Kidney cancer is NOT one disease.   Cancers of the kidney are actually a series of very different diseases that have different types of behavior.  While cancer is never totally predictable, the type of kidney cancer has a major impact on overall prognosis and the type of surveillance (follow-up) that a patient will require.  In the past, a physician may have referred to “kidney cancer” as a disease.  Today, it is clearly recognized as a group of different diseases and physicians divide these diseases into different categories.

In the past, the classification of kidney cancer was been based on “cytomorphological” characteristics.  Simply put, a small piece of the kidney cancer that was extracted was traditionally evaluated under a standard microscope.  The basic principles of microscopic evaluation of kidney cancers have changed little for a century. 

In 1996, an international group of experienced pathologist met in Heidelberg Germany to re-evaluate the manner in which kidney cancers are classified.  This group of pathologist combined traditional microscope findings with more advanced molecular studies to re-define kidney cancer types.  The published summary of this group’s findings is now known as the Heidelberg Classification.

The value of the Heidelberg consensus was that recent advances in our understanding of the genetic code underlying kidney cancer were incorporated into the thought process of physicians.  Indeed, it is the genetic changes in different kidney cancer types that results in the specific behavior of the disease.  Genetics affect the rate at which a kidney cancer will grow as well as the kidney cancer’s ability to spread to other parts of the body (metastasize).

The new understanding of the genetic changes which resulted in the creation of different types of kidney cancer allow for more precise stratification of the different types that exist.  The Heidelberg classification combines the traditional appearance of kidney cancer tissue under a microscope with genetic molecular classification to more precisely divide tumors.  This precise description of tumors allows physicians and patients to better understand and treat the different types of kidney cancer.  There are two major initial types of kidney cancer within the Heidelberg classification; benign and malignant kidney cancers. 

Benign Kidney Cancer Variants 
Generally, the definition of a benign tumor is that it does not have the ability to metastasize (spread via blood or lymphatic channels to other parts of the body).  Benign kidney cancers can still cause harm by growing and causing local damage to tissues.  However, compared to malignant kidney cancers, benign tumors are generally more easily cured.

Tumor Type	Description	Genetic Changes
Metanephric Adenoma	Consists of tubules and tissues that are like the kidney tissue of a human embryo.  Previously known as ‘nephrogenic adenofibroma’	Unknown
Papillary Renal Cell Adenoma	Mostly these tumors are incidental findings a the time of autopsy, or they are found in conjunction with papillary renal cell cancer variants in the tissue that has been removed	Trisomy of chromosomes 7 and 17 and loss of the Y chromosome
Oncocytoma	The most common of benign tumors removed by surgeons.  The pattern of growth consists of typical oncocytic cells	Loss of chromosome arms 1p, 14q, and loss of the y chromosome or translocation of 11q13 to other chromosomes

Malignant Kidney Cancer Variants
A malignant tumor is defined as a tumor type that has the capacity to metastasize.  Malignant kidney cancer variants are more common than benign kidney cancer types and have the ability to spread via the bloodstream, lymphatic channels, or direct growth into other parts of the body.  With any kidney growth that is discovered, a physician performs a “staging evaluation” prior to treatment.  A staging evaluation is a series of blood and imaging (radiographic tests) to determine if the tumor may have spread beyond the confines of the kidney.

Carcinoma Variant	Description	Genetic Changes
Conventional	Most common type of kidney cancer making up about 75% of all tumors.  Originally known as ‘clear cell carcinoma’ as it is primarily composed of cells with a clear appearance	Highly characteristic deletion on chromosome 3p known as the VHL gene.  Other changes include duplication of band 5q22 and deletions of chromosome arms 6q, 8p, and 14q
Papillary	Accounts for approximately 10% of all kidney tumors and is characterized by a ‘papillary’ growth pattern. These tumors are frequently multifocal (more than one in a kidney) and may be bilateral (involve both kidneys)	Trisomy of chromosomes 3q, 7, 8, 12, 16, 17, and 20, and loss of the Y chromosome are the most consistent changes
Chromophobe	Approximately 5% of all kidney growths and is has a pattern of solid sheets that are typically pale.	Loss of heterozygosity of chromosomes 1, 2, 6, 10, 13, 17, and 21. Often has multiple copies of chromosomes.
Collecting Duct Carcinoma	A rare kidney cancer variant consisting of only about 1% of all kidney growths.  Microscopically appears to have irregular channels.	Unknown
Unclassified	There are still a small number of kidney tumors that can not be classified under the microscope or by molecular genetic evaluation	Unknown

Copyright 2006 Kidney Cancer Institute