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.
For the vulva specifically, a lipoma is usually a soft, mobile labial mass. A deep, fixed, or rapidly enlarging vulval or pelvic fatty mass should never be assumed benign on clinical grounds alone.
Microscopic / histological features
The microscopy is the heart of this objective. A conventional lipoma is composed of sheets and lobules of mature white adipocytes that are essentially identical to normal fat:
- Each adipocyte is a large, uniform cell with a single large clear lipid vacuole (the fat is washed out in routine processing, leaving an empty space) that displaces the small, bland, flattened, peripheral, crescent-shaped nucleus against the cell membrane — the classic "signet-ring"-like adipocyte.
- The cells are monomorphic: uniform in size, with no nuclear atypia, no hyperchromasia, and no significant variation in cell size (in contrast to the cellular pleomorphism of liposarcoma).
- Mitotic figures are absent or extremely rare, and there is no necrosis.
- A thin fibrous capsule surrounds the lesion, and delicate fibrous septa carrying small, regular capillaries divide the fat into lobules.
- Critically, lipoblasts are absent in a conventional lipoma. A lipoblast — an immature fat cell with cytoplasmic lipid vacuoles that scallop and indent a hyperchromatic, often multilobated nucleus — is the histological hallmark of liposarcoma. (A note of caution taught to every trainee: the mere presence of vacuolated cells is not enough; degenerating adipocytes and macrophages can mimic lipoblasts, so the diagnosis of malignancy rests on genuine atypical lipoblasts plus atypical hyperchromatic stromal cells, not on a single suspicious cell.)
The diagnostic difficulty lies almost entirely at the lipoma / well-differentiated liposarcoma interface. Well-differentiated liposarcoma (ALT/WDLPS) is also composed predominantly of mature-looking fat, but it differs by containing scattered enlarged, hyperchromatic, atypical stromal cells, often within fibrous septa, together with variation in adipocyte size and (variably) lipoblasts. Because these diagnostic cells may be very focal, the practical rule is generous sampling of any large or deep fatty tumour. The variant lipomas each have their own microscopic signature — capillary proliferation with fibrin thrombi in angiolipoma; bland spindle cells with ropey "shredded-carrot" collagen and floret giant cells in spindle-cell/pleomorphic lipoma; a fibrous, myxoid, or chondroid matrix in the named variants.
Immunohistochemistry and molecular features
Adipocytes are mesenchymal and express S100 protein, which marks both benign and malignant adipocytic tumours and therefore does not distinguish a lipoma from a liposarcoma — its value is in confirming adipocytic differentiation, not in grading. The molecular separation is far more useful and is now central to soft-tissue diagnosis:
- Conventional lipoma characteristically harbours rearrangements of the HMGA2 locus (chromosome 12q14–15), frequently as a 12q13–15 translocation. These are clonal, confirming that a lipoma is a true neoplasm rather than simple fatty overgrowth — but they are benign aberrations.
- Well-differentiated and dedifferentiated liposarcoma are defined by amplification of the 12q13–15 region containing MDM2 and CDK4, detectable by FISH for MDM2 amplification or by MDM2/CDK4 immunohistochemistry. This is the single most decisive ancillary test in the lipoma-versus-WDLPS problem: MDM2 amplification is essentially absent in benign lipoma and present in well-differentiated liposarcoma, so a fatty tumour that is large, deep, or histologically equivocal should be tested for MDM2.
- Spindle-cell / pleomorphic lipoma shows loss of RB1 (13q14), a useful confirmatory marker for that variant.
- Myxoid liposarcoma (a malignant differential, not a lipoma) carries the FUS–DDIT3 (FUS-CHOP) translocation, a separate molecular entity.
This molecular framing is the modern way the WHO 2020 classification separates the adipocytic family, and it mirrors a theme the registrar will recognise from elsewhere in the oncology domain: pathology is increasingly defined by molecular class. The same principle that drives FIGO 2023 endometrial molecular staging (POLEmut / MMRd / NSMP / p53abn) underlies the HMGA2-versus-MDM2 distinction in fatty tumours — morphology sets the question, and molecular testing answers it.
Assessment
Figure D17.2 — Lipoma or liposarcoma? The reassuring features vs the worrying ones (large, deep, rapidly growing; atypia; MDM2/CDK4) that mandate imaging and referral.
Because the objective is pathological, "assessment" here means the diagnostic work-up that confirms a fatty mass is a benign lipoma and excludes its malignant mimic.
History. A long-standing (months to years), slowly enlarging, painless, soft lump is typical of lipoma. Ask specifically about the alarm features: rapid recent growth, new pain, fixation, and any constitutional symptoms. Angiolipoma is the benign exception that is often tender and multiple. A family history of multiple lipomas may point to familial multiple lipomatosis.
Examination. A classic superficial lipoma is soft, lobulated, mobile over deep tissues with freely mobile overlying skin, non-tender, and without skin discolouration. The clinical differential for a vulval/labial mass includes Bartholin and other genital-tract cysts, epidermal inclusion cysts, hernias (especially a labial hernia through the canal of Nuck), and — importantly — a soft-tissue sarcoma. The key clinical discriminators that move a fatty mass out of the reassuring category are: deep location, size greater than ~5 cm and growing, fixation to deep structures, hardness, and any rapid change.
Imaging. Ultrasound of a superficial lipoma typically shows a well-defined, compressible, mildly hyperechoic or isoechoic mass paralleling the skin, without significant internal vascularity on Doppler. MRI is the imaging modality of choice for any deep, large, or indeterminate fatty mass: a lipoma follows fat signal on all sequences (high on T1, suppressing on fat-saturated sequences) and lacks thick septa, nodular non-fatty components, or enhancement. Imaging features favouring liposarcoma — thick or enhancing septa, prominent non-fatty soft-tissue nodules, and large size — should trigger referral rather than local excision. The principle of using MRI to characterise a pelvic mass parallels its use in assessing other gynaecological masses for malignancy.
Biopsy and the role of histology. The definitive diagnosis is histological. For a typical small, mobile, superficial vulval lipoma, excisional biopsy is both diagnostic and curative. For any large, deep, or imaging-indeterminate fatty mass, the lesion should be managed on a sarcoma pathway: imaging first, then image-guided core biopsy (not enucleation) by an appropriate team, with histology including MDM2 FISH where the morphology is equivocal. In the South African public sector, soft-tissue histology and ancillary molecular testing such as MDM2 FISH are provided through the NHLS (National Health Laboratory Service); molecular tests may have restricted availability and turnaround at non-academic centres, so a high-suspicion deep fatty mass is best referred early to a centre with sarcoma multidisciplinary capability rather than excised locally.
Management
The management of a confirmed benign lipoma is principles-level and simple, and the contrast with sarcoma management is the teaching point.
Observation. A small, asymptomatic, clinically classic superficial lipoma can reasonably be observed with safety-netting, particularly where the diagnosis is confident and the patient is reassured. There is no oncological imperative to remove a typical small lipoma.
Excision. The standard treatment for a symptomatic, enlarging, cosmetically troublesome, or diagnostically uncertain lipoma is simple complete surgical excision (enucleation), removing the lesion with its thin capsule. A true lipoma shells out cleanly, recurrence after complete excision is uncommon, and the excised specimen must always be sent for histology — never discard a fatty lump on the assumption it is benign, because occult well-differentiated liposarcoma is the trap. For a vulval lipoma this is usually a minor local procedure under local or regional anaesthesia, performed with the same standards of informed consent as any gynaecological operation, with counselling that the histology determines whether any further action is needed.
If the histology is not a simple lipoma. Should pathology reveal atypical cells, MDM2 amplification, or a frank liposarcoma, management escalates entirely: the patient enters a sarcoma multidisciplinary pathway with consideration of wide local excision and, depending on grade and site, sarcoma-specific multimodal therapy. This is where the surgeon must resist the temptation of a "quick local excision" for a deep or large fatty mass — an enucleation of an unsuspected sarcoma compromises margins and worsens outcome.
It is worth being explicit about what lipoma management is not, because the rest of Domain D is dominated by complex oncological therapeutics that have no role whatsoever in benign lipoma. A lipoma is not screened for in any population programme — the WHO 90-70-90 cervical-cancer-elimination strategy and the South African SASOG/BetterGyn 2024 and NDoH cervical-screening pathways (HPV-based primary screening, screening HIV-positive women at diagnosis and approximately three-yearly thereafter regardless of age) target HPV-driven epithelial malignancy, not mesenchymal fat tumours. Likewise, none of the modern oncology therapeutics anchored elsewhere in this domain — concurrent chemoradiation with image-guided brachytherapy (EMBRACE-II), the immunotherapy regimens of KEYNOTE-A18 and KEYNOTE-826 (pembrolizumab), dostarlimab in RUBY, pembrolizumab–lenvatinib (KEYNOTE-775), or gestational-trophoblastic-neoplasia chemotherapy — has any application to a benign lipoma. Stating this explicitly is exam-relevant: it demonstrates that the candidate understands the lipoma's position as a benign lesion against the malignant backdrop of the domain, and it guards against the absurd error of over-treating benign fat.
Red flags / pitfalls
- Calling a deep or large fatty mass a "lipoma" on clinical grounds. Superficial small fatty tumours are almost always benign; deep, retroperitoneal, intra-abdominal, or large (>10 cm) fatty tumours have a materially higher chance of being a well-differentiated liposarcoma. Do not enucleate these — image and biopsy on a sarcoma pathway.
- Discarding the specimen. Every excised fatty lump goes to histology. Occult well-differentiated liposarcoma is the reason; the gross and even microscopic appearance can be deceptively bland.
- Over-relying on S100. S100 confirms adipocytic origin but does not separate benign from malignant. The decisive ancillary test is MDM2/CDK4 (FISH for MDM2 amplification), positive in well-differentiated/dedifferentiated liposarcoma and negative in lipoma.
- Mistaking degenerate cells for lipoblasts. A genuine lipoblast (vacuolated cytoplasm scalloping a hyperchromatic, indented nucleus) signals liposarcoma, but macrophages and degenerating adipocytes mimic them. Diagnose malignancy on atypical stromal cells plus genuine lipoblasts, not a single suspicious cell.
- Inadequate sampling. In a large fatty tumour the diagnostic atypical cells may be sparse and focal; under-sampling can miss a well-differentiated liposarcoma entirely. Sample firm, fibrous, gelatinous, or grey areas generously.
- Pain as false reassurance or false alarm. Most lipomas are painless, but angiolipoma is characteristically tender — tenderness alone does not equal malignancy. Conversely, new pain in a previously painless fatty mass is a genuine alarm feature.
- Confusing a labial fatty mass with other entities. A labial swelling may be a hernia through the canal of Nuck, a cyst, or a sarcoma — examine for reducibility, fixation, and growth before assuming lipoma.
Evidence anchors
- WHO Classification of Tumours of the Female Genital Tract, 5th edition (2020), and the companion WHO soft-tissue classification, are the source of truth for the histological typing of adipocytic tumours — defining the benign-to-malignant spectrum (lipoma → atypical lipomatous tumour/well-differentiated liposarcoma → dedifferentiated/myxoid/pleomorphic liposarcoma) and the molecular features (HMGA2 rearrangement in lipoma; MDM2/CDK4 amplification in well-differentiated/dedifferentiated liposarcoma; RB1 loss in spindle-cell/pleomorphic lipoma) used to separate them. This is the only verified-sources reference that speaks directly to lipoma pathology, and the molecular-class framing it embodies parallels the molecular pathology emphasised across the rest of the domain.
- Contrast with malignant disease is anchored by the domain's verified guidelines, cited here to make explicit that none apply to benign lipoma: the ESGO/ESTRO/ESP cervical (2023), endometrial (2025), and vulvar (2023) guidelines; FIGO 2018 cervical and FIGO 2023 endometrial molecular staging; the WHO Cervical Cancer Elimination Strategy (90-70-90) and South African SASOG/BetterGyn (2024) plus NDoH cervical-screening policy; and the medical/radiation-oncology trials (KEYNOTE-A18, KEYNOTE-826, RUBY/dostarlimab, KEYNOTE-775 pembrolizumab–lenvatinib, EMBRACE-II image-guided brachytherapy). These define the management of malignant gynaecological tumours and serve only to delineate, by contrast, that a benign lipoma requires none of them.
A note on the limits of the verified-sources list: there is no SA NDoH, RCOG, NICE, or ESGO guideline specific to the diagnosis or management of benign lipoma on the verified-sources reference, because it is a benign incidental lesion rather than a screened or staged malignancy. The pathology described here (gross enucleable yellow fat; monomorphic mature adipocytes with peripheral crescentic nuclei and no lipoblasts; HMGA2 versus MDM2 molecular separation; S100 positivity that does not grade) reflects standard WHO 2020 soft-tissue pathology and should be regarded as established histopathology rather than as resting on a single cited clinical guideline. Specific size thresholds (e.g. the ~10 cm and deep-location heuristics for raising suspicion of well-differentiated liposarcoma) are widely-taught practical guides rather than guideline-mandated cut-offs, and should be treated as clinical rules of thumb prompting referral, not as fixed diagnostic criteria.