[Replacement lipomatosis of renal tissue: a peculiar reaction to inflammation]

Replacement lipomatosis is an abnormal fatty proliferation of the renal sinus, hilus, perirenal and, sometimes, periureteral spaces. Our experience with 18 cases of replacement lipomatosis is reported: 6 patients had the massive form, one of them bilateral, 5 patients had associated xanthogranulomatous pyelonephritis and 7 had an initial focal form. After a thorough review of the literature of the last 30 years, we analyzed the etiopathogenetic bases of this abnormal chronic reaction. The cause of this abnormal fatty proliferation is thought to be related to peculiar individual reactivity; the association of predisposing factors (chronic obstruction and stones) with multiple infections (especially by Coli) does not seem to cause the abnormality. This hypothesis is confirmed by the frequent association of replacement lipomatosis and xanthogranulomatous pyelonephritis, where abnormal individual reactivity has been postulated. Replacement lipomatosis may develop even when the classic predisposing factors are not present or, in contrast may not develop even when they are present. The peculiarity of this pseudotumoral form, where the expansile trend of fatty tissue is apparent, is to mimic neoplastic mesenchymal disease. So far, few cases have been reported in the literature-i.e., less than 20 massive cases in the last 30 years. In this study, the clinical presentation and pathologic findings are correlated with imaging data, especially of CT; the diagnostic yield of this method is stressed, in one with the differential diagnosis with the other, neoplastic or not, forms with negative densitometry. The similarities and differences between replacement lipomatosis and xanthogranulomatous pyelonephritis, which are distinct conditions, are emphasized, so that a careful study of imaging findings may help make the correct diagnosis.