D. Classification

Classification of renal cell carcinomas has been a debatable field since Grawitz in 1883 emphasized their epithelial nature and favored an histogenesis from ectopic adrenal rests stressed by the term "hypernephroma". Based on electron microscopic studies, a tubular origin was assumed for all renal cell carcinomas, but a morphological subclassification of this complex of entities was not proposed by the WHO. In 1986 Thoenes et al. introduced a new classification of renal adenomas (RCAs) and carcinomas (RCC), which is based on two morphological criteria: 1. cellular characteristics, i.e. the different tumor cell types and 2. tissue characteristics, i.e. growth pattern in general. Meanwhile this proposal was completed and confirmed by cytogenetic and molecular genetic results and led to a new morphological classification.   More than 90% of tumors in the kidney which come to surgery are renal cell carcinomas and these cause approximately 2.4% of cancer deaths. There are no significant differences between the North American and Central European incidences, but in Scandinavia the frequency is higher and in Japan lower. The age-related peak incidence lies in the 6th decade with only a few cases occurring before the age of 40. The incidences shown in the box are drawn from the German Kidney Cancer Registry.   Frequency of Renal Cell Neoplasms in Surgical Series Clear cell renal cell carcinoma 75% Chromophil renal cell carcinoma 10-15% Renal oncocytoma 5% Chromophobe renal cell carcinoma 5% Collecting duct carcinoma <1% Renal cell carcinoma, unclassified <1% Neuroendocrine tumors <1% Metanephric adenoma <1% Interestingly, chromophil renal cell tumors predominate (82%) in patients with chronic renal insufficiency (hemodialysis). The distribution of the types differs with respect to tumor size. In autopsy and surgical series, more than 95% of renal epithelial tumors smaller than 10 mm in diameter are of the chromophil cell type. The incidence drops to a little bit less than 10% in the size group 5 -10 cm diameter, while the incidence of clear cell tumors rises to nearly 80%.