Saturday, February 28, 2015

The Frequency of Various Types of Cancers Explained

For more than a century, the medical world has known that certain kinds of cancers are far more prevalent than others.   The question, of course, that comes to mind is why is this so?  There has been much speculation concerning the answer to this important question.

Drs. Cristian Tomasetti and Bert Vogelstein form the Division of Biostatistics and Bioinformatics at the Department of Oncology at the Sidney Kimmel Cancer Center at John Hopkins University School of Medicine and the Department of Biostatistics at the John Hopkins Bloomberg School of Public Health in Baltimore MD conducted an exhaustive statistical analysis of patient data.  The following represents a summary of their results.

 The table below shows the relative lifetime risk of a number of different types of cancers.
Cancer by Tissue Type
Percentage of Lifetime Risk of Cancer
Brain and Nervous System
Pelvic Bone
Laryngeal Cartilage

Although some of these differences can be associated with certain risk factors such a smoking and alcohol use, ultraviolet light exposure and human papilloma virus (HPV) infection, such etiology only applies to specific populations.  In addition, environmental factors cannot explain the wide differences found in lifetime risks involving cancers of the alimentary tract – esophagus .51%, large intestine 4 82%, small intestine .20% and stomach .86%.
Interestingly, cancers of small intestine are three times less common than brain cancers even though the epithelial cells of the small intestine are far more exposed to environmentally dangerous substances than brain cells that are protected by the so-called blood –brain barrier.
Another factor that is often cited to explain differences in risk of various cancers is inherited genetic variation.  The statistical data shows, however, that this risk factor accounts for only between 5 and 10 percent of the etiology of cancer.

Therefore, there must be another cause that accounts for the wide variability shown in the table above.  The investigators went on to demonstrate that a very close correlation (81%) exists between the lifetime risk for a given cancer and the, “total number of divisions of the normal self-renewing cells (stem cells) maintaining the tissue’s homeostasis.”   From this perspective, it is the probability of sustaining deleterious random genetic mutations that transform a cell into a cancerous state that increases with the number of cell divisions of tissue-specific stem cells.

This may prove to be a very important finding in regards to understanding the etiology of cancer. 

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