This 60-year-old male presents with low back pain radiating to the anterior thigh and anterior calf within the last 6-8 months with no known injury.
Where is the mass located? Do you think the mass is aggressive or nonaggressive? What is the differential diagnosis? Can you name the four types of liposarcoma?
The mass is retroperitoneal and displaces the vascular tree (image 1, pink arrow) anteriorly. Based on its size, heterogeneity, and invasion of the ilium (image 3, green arrow), the mass can be deemed aggressive or malignant. The family of retroperitoneal sarcomas that should be considered includes:
- Malignant fibrocystiocytoma
- Synovial sarcoma
- Mesenchymal sarcoma
- Undifferentiated sarcoma
- Soft tissue Ewing sarcoma
- Synovial sarcoma.
Multiple foci of high signal are present within the lesion (images 1 and 3, orange arrows), and these foci could represent blood or fat. As fat, a diagnosis of liposarcoma would be favored. The four major types of liposarcoma are:
- Well-differentiated (looks mostly like fat with exuberant irregular septae)
- Myxoid liposarcoma (high signal intensity on T2 and cyst-like in character throughout)
- Round cell liposarcoma (mostly solid with scattered areas of fat)
- Undifferentiated liposarcoma (can look like anything and must be identified with pathologic staining)
For your educational and historic interest, malignant fibrous histiocytoma has been previously subdivided into several histologic and visual types, though these types have been reclassified. In the past, these have included:
- Pleomorphic with hemorrhage
- Malignant giant cell variant or giant MFH of small parts
- Myxoid or myxofibrosarcoma
Ewing sarcoma is one lesion that likes to attach itself to flat bone or arise directly from flat bone. This patient does have direct involvement of the flat bone either primarily or secondarily seen on image 3 (green arrow). These large Ewing sarcomas may be very pleomorphic-appearing on MRI and tend to bleed. Simple fibrosarcomas are often devoid of blood and certainly devoid of fat. When looking at a sarcoma, follow these clues for analysis:
- Large size = favors malignancy
- Bone invasion = favors malignancy
- Heterogeneity = favors malignancy
- Blood = favors malignancy
- Wide zone of transition = favors malignancy
- Transgresses compartments = favors malignancy
- Pain less than expected for lesion size = favors malignancy
MOST LIKELY DIAGNOSIS: Liposarcoma round cell variant
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