• Multicystic dysplasia is the most common form of renal cystic disease in childhood. The term dysplasia in this context refers to a developmental rather than a preneoplastic lesion. Because renal dysplasia is often associated with obstruction in the lower urinary tract, increased hydrostatic pressure in the developing kidney is thought to play a role in its development. The kidneys are usually grossly distorted; the cysts range from microscopic to several centimeters in diameter. The histologic hallmarks are ducts and tubules lined by epithelial cells and surrounded by cuffs of cellular mesenchyme.
Renal agenesis: Bilateral agenesis is incompatible with life and usually is encountered in stillborn infants. It is often associated with other congenital disorders (e.g., limb defects, hypoplastic lungs). Unilateral agenesis is uncommon and compatible with normal life if no other abnormalities exist.
Hypoplasia may occur bilaterally, resulting in renal failure in early childhood, but is more commonly encountered as a unilateral defect. The hypoplastic kidney shows no scars and has a reduced number of renal lobes and pyramids, usually six or fewer.
Many types of benign and malignant neoplasms occur in the urinary tract. In general, benign neoplasms such as small cortical papillary adenomas (<0.5 cm in diameter), which are found in up to 40% of adults in autopsies, have limited clinical significance. The most common malignant neoplasm of the kidney is renal cell carcinoma, followed in frequency by nephroblastoma (Wilms tumor) and by primary neoplasms of the calyces and pelvis. Other types of renal cancer are rare and are not discussed here. Neoplasms of the lower urinary tract are about twice as common as renal cell carcinomas. They are discussed in Chapter 18.
Oncocytoma, a benign neoplasm that arises from the intercalated cells of collecting ducts, represents about 10% of renal neoplasms. These neoplasms are associated with genetic changes—loss of chromosomes 1 and Y—that distinguish them from other renal neoplasms. Oncocytomas are characterized by a plethora of mitochondria, providing the basis for their tan color and their finely granular eosinophilic cytoplasm, seen histologically. A central stellate scar, which is another feature of oncocytomas, provides a characteristic appearance on imaging studies.
Renal cell carcinomas are derived from the renal tubular epithelium, and hence they are located predominantly in the cortex. These neoplasms represent 80% to 85% of all primary malignant neoplasms of the kidney and 2% to 3% of all cancers in adults. These data translate into about 65,000 cases per year in the United States; 40% of patients die of the disease. Carcinomas of the kidney are most common from the sixth to seventh decades, and men are affected about twice as commonly as women. The risk for developing these neoplasms is higher in smokers, hypertensive or obese patients, and those who have had occupational exposure to cadmium. The risk for developing renal cell cancer is increased 30-fold in individuals with acquired polycystic disease as a complication of chronic dialysis. The role of genetic factors in the causation of these cancers is discussed later.
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Renal cell cancers are classified on the basis of morphology and growth patterns. However, recent advances in the understanding of the genetic basis of renal carcinomas have led to a new classification that takes into account the molecular origins of these tumors. The three most common forms, discussed next, are clear cell carcinoma, papillary renal cell carcinoma, and chromophobe renal carcinoma.