Benignandmalignanttumorscanbedistinguishedfromoneanotherbasedonthedegreeofdifferentiation,rateofgrowth,localinvasiveness,anddistantspread.
Benigntumorsresemblethetissueoforiginandarewelldifferentiated;malignanttumorsarepoorlyorcompletelyundifferentiated(anaplastic).
Benigntumorstendtobeslowgrowing,whereasmalignanttumorsgenerallygrowfaster.
Benigntumorsarewellcircumscribedandhaveacapsule;malignanttumorsarepoorlycircumscribedandinvadethesurroundingnormaltissues.
Benigntumorsremainlocalizedtothesiteoforigin,whereasmalignanttumorsarelocallyinvasiveandmetastasizetodistantsites.
Study of cancer occurrence in populations has contributed substantially to knowledge about its origins. The now well-established concept that cigarette smoking is causally associated with lung cancer arose primarily from epidemiologic studies. A comparison of the incidence rates for colon cancer and dietary patterns in the Western world and in Africa led to the recognition that dietary fat and fiber content may figure importantly in the causation of this cancer. Major insights into the causes of cancer can be obtained by epidemiologic studies that relate particular environmental, racial (possibly hereditary), and cultural influences to the occurrence of specific neoplasms. Certain diseases associated with an increased risk for developing cancer also provide clues to the pathogenesis of cancer. The following discussion first summarizes the overall incidence of cancer to provide insight into the magnitude of the cancer problem and then reviews factors relating to the http://ebooksmedicine.net
Large Locally invasive Poorly demarcated Metastatic Rapidly growing with Poorly differentiated
Fig. 6.12 Comparisonbetweenabenigntumorofthemyometrium(leiomyoma)andamalignanttumorofsimilarorigin(leiomyosarcoma).
patient and the environment that influence the predisposition to cancer.
For the year 2012, the World Health Organization (WHO) estimated that there were about 14.1 million new cancer cases worldwide, leading to 8.2 million deaths (approximately 22,500 deaths per day). Moreover, due to increasing population size, by the year 2035 the WHO projects that the numbers of cancer cases and deaths worldwide will increase to 24 million and 14.6 million, respectively (based on current mortality rates). Additional perspective on the likelihood of developing a specific form of cancer can be gained from national incidence and mortality data. In the United States, it is estimated that the year 2016 will be marked by approximately 1.69 million new cases of cancer and 595,000 cancer deaths. Incidence data for the most common forms of cancer, with the major killers identified, are presented in
Fig. 6.13
Over several decades, the death rates for many forms of cancer have changed. Since 1995, the incidence of cancer in men and women in the United States has been roughly stable, but the cancer death rate has decreased by roughly 20% in men and 10% in women. Among men, 80% of the decrease is accounted for by lower death rates for cancers of the lung, prostate, and colon; among women, nearly 60% of the decrease is due to reductions in death rates from breast and colorectal cancers. Decreased use of tobacco products is responsible for the reduction in lung cancer deaths, while improved detection and treatment are responsible for the decrease in death rates for colorectal, female breast, and prostate cancers.