Which differentials to consider in this case?
This retroperitoneal mass in a young male adult raises concern for lymph node metastatic disease from testicular tumour or lymphoma. The appearances are favoures a dense mass displacing structures rather than invading, therefore, agressive retroperitoneal sarcomas are felt unlikely.
Extensive retroperitoneal mass on the left likely represents coalescent lymphadenopathy, partially encasing the anteriorly displacing the aorta and IVC, as well as causing proximal ureteral obstruction with severe left hydronephrosis. Most of the fat planes between the mass and adjacent retroperitoneal structures are not clearly identified. Small fat containing umbilical hernia.