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Soft Tissue Tumors
Atypical lipomatous tumor / well differentiated liposarcoma
Author: Nat Pernick, M.D., PathologyOutlines.com, Inc.
Reviewer: David Lucas, M.D., University of Michigan Health Systems (January 2009)
Revised: 22 October 2009, last major update June 2009
Definition
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● Low grade malignancy of relatively mature adipocytes that exhibit focal atypia
Terminology
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● 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
Epidemiology
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● Common liposarcoma subtype (40-45% of liposarcomas)
● Adults, peaks at ages 40-60 years
● Very rare in children
Sites
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● Usually lower limbs, retroperitoneum; also paratesticular, mediastinum or subcutaneous tissue
Case reports
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● Gluteal mass presenting as sciatic hernia (World J Surg Oncol 2006;4:33)
Treatment and prognosis
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● 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 (AJSP 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
Gross description
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● Circumscribed or infiltrative
● Bulging, large (particularly in retroperitoneum), yellow-white
● Resembles lipoma
● Coarsely lobulated with pale, firm areas
● Fat necrosis in large lesions
Clinical images
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Intraoperative thigh mass
Pelvic tumor during surgery
Gross images
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Thigh mass
Retroperitoneal tumors
Pelvic tumor Well circumscribed tumor
Other images: tumor of mesocolon
Micro description
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● 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
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 - 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
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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
Sclerosing subtype Sclerosing subtype Metaplastic Metaplastic
(figure 2) bone smooth muscle
Cytology description
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● Large cells with multilobulated nuclei and mature appearing adipocytes
● May have bizarre tumor cells (Acta Cytol 2000;44:459)
Positive stains
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● MDM2 and CDK4 (both together are sensitive and specific, AJSP 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
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● HMB45
Electron microscopy
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● Pseudoinclusions due to invaginations of nuclear membrane
Molecular / cytogenetics
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● Ring or giant marker/rod chromosomes derived from 12q13-15 in almost all cases
● MDM2 and CDK4 by FISH or real time PCR can differentiate from other sarcomas (Hum Path 2006;37:1123, AJSP 2007;31:1476)
● Rarely have other structural rearrangements (Oncol Rep 2005;13:649)
Molecular images
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(A) ring chromosome, (B) giant marker chromosome, (C) anaphase bridge
Differential diagnosis
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● 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
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End of Soft Tissue Tumors > Atypical lipomatous tumor / well differentiated liposarcoma
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