Lipoma gross and microscopy size measuring 4.1x3.2x1.8 cms

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Lipoma - Gross and Microscopic Pathology

Specimen size: 4.1 x 3.2 x 1.8 cm

GROSS EXAMINATION

Size: 4.1 x 3.2 x 1.8 cm (within the typical range for conventional subcutaneous lipoma; most present as small, solitary masses, though they can reach several centimeters)

Shape: Well-defined, ovoid to lobulated mass

Surface / Capsule: Surrounded by a thin, glistening fibrous capsule - the tumor is well-encapsulated, clearly demarcated from surrounding tissues

Cut section:

Uniform yellow, greasy, glistening surface - identical in appearance to normal subcutaneous fat
Soft, compressible consistency
Lobulated architecture separated by thin whitish fibrous septa
No areas of necrosis, hemorrhage, or cystic change (in uncomplicated cases)

Margins: Pushing, well-circumscribed borders; slips cleanly out of the surrounding tissue at surgery ("enucleation" plane is easily established)

MICROSCOPIC EXAMINATION (H&E)

Low power (scanning):

Lipoma - low power H&E showing well-encapsulated lobulated lesion with a thin fibrous capsule

Figure 122-1A (Fitzpatrick's Dermatology): Well-circumscribed, encapsulated lipogenic lesion in the subcutis with a thin fibrous capsule

Well-circumscribed, encapsulated, lobulated lesion
Lobules of mature adipose tissue separated by thin, hypocellular fibrous septa
Septa contain thin-walled capillaries
Tumor is clearly demarcated from the surrounding dermis/subcutis

High power:

Figure 122-1B (Fitzpatrick's Dermatology): Mature, univacuolated adipocytes showing only mild variation in size and shape

Adipocytes: Large, mature, univacuolated cells - each cell contains a single large lipid vacuole that displaces the nucleus peripherally ("signet ring" appearance is the mature adipocyte form)
Nuclei: Small, compressed, peripherally placed, without atypia or hyperchromasia
Size/shape variation: Only slight variation in adipocyte size and shape - no significant pleomorphism
Nuclear atypia: Absent
Mitoses: Absent (or extremely rare)
Fibrous septa: Thin, hypocellular with thin-walled capillaries
Necrosis: Absent in uncomplicated cases (post-traumatic cases may show fat necrosis, lipid-laden histiocytes, and chronic inflammation)

IMMUNOHISTOCHEMISTRY (if performed)

Marker	Result
S-100	Positive (adipocytes)
p16	Negative
MDM2	Negative
CDK4	Negative

The negativity for p16, MDM2, and CDK4 is important to distinguish lipoma from atypical lipomatous tumor (ALT)/well-differentiated liposarcoma, which shows focal positivity for these markers.

DIAGNOSIS

Benign lipoma, conventional type

KEY DISTINGUISHING FEATURES FROM MALIGNANCY

Feature	Lipoma	Atypical Lipomatous Tumor / WD Liposarcoma
Encapsulation	Well-encapsulated	May be poorly marginated (especially retroperitoneal)
Adipocyte atypia	None	Striking variation in size; hyperchromatic nuclei
Fibrous septa	Thin, hypocellular	Often contain atypical/enlarged stromal cells
Lipoblasts	Absent	May be present
MDM2/CDK4 IHC	Negative	Focally positive (nuclear)
MDM2 FISH	No amplification	12q13-q15 amplification

NOTES FOR THIS SPECIMEN

At 4.1 cm, this lipoma is moderately sized. Most conventional lipomas are < 5 cm. Lipomas > 5-10 cm or those in deep locations (intramuscular, retroperitoneal) warrant extra scrutiny to exclude ALT/WD liposarcoma.
Intramuscular lipomas can be poorly circumscribed with infiltration of skeletal muscle fibers (which show secondary atrophy).
If any worrisome histologic features are present (atypia, lipoblasts, thick septa with atypical cells), MDM2 FISH should be performed.

Sources:

Robbins, Cotran & Kumar - Pathologic Basis of Disease, p. 1117
Robbins & Kumar Basic Pathology (Robbins Pathology), block 8
Fitzpatrick's Dermatology, Vol. 1-2, pp. 2202-2203 (Fig. 122-1)

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