiple specialities. Surgery is may effective, most in infiltrative cases, also radiotherapy can be effective in these cases [7]. Also pharmacological treatment such as intralesional steroid injection in patients who had recurrence of fibromatosis and inmmunosuppressive drugs could result in improved control of the disease [8].
There is a strong tendency toward local recurrence; however, these lesions rarely metastasise.