Discussion:
The RLS is a rare mesenchymal tumor representing 0.07% to 0.2% of all neoplasia. This tumor mainly affects people aged 40 to 60 years and its prevalence seems equivalent in both sexes [3] .
This tumor generally remains asymptomatic and can reach a large size (over 30cm) with possible invasion or displacement of neighboring organs. Indeed, the diagnosis is suggested when there is an abdominal mass syndrome, a significant increase in abdominal volume or when there are signs of digestive or urinary compression. This is explained by the fact that the retroperitoneum is a large expandable space without any bony boundaries [4 ,5] .
Differential diagnosis of RLS includes tumors with fatty components such as renal angioleiomyolipoma, adrenal myelolipoma, retroperitoneal lipoma, and teratoma [1] .
There are four histologic subtypes of liposarcoma: Undifferentiated, pleomorphic, well-differentiated and myxoid. The undifferentiated and pleomorphic types are associated to a high degree of malignancy[6] .
Computed tomography (CT) is the most common used imaging method for the diagnosis and preoperative evaluation of RLS. Indications for Magnetic resonance imaging are Musculo-skelettal involvement or pelvic extension[6] .
Surgery is currently the only potentially curative treatment. In fact, complete surgical resection is the cornerstone of non-metastatic RLS treatment. Furthermore, R0 resection is the most consistent prognostic factor [5] .
It is usually technically challenging to achieve clear resection margins. In most cases, it is necessary to proceed to an en-bloc resection including one or more organs with the RLS [7] . This means that surgeons tend to resect certain organs, especially the kidney, in the case of giant RLS, to avoid difficult dissection and mainly R1 resection margins [5] . For these reasons, our patient had an en-bloc resection of the RLS with the right kidney.
The only definite indication for nephrectomy is kidney invasion or perirenal fat infiltration. This should at best be suspected on preoperative imaging and must be confirmed intraoperatively[8-10] .
A RLS tumor that encases the kidney is a relative indication for nephrectomy. In this case, kidney conservation may be attempted but there is always a risk of incomplete resection [11, 12] . Some authors reported an en-bloc resection of the RLS with kidney and showed that there was no histologic tumoral involvement of the kidney[13, 14] as was the case of our patient. This means that nephrectomy could be avoided.
The absence of tumor (RLS) capsule break-in is a crucial condition for kidney conservation [15] . Avoiding nephrectomy seems also possible when the RLS is widely displacing the kidney [4, 16] . In addition to these criteria, the patient’s advanced age is another argument for trying to preserve the kidney [11] .
An ureter encasement is not a formal indication for nephrectomy. In fact, a thorough dissection can be attempted with a prior insertion of a double J catheter to reduce the risk of a possible ureteral injury. Ureteral tumoral infiltration must lead to its resection with renal auto-transplantation or to an ureterostomy [4, 17, 18].
The long-term prognosis of RLS with incomplete resection (R1) is poor with 5- and 10-years average survival rates at 16.7% and 8% respectively [5] .
Metastatic and/or unresectable RLS (invading vital structures) have obviously a very poor prognosis.
Neoadjuvant therapy could be an option to downstage the tumor before surgical intervention. Patients with positive surgical margins or high grade liposarcoma also need the consolidation from adjuvant therapy. Studies have shown that radiotherapy reduces the risk of local recurrence and improves the recurrence-free interval. However, data supporting positive impact of radiotherapy on overall survival is limited [19] . At present the role of chemotherapy in the management of RLS remains controversial [3] .
Multidisciplinary cooperation is particularly important in the treatment of giant retroperitoneal liposarcoma. Long-term follow-up CT scan is important, especially within 2 years after initial resection, which is peak recurrence period [5] .