65-year-old man, with a history of transurethral resection (TUR) for high-grade superficial transitional cell bladder neoplasia (pT1 G3), with subsequent low-grade recurrence, with the following history of interest: smoker of more than 1 pack/day, AHT, severe COPD and dilated cardiomyopathy.
During follow-up for his bladder neoplasia, an abdominal computed tomography (CT) scan was performed without intravenous (IV) contrast, in which a heterogeneous nodular lesion with fatty content (hypodense), 2 cm in diameter, well-defined borders and retroperitoneal location, posterior to the inferior vena cava, was observed as a casual finding. The study was completed with an abdominal ultrasound scan showing a retroperitoneal hyperechoic lesion in contact with the right lateral aspect of the inferior vena cava.

A wait-and-see approach was adopted and a follow-up CT scan was performed one year later, showing an enlargement of the mass (4 cm maximum diameter) which displaced and compressed the inferior vena cava medially without showing signs of invasion. In view of the suspicion of retroperitoneal liposarcoma, magnetic resonance imaging (MRI) with IV gadolinium was performed, confirming the fat content of the lesion and a discrete heterogeneous contrast uptake. The T1 sequences showed a hyperintense lesion, although slightly inferior to the retroperitoneal, mesenteric and subcutaneous fat, with an area of lower intensity in the upper pole of the lesion. The fat suppression technique showed a discrete hyperintensity compared to perilesional fat mainly in the upper half.

In the opposite phase images the lesion was hypointense with a nodular area at its upper pole of higher signal intensity corresponding to the area of lower intensity in the T1 sequence and higher intensity in the fat suppression. After gadolinium administration, a discrete gadolinium uptake was observed.
With the radiological diagnosis of a fatty tumour, probably liposarcoma (given its predominant frequency among retroperitoneal masses), surgery was decided. A reniform mass was surgically removed, posterior to the inferior vena cava and anchored in the retroperitoneum to the anterior vertebral ligament, 2 cm from the right renal vein at its lower edge. The lesion had no vascular pedicle, showing only small peripheral lymphatics.
A pseudoencapsulated lobular tumour measuring 6 x 5 cm, with an adipose consistency and a greyish-yellowish colour was referred for pathological examination. Histology of the specimen revealed a well-demarcated adipose tumour, showing extensive haematopoietic islets with the presence of the 3 series together with rounded lymphoid aggregates, diagnostic of lumbar prevertebral myelolipoma, although a bone marrow study was recommended in order to rule out its primary pathology (extramedullary haematopoiesis in the context of a myelodysplastic-myeloproliferative syndrome). The latter possibility was ruled out by analysis of the bone marrow, confirming the diagnosis of retroperitoneal extra-adrenal myelolipoma.