Atypical Lipomatous Tumor
Definition
- Neoplasm of mature fat exhibitng at lest focal cytologic atypia occurring at sites other than the subcutis of posterior neck, back and shoulders
Alternate/Historical Names
- Well differentiated liposarcoma
- Lipoma-like liposarcoma
- Atypical lipoma
- Pleomorphic lipoma
- Sclerosing liposarcoma
- Inflammatory liposarcoma
- Lymphocyte rich liposarcoma
- Spindle cell liposarcoma
Diagnostic Criteria
- Contains mature adipose tissue
- Fat may appear grossly normal
- Fat cell size may range from smaller to larger than normal
- Atypical cells required for diagnosis
- Atypical cells frequently located in fibrous areas
- May also be located in fat, myxoid areas or in walls of blood vessels
- Atypical cells usually scattered, not in nests or confluent
- Large irregular nuclei should be visible at low magnification
- Dense, smudgy chromatin
- Frequent clear nuclear pseudoinclusions
- Frequent floret cells, defined by a ring or semicircle of nuclei
- Lipoblasts sufficient but not required for diagnosis (in the proper clinical context)
- Atypical nuclei indented or scalloped by vesicles containing fat
- Cytoplasmic vacuole(s) that indent and distort nucleus
- Atypical cells frequently located in fibrous areas
- Frequently contains fibrous or myxoid areas
- Collagen may be fine or coarse
- Frequently extends between adjacent lipocytes at edge of fibrotic areas
- May predominate
- Has been termed sclerosing liposarcoma or sclerosing variant
- No clinical significance
- Myxoid pattern may predominate and simulate myxoid liposarcoma
- Primary retroperitoneal tumors with this appearance appear to be atypical lipomatous tumors genetically (de Vreeze)
- Collagen may be fine or coarse
- Usually occurs in deep soft tissue, especially the retroperitoneum
- By definition, it does not occur in the dermis or subcutis of the posterior neck, upper back or shoulders
- Fatty tumors with atypia in those locations are termed pleomorphic lipoma
- They do not recur aggressively or dedifferentiate
- Atypical lipomatous tumor does occur rarely in other subcutaneous sites
- Fatty tumors with atypia in those locations are termed pleomorphic lipoma
- By definition, it does not occur in the dermis or subcutis of the posterior neck, upper back or shoulders
- If a high grade sarcomatous component is present, designate the process as dedifferentiated liposarcoma
- Chronic inflammation may be prominent
- Lymphocytes, plasma cells, germinal centers
- May obscure underlying atypical lipomatous tumor
- Fat necrosis may be present
- Has been termed inflammatory or lymphocyte rich liposarcoma
- No clinical significance
- Lymphocytes, plasma cells, germinal centers
- Foci of moderately increased cellularity and mitotic activity rarely may be seen
- Non-lipogenic component with features of fibromatosis, low grade fibrosarcoma or hemangiopericytoma
- Mitotic rate <5/10 hpf
- No difference in survival from usual atypical lipomatous tumor
- No metastases without recurrence as dedifferentiated liposarcoma
- Has been termed low grade dedifferentiated liposarcoma
- We do not use that term or separate them from usual atypical lipomatous tumors
- Cellular atypical lipomatous tumor (Evans 2007) is an acceptable term
- Spindle cell variant is predominantly composed of bland spindle cells
- Fibrosarcoma-like lipomatous lesion has been proposed as an alternate term
- Heterologous mesenchymal differentiation rarely may be seen
- Cartilage
- Bone
- Smooth muscle
- Well Differentiated Liposarcoma is an equivalent term, we generally designate such tumors, regardless of location as Atypical Lipomatous Tumor / Well Differentiated Liposarcoma
Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342
Original posting : 8/4/07
Last update: 12/28/13
