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| Detecting and Diagnosing Cancer |
The most common techniques for detecting cancer are imaging techniques such as MRI, X-rays (such as mammograms), CT, and ultrasound, which can provide an image of a tumor. Endoscopy allows a physician to insert a lighted instrument to look for tumors in organs such as the stomach, colon, and lungs. Most of these techniques are used to detect visible tumors, which must then be removed by biopsy and examined microscopically by a pathologist. The pathologist looks for abnormalities in the cells in terms of their shape, size, and structure, especially the nucleus. In addition, the pathologist looks at the borders of the tumor to see whether those cells are normal. Based on examination of the tumor cells, the pathologist determines whether the tumor is benign or malignant, and determines whether is in an early or late stage of development. Diagnosis may also include the removal and examination of lymph nodes to determine whether the cancer cells have spread.
Tumor markers are proteins found more often in the blood of individuals with the tumor than in normal individuals. These are not ideal compounds for diagnosing of cancer for two reasons. First, individuals without cancer may have elevated levels of the marker, leading to false positives. Second, tumor markers are not sufficiently elevated in all individuals with cancer to allow their detection. This leads to false negatives. One of the most commonly used tumor markers is prostate-specific antigen (PSA). It is present in all adult males, but its level is increased after both benign and malignant changes in the prostate. Therefore, high levels of PSA indicate only that further tests are required to determine whether the condition is cancer. If prostate cancer is diagnosed, the levels of PSA can help to determine the effectiveness of treatment and detect recurrence. Another tumor marker is CA125, which is produced by a number of different cells, particularly ovarian cancer cells. It is used primarily to monitor the treatment efficacy of ovarian cancer. When the cancer is responding to treatment, CA125 levels fall. It is not used as a routine test for ovarian cancer because many common conditions that cause inflammation also increase the level of CA125, leading to a high incidence of false positives.
The earlier a cancer is found the more effectively it can be treated; however, early stage cancers typically produce no symptoms. Scientists are developing molelcular techniques to detect very early cancer. Using techniques such as mass spectrometry, they are also developing specific blood tests to identify a pattern of new proteins in the blood of individuals with a particular type of cancer. (See the Proteomics unit.) In addition, scientists are developing DNA microarrays to identify genes expressed in particular types of cancer cells. (See the Genomics unit.)
With the sequencing of the human genome and the mapping of single nucleotide polymorphisms (SNPs) (see the Genomics unit), it may be possible to diagnose particular cancers by identifying cells with known gene alterations. In 2002 scientists detected ovarian cancer by testing blood for the presence of DNA released by tumor cells. They looked for changes in certain alleles at eight SNPs that are characteristic of cancer. Using this technique, they successfully identified eighty-seven percent of patients known to have early-stage of ovarian cancer and ninety-five percent of those with late-stage ovarian cancer. The ability to determine which genetic alterations are associated with various cancers opens up the possibility of identifying cancerous cells while the cancer is in an early, treatable stage.