A biopsy is simply a piece of cancer tissue or fluid that is removed from a person and verified for malignancy by a pathologist. Certain characteristics confirm whether or not a person has cancer, and if so, the pathologist determines the histological properties of that cancer. Doing so allows the oncology professional (surgeon, medical or radiation oncologist) to plan treatment for the best possible outcome for the patient in terms of overall survival and quality of life.
A biopsy can be performed on any area of the body wherever there is a suspected tissue that might contain cancer cells. Further, those cells have cellular characteristics that are common for malignant cell types. For example, adenocarcinoma tissue cells contain Creatine Kinase and Carcinoembryonic Antigen, while those of melanoma do not. However, this process is not an exact science and a histopathological diagnosis of malignancy can be made in error due to the sampling, processing, and interpreting steps along the way. Also, something that is benign might not remain so over time, and something that is malignant can remain inactive clinically for a long time.