The Idea of Cancer and the Idea of Screening

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The public's understanding of how cancer works is central to its understanding of how screening works and thus to its strong interest in screening. Especially relevant is the public's perception of the development and progression of cancer and of the degree of homogeneity of cancers with the same name (e.g., breast cancer) in their malignant potential.

Although the process of cancer development is not completely understood, it is clear that a normal cell does not become cancer suddenly, all at once. Rather, cells undergo a number of assaults over time, with various results.2 Some of these assaulted cells develop various abnormal forms, or "intermediate lesions," such as cervical intraepithelial neoplasia (CIN), colonic polyps, or ductal carcinoma in situ of the breast (DCIS). Although not cancer themselves, these intermediate lesions do at times develop into cancer.

As screening frequently detects intermediate lesions, their natural history is important. If nearly all intermediate lesions progress to malignant cancer, then early detection and treatment would appear to be an effective strategy for cancer control. The detection of intermediate lesions would be a triumph. By interdicting the developing cancer at this early point (i.e., even before it can be called a cancer), treatment could eradicate a lesion that would have caused major health problems in the years to come.

With many intermediate lesions, however, the majority (most often, the great majority) never progress to invasive cancer. Thus, screening often results in detecting and treating intermediate lesions that do not need to be detected or treated. If there are any harms to this early detection and treatment, the magnitude of these harms must be counted against the magnitude of the benefit. It is doubtful if many people understand this result of screening, or at least the frequency with which it occurs.

After cancer develops, a critical issue is the extent to which it uniformly progresses in a linear and inevitable manner to cause symptoms and death. If cancer is always an inexorably progressive condition, it is intuitively appealing to think that early detection is an effective strategy for cancer control. The experiences of people who have cancer with the same name (e.g., breast cancer) would vary little; all would be destined for a difficult death because the cancer had progressed too far for effective treatment. Again, the facts are otherwise.

Cancers, even cancers with the same name (e.g., breast cancer), vary widely in their growth rate and malignant potential. Studies have found that cancers that vary with respect to certain cell markers have different prognoses.3 Gene expression profiling using DNA microarrays4,5 has shown the genetic heterogeneity of individual breast cancers. There is not one type of breast (or colorectal or prostate) cancer, but a number of types, each with a different natural history. Together with the probable but largely unknown ways in which individual susceptibility varies, these cancer types produce great variation in the ways a particular cancer is expressed. Some cancers in certain individuals grow rapidly and are lethal within a short time, regardless of our best treatments. Screening is unlikely to make a difference for people with such cancers, which may metastasize from the first cell.

Other cancers with the same name grow more slowly, or not at all. People with some of these latter cancers may be greatly helped by early detection and treatment; others have cancers that do not need to be detected and treated at all. In some cases, lesions that clearly meet histologic criteria for cancer do not cause important clinical problems. Experts have termed this last group pseudodisease, lesions that appear to be cancer but do not progress to clinically important disease. It is the existence of this type of cancer, less malignant and less requiring of treatment, that gives pause to the push for screening. Here are cancers that do not need to be found early; some of them do not need to be found at all.

Much of the public has another conception of how cancer works. The word cancer usually means a condition that universally and inevitably progresses, a condition that is potentially fatal in every case. The fact that some people have long-term survival after cancer diagnosis is attributed to some exceptional characteristic of the individual or to effective treatment. Intermediate lesions are called premalignant; the popular conception is that they too inexorably progress to cause major clinical problems. This incorrect view of the nature of cancer is an important underlying reason for the popularity of cancer screening. As people have commented to the authors, cancer screening "simply makes sense." Given this view of cancer, one can understand their thinking.

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10 Ways To Fight Off Cancer

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