Benign adipocytic tumors
The best-known representative of this group is the lipoma (Lipoma). On MRI, it appears as a fatty mass with a thin capsule and thin internal septa. Intramuscular lipomas are also distinguished, representing large masses with muscle fibers at the periphery.
In young children (predominantly under 3 years of age), lipoblastoma (Lipoblastoma) occurs. It consists of mature and immature fat cells (lipoblasts). Due to myxoid and cystic areas, its MRI appearance is heterogeneous and may resemble myxoid liposarcoma, but over time it matures and transforms into a conventional lipoma.
A rare brown fat tumor is hibernoma (Hibernoma). Most commonly, it is localized in the thigh. It differs from subcutaneous fat by a hypointense signal on T1-weighted images and incomplete signal suppression on fat-suppressed sequences. A characteristic sign is the presence of tortuous vessels within the mass.
Atypical lipomatous tumor and well-differentiated liposarcoma
The terms atypical lipomatous tumor (Atypical Lipomatous Tumour, ALT) and well-differentiated liposarcoma (Well-Differentiated Liposarcoma, WDL) describe morphologically and genetically identical tumors. The choice of term depends on the location:
- ALT — used for tumors of the extremities and trunk where complete surgical removal is feasible.
- WDL — used for deep locations (retroperitoneum, mediastinum) where complete resection is difficult and the risk of uncontrolled recurrence is high.
These are locally aggressive tumors that do not metastasize until the process of dedifferentiation begins. On MRI, a deep lipoma and ALT/WDL may look similar; therefore, both are managed by primary wide-margin excision without a prior biopsy.
MRI signs of malignancy
When identifying a lipomatous mass, it is important to assess its malignant potential. The presence of liposarcoma (ALT/WDL) may be indicated by the following signs:
- Size greater than 10 cm.
- Presence of thick, nodular septa.
- Globular or nodular non-adipose areas.
- Fatty tissue content in the tumor is less than 75%.
- Incomplete fat signal suppression.
- Contrast enhancement of the septa (a more reliable sign than mere thickness).
Other malignant liposarcomas
As the histological grade increases, the fat component in tumors becomes less prominent on MRI.
Dedifferentiated liposarcoma arises from ALT/WDL and represents a high-grade malignancy. The risk of dedifferentiation is higher in retroperitoneal locations. On imaging, it appears as a non-lipomatous mass within a fatty tumor.
Myxoid liposarcoma is the second most common type of liposarcoma in adults. It contains a myxoid stroma (resembling fluid on MRI) and thin fatty septa visible as hyperintense lines on T1-weighted images.
Briefly: answers to topical questions
What are the WHO stages and histological subtypes of liposarcomas?
The histological classification of liposarcomas includes several subtypes differing in malignancy grade, although specific WHO stages are not described in the text. The main variants are:
- Well-differentiated (well-differentiated) — includes lipoma-like, sclerosing, inflammatory, and spindle cell subtypes.
- Dedifferentiated (dedifferentiated) — represents a mixture of atypical adipocytes with sarcomatous elements of different structure.
- Myxoid (myxoid) — consists of myxoid stroma containing primitive mesenchymal cells and lipoblasts.
- Pleomorphic (pleomorphic) — a rare, high-grade malignant tumor.
- Round cell (round cell).
What are the principles of surgical treatment and indications for radiation therapy in extremity liposarcomas?
The principles of surgical treatment for well-differentiated extremity liposarcomas consist of their complete removal.
- Primary excision (primary excision) — performed with the widest possible margins.
- Biopsy (biopsy) — preliminary biopsy is not required.
Complete excision of such neoplasms in surgically accessible sites leads to patient cure. Indications for radiation therapy in extremity liposarcomas are not described in these materials.
Class and exam questions
What is the difference between atypical lipomatous tumor (ALT) and well-differentiated liposarcoma (WDL)?
They are genetically and histologically identical. The difference lies only in terminology depending on location: ALT for surgically accessible sites (extremities), WDL for surgically difficult sites (mediastinum, retroperitoneum).
How to distinguish a benign lipoma from a liposarcoma on MRI?
Liposarcoma is favored by a size over 10 cm, thick contrast-enhancing septa, the presence of non-adipose nodules, and incomplete fat suppression.
Why can lipoblastoma in children cause diagnostic challenges?
Due to myxoid areas, it is heterogeneous on MRI and mimics myxoid liposarcoma, although it is benign and eventually matures into a lipoma.
What else to study on this topic
- Morphological structure and grading of sarcomas
- Features of STIR and fat-suppressed sequences in MRI
- Management of nodular fasciitis
- Surgical approaches and the concept of "wide margin" in excision
- Genetic markers of liposarcomas
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