Clinical overview
A lipoma is a benign neoplasm of mature white adipocytes — the commonest soft-tissue tumour in the human body and, in the gynaecological context, an occasional but reassuringly benign finding of the vulva, mons pubis, labia majora, and rarely the round ligament, broad ligament, vaginal wall, or retroperitoneum. In a domain dominated by the hard problems of cervical, endometrial, and vulvar carcinoma, the lipoma sits at the opposite end of the spectrum: a slow-growing, soft, mobile, painless mass that almost never metastasises and almost never recurs after simple excision. Its place on the FCOG(SA) blueprint is precisely as a contrast lesion. The registrar must be able to recognise a lipoma confidently enough to reassure the patient, distinguish it pathologically from its malignant mimic the liposarcoma, and avoid the twin errors of either over-treating a benign lump or under-investigating a deep, fixed, rapidly enlarging mass that is in fact a sarcoma.
Vulval lipomas typically present in adult women as a soft, slowly enlarging swelling of the labium majus, often present for months to years before review, frequently noticed incidentally. They are usually solitary, mobile over deeper tissues, non-tender, and without overlying skin change. The clinical teaching point for this objective is that the diagnosis is fundamentally a pathological one — the gross and microscopic features of mature fat in a thin fibrous capsule define the lesion, and it is the deviations from those features (deep location, large size, fixation, pain, recent rapid growth, atypical cells, abnormal cytogenetics) that should prompt suspicion of a sarcoma rather than a lipoma.
Core knowledge
Definition and classification
A lipoma is a benign tumour composed of mature white adipocytes, classified within the adipocytic tumour family of the WHO Classification of Tumours, 5th edition (2020), which is the source of truth for soft-tissue and female-genital-tract histological typing. The adipocytic spectrum runs from clearly benign (lipoma and its variants), through an intermediate locally-aggressive category (atypical lipomatous tumour / well-differentiated liposarcoma, ALT/WDLPS), to frankly malignant (dedifferentiated, myxoid, and pleomorphic liposarcoma). Understanding this continuum is the central pathology lesson: the benign end and the intermediate end can look almost identical on a thin section, and they are separated reliably only by location, size, cytogenetics, and the presence of lipoblasts and atypical hyperchromatic stromal cells.
Conventional lipoma is the prototype. Recognised variants relevant to soft-tissue and gynaecological practice include:
- Conventional (ordinary) lipoma — mature white fat, the great majority of cases.
- Angiolipoma — mature fat plus a prominent capillary network, classically containing fibrin microthrombi; characteristically subcutaneous, often multiple, and frequently tender (one of the few painful benign fatty tumours).
- Spindle-cell / pleomorphic lipoma — mature fat admixed with bland spindle cells, ropey collagen, and (in the pleomorphic variant) floret-type multinucleated giant cells; characteristically arising on the posterior neck, shoulder, and back, with loss of the RB1 gene (13q14).
- Fibrolipoma / myxolipoma / chondroid lipoma — variants defined by a prominent fibrous, myxoid, or chondroid component.
- Angiomyolipoma — a perivascular epithelioid (PEComa) family lesion of fat, smooth muscle, and thick-walled vessels, classically renal and associated with tuberous sclerosis, mentioned here because it is a fat-containing lesion in the differential, not a true lipoma.
- Lipomatosis — diffuse, ill-defined overgrowth of mature fat (e.g. pelvic lipomatosis) rather than a discrete encapsulated tumour.
Gross / macroscopic features
Figure D17.1 — Lipoma pathology: a benign, soft, well-circumscribed tumour of mature adipocytes — commonest on the vulva/labia majora — with bland histology and no atypia.
On naked-eye examination, a lipoma is a soft, well-circumscribed, lobulated mass. It is typically encapsulated by a thin, delicate fibrous pseudocapsule that allows it to be "shelled out" or enucleated cleanly at surgery — a feature with direct operative relevance, because complete excision of a true lipoma is usually straightforward and curative. The cut surface is uniformly soft, greasy, and yellow, indistinguishable on gross inspection from normal subcutaneous fat, and it floats in formalin owing to its lipid content. Most subcutaneous lipomas are small (a few centimetres), though they may reach considerable size.
The gross deviations that should worry the pathologist and surgeon are exactly those that distinguish a lipoma from a well-differentiated liposarcoma:
- Size — a fatty tumour larger than roughly 10 cm, especially in a deep location, is statistically more likely to be ALT/WDLPS than a benign lipoma.
- Location — superficial subcutaneous fatty tumours are nearly always benign; deep-seated (intramuscular, retroperitoneal, intra-abdominal, deep within the vulva or pelvis) fatty tumours carry a materially higher risk of being a well-differentiated liposarcoma and must be sampled generously.
- Texture and colour — firm, fibrous, gritty, gelatinous/myxoid, or grey-white/necrotic areas within an otherwise fatty mass are red flags for a liposarcomatous component and demand extensive sampling, because the diagnostic cells may be sparse and focal.