Differential diagnosis, investigations and treatment:
Upon this admission, routine lab tests and imaging were performed. Laboratory examination revealed elevated ESR (100 mm/hour) and increased serum creatinine (1.6 mg/dl). Non-enhanced chest and abdominopelvic CT with reconstruction of volumes at 1mm slice thickness showed bilateral isodense renal pelvis masses without calcification and with a maximum diameter of 80 x 75 x 92 mm on the right and 90 x 102 x 126 mm on the left side. Multiple non-significant mediastinal and periaortic lymph nodes were also detected with a maximum short-axis diameter (SAD) of 8 mm. No skeletal blastic or lytic lesions were noted (Figure 1).
For further evaluation of the renal lesions, abdominal magnetic resonance imaging (MRI) was carried out by a 1.5 Tesla scanner (Magnetom Avanto; Siemens Healthineers, Erlangen, Germany) using a sixteen-channel phased array coil. MRI images included standard sequences such as T1-weighted, T2-weighted, diffusion-weighted imaging (DWI) with b-value of 50-400-800 s/mm2 and apparent diffusion coefficient (ADC) maps. In addition, contrast-enhanced 3-dimensional T1-weighted gradient-echo was performed in axial and coronal planes. The renal pelvis lesions displayed hypointense to isointense signals on T1-weighted images and were isointense to hyperintense on T2-weighted images with slight heterogeneity. On ADC maps, mild restricted diffusion with low signal intensity was exhibited. Masses had gradual enhancement without washout in the delayed phase. Bilateral hydronephrosis along with signs of parenchymal loss (predominantly in the left kidney) was also noted but neither perinephric nor subcapsular mass-like lesions were seen. Other vital organs were preserved without any signs of infiltration and no abnormal signal or enhancement was found in the periaortic lymph nodes (Figure 2). To reach a definite diagnosis, CT-guided core needle biopsy was performed, which showed marked inflammatory cell infiltration with predominance of lymphocytes, plasma cells and activated histiocytes without any evidence of granuloma formation; however, the histopathological findings were inconclusive.
Due to persistently raised creatinine, the patient subsequently became a candidate for surgery and underwent right partial nephrectomy and left radical nephrectomy with a small residual mass remaining due to technical difficulties of complete resection. Histopathologic examination of surgical specimens revealed diffuse histiocytic proliferation and plasma cell infiltration, which was in favor of a histiocytic proliferative disorder, and the subtle presence of emperipolesis raised the suspicion for RDD. Finally, immunohistochemical (IHC) assay stained positive for S100 protein and negative for CKAE1/AE3 and CD-1a, confirming the diagnosis of RDD. Approximately 6-7 % of the plasma cells also stained positive for IgG4.