Some lesions are not malignant in themselves but have the potential to develop into malignant tumors; these lesions are called precancerous lesions or precancerous states. It may take a short time or a long time to develop from precancerous lesions to cancer. Therefore, an important task of tumor prevention and treatment is to detect precancerous lesions and pre-cancerous states. The following are some precancerous lesions: 1. Adenomas of the colon, especially villous adenomas and familial adenomatous polyposis are almost always cancerous. 2. chronic cervicitis with cervical erosion 3. breast fibrocystic disease also known as cystic hyperplasia 4. chronic atrophic gastritis with intestinal epithelial metaplasia 5. ulcerative colitis 6. chronic skin ulcers 7. mucosal leukoplakia white patches often occurring in the oral cavity and vulvar mucosa.