A 54-year-old woman, with no medical history of interest, was referred to the plastic surgery department for a condition of 2 months' duration that started with pain and paraesthesia in the right leg. In the last few weeks she had worsened, presenting with foot drop and difficulties in walking. The patient's symptoms were suggestive of right sciatic nerve neuropathy.
Physical examination revealed an increase in size on the posterior aspect of the right thigh and palpation of a large, deep, soft and painful tumour.
The patient's symptoms and physical examination suggested a neuropathy of the sciatic nerve, probably due to extrinsic compression by the tumour.
The following complementary studies were performed to confirm the diagnosis:
- Magnetic resonance imaging (MRI): a mass with maximum diameters of 14 x 9 x 5 cm (craniocaudal x transverse x anteroposterior) was visualised deep in the posterior compartment of the distal half of the right thigh, mostly with a signal similar to fatty tissue, although there were some septa and micronodular images with intermediate signal intensity in T1 and high in STIR and T2-SPIR. This mass subsequently displaced the sciatic nerve, findings which, although they could be related to a lipoma, also suggest the need to rule out a low-grade liposarcoma.

- Electromyography (EMG): compatible with compression of the sciatic nerve at the level of the distal third of the right thigh.
Given the patient's symptoms and the finding of a tumour in the MRI, it was decided to carry out surgical treatment of the tumour.
We performed surgery under spinal anaesthesia, resecting the tumour and releasing the sciatic nerve at the level of the tumour. The patient did not present any postoperative complications.

The anatomopathological study of the surgical specimen revealed macroscopically a large tumour of lipomatous appearance, yellow, rounded, well delimited, 12 x 10 x 4 cm in size and weighing 266.7 g, with no haemorrhagic or necrotic areas on section.

Microscopically it is mature adipose tissue, separated into lobules by fine fibrous tracts and numerous small vessels at the periphery. The adipose cells show no atypia or mitosis.
The patient's evolution was satisfactory, with a gradual decrease in pain and paraesthesia and a gradual recovery of muscle strength to normal. After a postoperative follow-up of 15 months, the patient is asymptomatic, with no paresthesia, no pain, no gait disturbance and no evidence of recurrence.