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TOPIC 9.docx

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HLTH 101
Glenn Ward

TOPIC 9: INTRODUCTION TO CANCER 1. INTRODUCTION  the terms cancer, neoplasia and malignancy are often used interchangeably o generally, cancer is used to describe the disease involving neoplasia (roughly meaning growth of new tissue) o malignancy refers to the later stage of cancer in which the growth has spread to other tissues 2. IMPACT OF CANCER  the risk is higher than the mortality rate  in Canada: o the lifetime cancer risk is: greater than 40% for women greater than 45% for men o cancer is also the cause of 25% of all deaths o in 2009 there were 171,000 new cases of cancer and 75,300 deaths due to cancer o incidence and mortality rates reflect that fact that cancer is a late-onset disease*  incidence rate is highest for those in their 60’s  mortality rates are highest for those in their 70’s and 80’s*  rates generally decline after age 90 o cancer is the 2nd leading cause of death in Canada and most of the developed world o after age 75 it is the leading cause of death o cancer has the greatest numbers of YLL*  die before what would have been their normal life span  between ages 45-64 cancer deaths are greater than the next 3 causes combined  1971: U.S. President Nixon signs the “National Cancer Act”, commonly known as the "war on cancer"* o after 40 years, has the war been won? o in fact, it has been difficult to show beyond a doubt that the “war” has been won  this can be contrasted with the case of cardiovascular disease, for which deaths have declined by 50% or more in Canada over the past 30 years o if it’s not clear that we have made substantial gains in reducing incidence rates and mortality due to cancer, this is likely due mainly to three unique challenges to the study of cancer 3. UNIQUE CHALLENGES TO THE STUDY OF CANCER i) determination of the size of the problem*  recall that incidence and mortality rates are often used to establish the “size” of a health problem a) incidence rates* o cancer incidence rates actually increased for at least 25 years after the war on cancer was initiated  a little over a decade ago, the age-adjusted incidence rate for cancer finally started to decrease  could the initial increase in incidence rates actually be due to the effects of increases in the diagnosis of cancer rather to a real increase in the cancer rate? o there are at least two possible reasons for an increase in diagnosis* 1) publicity campaigns heightening the awareness of early detection for cancer can often lead to more diagnoses of cancer* 2) the development of early detection techniques that may detect cancers that would have been missed in earlier times* b) mortality rates* o although the incidence rate of cancer has increased since the early 1970’s, the 5- year survival rate (the rate of people with cancer who survive at least 5 years following the diagnosis) has also increased  is this an indication of successful treatment or is it due to early detection? o we know that cancer screening is far more common today, and increased cancer screening can lead to at least 3 potential biases:* 1) screening bias*  since cancer screening is generally done on a volunteer basis, it has been suggested that those who undertake cancer screening are also those who take other health precautions and are therefore likely to be more healthy than the general population  if they are more healthy, then they may have a better prognosis  therefore, could there be an over-diagnosis of cancers for which the patient is likely to survive longer? 2) lead-time bias*  in lead time bias, the cancer has been identified during the asymptomatic period  as screening methods are refined, cancer can be identified earlier  but the earlier cancer is identified, the longer the interval between diagnosis and death, making it appear that people with cancer are surviving longer 3) length bias* o if a tumour is particularly aggressive, it will develop and grow at a relatively rapid rate  these types of tumours are less likely to be detected early due to the fact that its growth may extend beyond the treatable period within the latency period between testing  slow tumours, on the other hand, are more likely to be detected at early stages by screening o slower, more easily treatable tumours may be the ones most detected by screening and, therefore, people who have cancer detected by screening may not be a representative sample but may be those who’ve been diagnosed with a cancer that is unlikely to cause death within 5 years  the U.S. National Cancer Institute has reported that there have been slight gains in survival from cancer since 1971, even when taking into account the biases of screening, etc.  the improvements have mostly been for childhood cancers (Hodgkin’s disease, leukemia, etc.), prostate cancer, breast cancer, and lung cancer in males ii) identification of the causes of cancer*  since cancer is a chronic disease and not a point-onset disease, it’s very difficult to determine the nature of the agent that may have caused the initial changes that eventually became cancer o therefore the search for the causes of cancer requires the skilled use of epidemiology*  in this approach, groups of individuals who develop specific cancers are investigated in order to find the factor or factors they had in common at prior times in their lives  however, this approach leads only to associations that, while they may be inferred to be the “causes” of a cancer, are still often controversial  furthermore, the discovery of possible causes of specific cancers can lead to the desire to reduce or eliminate the factors that someone has discovered may be causing cancer  often, these factors cannot be reduced without great social or political cost o therefore, the search for causes of cancer can often have political and social considerations* iii) prevention/treatment*  as is the case with many serious diseases, we need to be able to evaluate the effectiveness of proposed treatments and prevention strategies  standard research methods to evaluate this effectiveness generally involve intervening in some cases of cancer but not others, in order to observe subsequent differences in outcomes  this approach is rife with ethical issues that can sometimes compromise the ability to obtain the information we need to make informed decisions about treatment and prevention strategies 4. DEFINING CHARACTERISTICS OF CANCER i) Clonality:  all tumour cells derive from a single somatic cell which proliferates beyond the influence of the normal controls on cell division* (monoclonal)  therefore, the original tumour cell is a single cell that forms clones of itself Evidence:  females who are heterozygous for alleles on the X chromosome* o females have two X chromosomes but only one is active in each cell o the process of inactivation occurs early in embryological development and is random, so that one X chromosome is active in some cell lines and the other is active in the rest of the cell lines  therefore, females are mosaics for genes having different alleles on each X chromosome* o therefore, if females are heterozygous for alleles on the X chromosome, approximately half of their cells should express one allele, and half should express the other o if tumours arose from more than one cell (i.e., if the tumours were polyclonal):  then the tumours would show expression of both alleles o therefore, we should look for women who are known to be heterozygous for alleles of genes residing on the X chromosomes  African women are good candidates for such a study because they have a relatively high frequency of gluc
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