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Soft Tissue Tumors

Adipose tissue

Atypical lipomatous tumor / well differentiated liposarcoma

Reviewer: Vijay Shankar, M.D. (see Reviewers page)
Revised: 12 November 2012, last major update August 2012
Copyright: (c) 2003-2012, PathologyOutlines.com, Inc.


● Low grade malignancy of relatively mature adipocytes that exhibit focal atypia


● Called atypical lipomatous tumor in periphery
● Use of “atypical lipoma” is not recommended
Suggestion: call “well differentiated liposarcoma” in retroperitoneum or central body site and “atypical lipomatous tumor” elsewhere


● Common liposarcoma subtype (40-45% of liposarcomas)
● Adults, peaks at ages 40-60 years
● Very rare in children


● Usually lower limbs, retroperitoneum, trunk, head and neck region, also paratesticular, mediastinum or subcutaneous tissue
● Unusual sites include parotid gland, esophagus, mesentry, breast

Case reports

● 36 year old woman with gluteal mass (World J Surg Oncol 2006;4:33)
● 43 year old woman with abdominal fullness and weight gain (Hinyokika Kiyo 2010;56:697)
● 73 year old man with mesenteric tumor (Case Rep Oncol 2011;4:178)

Treatment and prognosis

● Excision; negative margins are particularly important for sclerosing subtype (Ann Surg Oncol 2004;11:78)
● Does not metastasize unless it dedifferentiates, which is associated with shorter survival (Am J Surg Pathol 2007;31:1)
● Subcutaneous or intramuscular tumors may recur but typically do not dedifferentiate or metastasize
● Retroperitoneal or central body site tumors are difficult to resect with a clear margin; frequently recur, dedifferentiate and cause death
● Sample carefully to look for dedifferentiated component
● Retroperitoneal / intra-abdominal location is associated with the worse outcome (J Surg Res 2012;175:12)

Clinical images

Intraoperative thigh mass

Pelvic tumor during surgery

Gross description

● Circumscribed or infiltrative
● Bulging, large (particularly in retroperitoneum), yellow-white
● Resembles lipoma
● Coarsely lobulated with pale, firm areas
● Fat necrosis in large lesions

Gross images

Thigh masses

Retroperitoneal tumors

Mesenteric tumor

Pelvic tumor

Other images: tumor of mesocolon

Micro description

● Mature fat plus variably sized adipocytes and fibromyxoid stroma containing spindle cells with large, deep-staining nuclei and marked nuclear enlargement or pleomorphism
● Cellularity is low and mitotic figures are uncommon
● Usually fibrous tissue septa are present that may contain spindle cells or highly pleomorphic cells
● Some nuclei have sharply outlined vacuoles (Lochkern)
● Rarely heterologous differentiation
● No/few lipoblasts
● Rarely have no/minimal adipocytes except in recurrences
● Some authors do not require any atypia for diagnosis of retroperitoneal tumors
● May be associated with metaplastic bone formation (Hinyokika Kiyo 2010;56:697)
● Rarely has low grade osteosarcomatous component (Am J Surg Pathol 2010;34:1361)

Tumors are subtyped, but this has no clinical significance (Semin Diagn Pathol 2001;18:258)
Inflammatory subtype: rare; lymphoid nodules in cellular stroma with atypical multinucleated cells; may obscure lipocytes (see inflammatory liposarcoma)
Lipoleiomyosarcoma subtype: liposarcoma with leiomyosarcomatous differentiation (see lipoleiomyosarcoma)
Lipoma-like subtype: most common subtype; scattered atypical cells; rare to frequent lipoblasts
Sclerosing subtype: retroperitoneal or paratesticular lesions; collagenous fibrous tissue with scattered adipocytes and atypical multinucleated stromal cells; few lipoblasts
Spindle cell subtype: CD34+ spindle cells in short fascicles, whorls or storiform pattern mixed with well differentiated liposarcoma component with hyperchromatic nuclei but only mild atypia
Mixed subtype: very common

Micro images

Various images

Well circumscribed tumor

Enlarged, dense irregular nuclei

Atypical cells within fibrous tissue

Bizarre shaped cells

Lochkern cells with sharply outlined nuclear vacuoles

Lipoblasts are multivacuolated with atypical nuclei

Floret cells

Thick collagen fibers

Thin collagen fibers

Moderately cellular (but not dedifferentiated)

Atypical cells within blood vessel walls

Subcutaneous tumor

Metaplastic bone

Metaplastic smooth muscle


Cytology description

● Large cells with multilobulated nuclei and mature appearing adipocytes
● May have bizarre tumor cells (Acta Cytol 2000;44:459)

Positive stains

● MDM2 and CDK4 (both together are sensitive and specific, Am J Surg Pathol 2005;29:1340)
● S100 (adipocytes in 2/3 of cases)
● CD34 (some spindle cells)
● Usually p16 (Appl Immunohistochem Mol Morphol 2009;17:51)

Negative stains

● HMB45

Electron microscopy description

● Pseudoinclusions due to invaginations of nuclear membrane

Molecular / cytogenetics description

● Ring or giant marker/rod chromosomes derived from 12q13-15 in almost all cases
● Amplifications of the 12q12-21 and 10p11-14 regions (Virchows Arch 2012;461:67)
● MDM2 and CDK4 by FISH or real time PCR can differentiate from other sarcomas (Hum Pathol 2006;37:1123, Am J Surg Pathol 2007;31:1476)
Carboxypeptidase M amplification may be alternative diagnostic tool (Mod Pathol 2009;22:1541)
● Other rearrangements include amplifications of 12q12-21 and 10p11-14 regions (Virchows Arch 2012;461:67), hypertriploid, #8 rearrangements (Oncol Rep 2005;13:649)

Molecular images

(A) ring chromosome, (B) giant marker chromosome, (C) anaphase bridge

(A) ring chromosome, (B) giant marker chromosome, (C) anaphase bridge

Differential diagnosis

Lipoma: no atypical nuclei
Lipomatous angiomyolipoma: HMB45+ epithelioid cells, MDM2 and CDK4 negative (Pathol Int 2006;56:638)
Lipoblastoma: age 3 or younger, MDM2 and CDK4 negative (Virchows Arch 2002;441:299)
Paraffinoma: history of paraffin injection, no atypical nuclei, MDM2 and CDK4 negative (Br J Radiol 2003;76:264)
Malignant fibrous histiocytoma: no adipocytes, MDM2 and CDK4 negative
Retroperitoneal round cell/myxoid liposarcoma: morphology may be similar, but negative for MDM2, CDK4 and 12q13-15 amplification (Mod Pathol 2009;22:223)

Additional references

Stanford University

End of Soft Tissue Tumors > Adipose tissue > Atypical lipomatous tumor / well differentiated liposarcoma

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