Clinical History
The patient presented with left knee pain of several months' duration and a popliteal palpable mass.
Imaging Findings
The patient presented with left knee pain of several months' duration and a popliteal palpable mass. She had a previous history of subcutaneous lipomas.
MRI of the knee was performed using an MR scanner (0.2T) with dedicated coil. The examination consisted of T1-weighted spin-echo (SE), T2*-weighted gradient-echo (GE) and GE-STIR sequences, performed in the axial, sagittal and coronal planes. The study revealed an intramuscular oval encapsulated lipomatous mass (6cmm x 2cm x 3.5cm), located deep within the femoral biceps. Within the mass, multiple septae were observed. The appearance suggested an intramuscular lipoma.
The patient underwent a complete surgical excision of the mass. The final histological diagnosis was of intramuscular lipoma.
A year later the patient complained of elbow pain and MRI showed a lipomatous septate formation in the brachial muscle with well-defined borders (4cm x 2cm x 2.5cm), contiguous with the radial nerve.
Discussion
Lipomas are the most frequently occuring soft-tissue tumours and they usually appear in the fifth or sixth decade of life; they are homogeneous and clearly marginated, with or without internal fibrous septations. Rarely (in 5% of cases) lipomas are multiple.
They may be divided into subcutaneous or superficial and deep-seated (i.e. intramuscular, mediastinal, retroperitoneal). Deep lipomas and multiple lipomas are more frequent in males.
Intramuscular lipomas are benign and relatively rare. Histologically they may be divided into two types: well-circumscribed and infiltrative. In the former, fatty tissue is clearly delineated from the surrounding muscle, whereas in the latter there is replacement of the muscle tissues in a bland fashion by lipocytes, while longitudinal sections demonstrate a striated appearance of muscle fibres caused by the proliferation of fat cells.
Growth is slow or very slow; in deep locations they grow slowly but steadily up to very large volumes and they may cause compression on a nerve (e.g. the deep branch of the radial nerve in the forearm), manifesting progressive nerve palsy.
On MRI, lipomas have bright signal intensity on T1-weighted images and do not increase in signal intensity on T2-weighted sequences; on STIR images the fat signal is nulled. Low signal intensity septations may occur within these lesions on T1- and T2-weighted images.
Marginal excision is curative; recurrence is rare but more frequent in deep lipomas and in intramuscular ones in which removal may have been incomplete.
D
Differential diagnosis is with well-differentiated liposarcoma.
Differential Diagnosis List
Multiple intramuscular lipomas detected by MRI
Final Diagnosis
Multiple intramuscular lipomas detected by MRI
