By A. Vaglio
Idiopathic retroperitoneal fibrosis is characterised by a fibro-inflammatory retroperitoneal mass, which often entraps the ureters and is usually localised in the peri-aortic and peri-iliac regions, but it rarely affects the perirenal space. Histologically, lymphocytes, plasma cells and macrophages are found within variable amounts of fibrosis, but “foamy” histiocytes and multinucleated giant cells are usually absent 2,3.
Thus, in our case, on the basis of the diffuse distribution of the retroperitoneal infiltration and of the presence of histiocytes, the diagnosis of histiocytosis seemed to be the most likely.
Histiocytes can be divided into two groups: the first derives from the monocyte-macrophage group and includes monocytes, macrophages, and Kupffer cells of the liver. The second group derives from the Langerhans cell-dendritic cell group and is the histiocytic cell type involved in histiocytosis X, now called Langerhans-cell histiocytosis (LCH); LCH includes Hand-Schüller-Christian disease, Letterer-Siwe disease and eosinophilic granuloma 4,5. Langerhans-like histiocytes usually stain positive for S-100 protein and CD1a leukocyte antigen, and electron microscopy of their cytoplasm discloses Birbeck granules in more than 20% of cells. In addition to the above cited forms of histiocytosis, Erdheim-Chester disease (ECD) also had to be considered in the differential diagnosis of our case: ECD is a rare, non-Langerhans form of histiocytosis which often affects the retroperitoneal peri-aortic and peri-renal regions. In ECD, histiocytes differ from LCH because of their negative stain for S-100 and CD1a, and usually do not show Birbeck granules.
Therefore, we decided to perform an immunohistochemical analysis of the histologic sections of the retroperitoneal biopsies. The immunohistochemistry results showed that the foamy histiocytes stained negative for S-100 and CD1a. Unfortunately, histological sections for electron microscopy were not available in our case. Thus, the hypothesis of ECD seemed to be quite reliable, but it had to be confirmed by other diagnostic tests.