Case 1: Dedifferentiated liposarcoma

Case 1: A 55 years old man presented with a left paratesticular mass

Educational notes:
1. This para-testicular mass is a dedifferentiated liposarcoma (LPS) showing transition of a well differentiated liposarcoma (LPS) to non-lipogenic sarcoma. The dedifferentiated lipomatous  component is composed of scattered marked pleomorphic malignant cells set in an edematous inflammatory stroma. Some of the malignant cells are in multinucleated form. 
The nature of this malignant tumor is revealed with the presence of the adjacent well differentiated LPS. The well-differentiated LPS is composed of mostly mature adipose showing marked variation in size and shape. Atypical stromal cells displaying enlarged  hyperchromatic nuclei are seen within the fibrous septa. Occasional adipocytes with atypical enlarged hyperchromatic nuclei are also seen.
2. Dedifferentiated LPS is usually a non-lipogenic sarcoma of variable histological grades. The diagnosis of dedifferentiated LPS lies in recognition of progression of an atypical lipomatous tumor/ well-differentiated LPS in the primary or in a recurrence.
3. Nonetheless, the well-differentiated component may not be identifiable in some cases. 
Diffuse nuclear expression of MDM2 and/or CDK4 immunohistochemistry or demonstration of MDM2 gene amplification by FISH would be helpful to differentiate dedifferentiated LPS from other high-grade sarcomas in the appropriate setting, especially in the case of sarcomas arising in the retroperitoneum and spermatic cord. 
4. Dedifferentiated LPS has a high local recurrence (at least 40%) and develops distant metastases in 15-20%. Nevertheless, as compared to the other high-grade sarcomas, dedifferentiated LPS has a less aggressive clinical course.

Reference:
1. WHO Classification of Tumours Editorial Board. Soft tissue and bone tumours [Internet]. Lyon (France): International Agency for Research on Cancer; 2020