It showed a well-defined 14 cm × 8 cm × 8 cm maximum diameter mass that traversed the greater sciatic foramen through the infra-piriformis area. It lay partly within the pelvis and partly within the right ischiorectal fossa and upper thigh.

The tumour was homogenously iso-intense with fat except for a small focus of low signal intensity at its superior margin. There was no abnormal signal on the STIR sequence, which demonstrated normal fat suppression.

The size and situation of the mass displaced the pelvic organs, resulting in the bladder being pushed anteriorly, the uterus superiorly and the rectum to the left. Inferiorly, the mass extended through the ischio-rectal fossa, medial to the ischial tuberosity, to come to lie in the subcutaneous fat medial to sartorius and the hip adductors (Fig. ​(Fig.22 and ​and3).3). Except for its size and deep position, there were no sinister features, although an atypical lipoma or well differentiated liposarcoma could not be excluded.

Sciatic hernias may present as a gluteal mass, as a rare cause of sciatica (following compression of the sciatic nerve) [5], or with complications of their contents. Such contents have included small bowel (sometimes leading to obstruction) [6,7], the ureter or bladder (causing urinary tract symptoms) [3,4], ovaries and fallopian tubes (causing pelvic pain syndromes) [8], colon, omentum, and Meckel's diverticulum. To our knowledge, the only previous description of a lipoma herniating through the sciatic foramen occurred in 1964 [9].

Symptomatic hernias should be surgically repaired as soon as possible (with mesh, if required), through either a transabdominal or transgluteal approach [10,11].

Well-differentiated liposarcoma accounts for about 40% to 45% of all liposarcomas [12], and therefore represents the larger subgroup of adipocytic malignancies. It tends to occur equally in the retroperitoneum or the limb, followed by the paratesticular area and the mediastinum, with a peak incidence between the fifth and seventh decades.

Well-differentiated liposarcoma is further subdivided into the lipoma-like (adipocytic), sclerosing, inflammatory and spindle cell subtypes, of which the first two are by far the commoner. Our case was an example of the lipoma-like variant of well-differentiated liposarcoma. These tumours, as they sound, are composed mainly of univacuolated adipocytes showing some variation in size and shape, associated with only scattered multivacuolated lipoblasts (the presence of which are an absolute prerequisite for the diagnosis of liposarcoma) and occasional hyperchromatic spindle or stellate cells, which are often found within dense fibrous septa [13]. It has been suggested that these thick septa, which contain skeletal muscle elements, may be one method of differentiating malignant tumours from simple lipomas, on fat-suppressed T1-weighted MR images after gadolinium (III) diethyltriaminepentaacetic acid (Gd-DTPA) administration [14]. Other features that suggest malignancy include increased patient age, large lesion size, presence of nodular/and or globular or non-adipose mass-like areas, and decreased percentage of fat composition [15].

Cytogenetically, well-differentiated liposarcomas appear to be relatively homogenous, exhibiting characteristic ring, as well as giant marker chromosomes, containing amplified genetic material derived from the 12q 13–15 chromosome region [16].

It is important that all well-differentiated liposarcomas are extensively sampled, both to ensure identification of lipoblasts (hence avoiding confusion with, for example, intramuscular lipoma), and to exclude the presence of a dedifferentiated element. Such an area of dedifferentiation confers a much worse prognosis, with up to a 50% chance of metastasis [13]. Dedifferentiation is largely a time-dependent phenomenon that occurs in sites in which there is a high likelihood for clinical persistence of disease (e.g. the retroperitoneum) [16]. One study demonstrated that the time interval between diagnosis and dedifferentiation might be as long as 18 years [17]. However, the progression of the disease following dedifferentiation may be highly variable and probably depends on a number of factors, including the amount of dedifferentiation and type of therapy.

The classification of these tumours has been a source of debate for pathologists in recent years, but the current trend is to classify all pure well-differentiated cases arising in a limb as atypical lipoma, since, although they commonly recur, they never metastasise and therefore wide excision should be curative. Liposuction alone has been advocated as a possible treatment for these tumours [18]. However, tumours of the retroperitoneum, irrespective of microscopic appearance, have a five-year survival of approximately 35%, owing to their usual incomplete excision and repeated local recurrence with involvement of local structures [13].

GHMS is a surgical trainee who was involved in this patient's management. He was also involved in drafting and revising the manuscript.

KJS was the Plastic Surgery Consultant in charge of this patient's surgical management. He was also involved in drafting and revising the manuscript.

DNA was the General Surgery Consultant in charge of this patient's surgical management. He was also involved in drafting and revising the manuscript.