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ADIPOSE TISSUE NEOPLASMS, lobularhibernoma, fibrolipomatoushamartomaofnerv…
ADIPOSE TISSUE NEOPLASMS
KEY POINTS
1) Atypical lipomatous tumor and well-differentiated liposarcoma are the same thing but in different locations
2) Atypical lipomatous tumor/well-differentiated liposarcoma is the low-grade form of dedifferentiated liposarcoma and has the same root genetic alterations (MDM2 amplification is characteristic of both). These tumors lack a characteristic translocation/fusion
3) Dedifferentiated liposarcoma has many appearances and it is the most common sarcoma in the retroperitoneum
4) Myxoid liposarcoma is genetically distinct from the other liposarcomas and is a translocation sarcoma so it has monotonous nuclei and no atypical mitoses. It lacks MDM2 amplification but instead has gene rearrangements/translocations of FUS-DDIT3 or EWSR1-DDIT3
5) Round cell liposarcoma is an old term for the high-grade form of myxoid liposarcoma
6) Pleomorphic liposarcoma is unrelated to the other liposarcomas. It has a complex karyotype but does not have MDM2 amplification. Homologous dedifferentiation: Rarely, dedifferentiated liposarcoma can show an overlapping pattern
TUMORS
LIPOBLASTOMA
- Benign tumor of babies
- Can resemble myxoid liposarcoma
- Has rearrangements of PLAG1 and lacks nuclear pleomorphism
- Can do testing for DDIT3 rearrangements to rule out myxoid liposarcoma
- Microscopic features:
1) Superficial lobulated lesions
2) Consist of a mixture of mature fat and primitive
3) The primitive areas are richly vascularized and contain lipoblasts (adipocytic cells in which the nuclei are indented by lipid droplets)
ANGIOLIPOMA
- Benign lesion
- Arise in young adults
- Superficial
- Microscopic features:
1) Well-marginated small nodules composed of a mixture of fat and capillaries that display fibrin thrombi
2) There can be mitoses in the cells that support the capillaries but not in the endothelial cells themselves
SPINDLE CELL AND PLEOMORPHIC LIPOMAS
- Tumors are 'cousins' with a lot of overlapping feautres
- Tumors of middle-aged adults
- Neck and shoulder region
- Microscopic features:
1) Superficial and well-marginated when found in classic location but can be slightly in filtrative when found in areas with little subcutaneous tissue e.g. the shins
2) Consist of spindle cells, wiry collagen ('kinky' collagen), mast cells and variable amounts of stroma
3) Sometimes have striking cleft-like spaces
4) Some examples have little fat
5) Pleomorphic lipoma has the additional feature of multinucleated hyperchromatic cells called fleurette cells
6) These lesions express CD34 and the spindle cells lack S-100 protein expression but have areas of fat
7) Both lesions have lipoblasts
MYXOID LIPOSARCOMA
- Arises in deep soft tissue (often extremities)
- In young adults
- Low-grade and high-grade forms
- High-grade form is also known as round cell liposarcoma. These tumors are unusual as the have a tendency to spread to other soft tissue sites and even bone
- Translocation sarcoma. As such, has uniform nuclei and lacks atypical mitoses
- Can have cystic spaces and loculation as well as areas with mature-appearing fat but the nuclei are uniform
- Lipoblasts tend to be found at the periphery
- Vascular pattern characteristic: Has delicate capillaries in a complex network, likened to chicken wire (seen in top image, low-grade form)
- High-grade form has solid areas of cells that obscure the vascular pattern (bottom image)
- Usually report % of high-grade component, as even 5% can have a negative affect on the outcome
ATYPICAL LIPOMATOUS TUMOR/WELL-DIFFERENTIATED LIPOSARCOMA
- Atypical lipomatous tumor is the term used for lesions in readily accessible tissues
- Well-differentiated liposarcoma is the term when the lesion is retroperitoneal, as these are more likely to dedifferentiate (evolve to a higher grade form)
- Occur in middle-aged and older adults
- Deep lesions, either involving the deep muscles of the extremities or the mediastinum, retroperitoneum or abdomen
- ALT is Dx at 4x and then confirmed at higher magnification
- Microscopic features:
1) Bands of collagen are punctuated by cells with enlarged hyperchromatic nuclei
2) These nuclei are also seen suspended in 'naked' surrounding fat
3) Some tumors contain abundant lymphoid tissue and some are sclerotic
MASSIVE LOCALIZED LYMPHEDEMA IN THE MORBIDLY OBESE
- Process can mimic liposarcoma but differs in several ways
- Lesions tend to arise in one of the patient's thighs, resulting in a pendulous, superficially weight-based mass that has overlying skin stasis changes with thickening of the skin and an elephantiasis-like appearance
- These lesions can also arise in the scrotal area
- Microscopic features:
1) Superficial edematous process
2) Abundance of dilated lymphatic spaces
3) Inflammation overlying the skin and around vessels
HIBERNOMA
- Tumors with differentiation along the lines of brown fat
- Arise in young adults
- Superficial
- Microscopic features:
1) At high magnification, the brown fat adipocytes resemble lipoblasts but have a microvesicular pattern of fat droplets
2) The brown fat appearance mimic liposarcoma and the rich vascularity can appear concerning
DEDIFFERENTIATED LIPOSARCOMA
- The high-grade form of atypical lipomatous tumor/well-differentiated liposarcoma
- Occurs in deep soft tissue (usually retroperitoneum, occasionally extremities)
- Occurs in adults
- Lesions that were known as inflammatory malignant fibrous histiocytoma in the past are now considered dedifferentiated liposarcoma
- To Dx on H&E alone, need both the well-differentiated component and the high-grade
- Microscopic features:
1) A high-grade liposarcoma arising in association with a well-differentiated liposarcoma
2) The well-differentiated liposarcoma can differentiate into any type of high-grade component including: whorls, smooth muscle differentiation, skeletal muscle, osseous (heterologous) differentiation
LIPOMA
- Resembles adipose tissue but forms a mass
- Unusual in children and young adults (especially deep ones)
- Prone to fat necrosis. This results in nuclei that can be seen at 4x as large individual cells imparted by either histiocytes or by nuclei with intranuclear lipid invaginations (lochkerns). These contrast with the atypical nuclei of atypical lipomatous tumor/well-differentiated liposarcoma or with lipoblasts
FIBROLIPOMATOUS HAMARTOMA OF NERVE
- Diagnosed on H&E alone
- Presents in patients <30 years, in the fingers, hand or wrist. Some patients have macrodactyly
- Lesions are associated with the median nerve and produce a swelling
- Microscopic features:
1) Fat with fibrous stands intimately wrapped around nerves
2) Onion skin appearance where the fatty component meets the nerve
PLEOMORPHIC LIPOSARCOMA
- In contrast to dedifferentiated liposarcoma, pleomorphic liposarcoma is an H&E Dx because it is Dx by finding the areas of pleomorphic lipoblasts and it has no characteristic molecular or immunolabeling signature
- Aggressive sarcoma in older adults
- Typically arises in the deep thigh
- Microscopic features:
1) Undifferentiated areas and variable areas with extremely pleomorphic lipoblasts
2) Lipoblasts contain bizarre nuclei
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