In situ

Ductal carcinoma in situ (DCIS) of the breast, cervical intraepithelial neoplasia found by smear tests, and high-grade dysplasia in Barrett's oesophagus or colon polyps are all in situ or precancerous lesions; the basement membrane beneath the epithelium is still intact, so the cells have no access to blood or lymph vessels. Screening programmes detect many such lesions, which is both their strength (removing them prevents cancer) and their difficulty (many would never have progressed, so some treatment is overtreatment). Trials such as LORIS and COMET test whether low-risk DCIS can safely be watched rather than removed.