Infiltrative renal lesions: Radiologic-pathologic correlation

Most renal masses exhibit an expansile growth pattern characterized by radi al tumor enlargement that displaces normal renal parenchyma and forms spher ical, often exophytic, lesions. These expansile masses have pushing margins that impress adjacent normal renal parenchyma but do not infiltrate it; th is behavior results in a well-defined, encapsulated appearance at both radi ologic and gross pathologic examination. In contrast, certain disease proce sses involve the kidney in an infiltrative fashion by using the normal rena l architecture as scaffolding for interstitial growth. These infiltrative r enal lesions lack a sharp border of demarcation with the normal parenchyma and therefore demonstrate ill-defined zones of transition between the lesio n and normal parenchyma. Although infiltrative lesions frequently enlarge t he kidney, its reniform shape is usually maintained. Cross-sectional imagin g can often help distinguish infiltrative from expansile growth patterns th rough analysis of the parenchymal interface between the process and the kid ney, the effect of the lesion on the collecting system and renal sinus, and the overall renal morphology. A wide variety of neoplastic and inflammator y conditions characteristically involve the kidney by cellular infiltration . Although considerable overlap of the imaging features exists among the va rious infiltrative processes, the correct diagnosis may be suspected when t he clinical data and associated radiologic findings are considered together .