Unlike diagnosis efforts prompted by symptoms and medical signs, cancer screening involves efforts to detect cancer after it has formed, but before any noticeable symptoms appear. This may involve physical examination, blood or urine tests, or medical imaging. Cancer screening is currently not possible for many types of cancers, and even when tests are available, they may not be recommended for everyone. Universal screening or mass screening involves screening everyone. Selective screening identifies people who are known to be at higher risk of developing cancer, such as people with a family history of cancer. Several factors are considered to determine whether the benefits of screening outweigh the risks and the costs of screening. These factors include:   1. Possible harms from the screening test: for example, X-ray images involve exposure to potentially harmful ionizing radiation. 2. The likelihood of the test correctly identifying cancer. 3. The likelihood of cancer being present: Screening is not normally useful for rare cancers.   Possible harms from follow-up procedures.   1. Whether suitable treatment is available. 2. Whether early detection improves treatment outcomes. 3. Whether the cancer will ever need treatment. 4. Whether the test is acceptable to the people: If a screening test is too burdensome. 5. Cost of the test.