When does a benign tumor become malignant?

A benign tumor becomes malignant when it spreads to new sites and continues to proliferate. Malignant tumors, or cancers, are characterized by their ability to invade surrounding tissues and metastasize, which means they can travel through the bloodstream or lymphatic system to establish new tumors in different parts of the body. This invasive growth and spread signify a loss of the regulated growth control seen in benign tumors.

While factors like size or influence from hormones might affect a tumor's growth, they do not define the transition from benign to malignant. The defining characteristic of malignancy is the ability to metastasize and invade other tissues, leading to further complications and a more aggressive disease state. Thus, the key aspect of malignancy is the tumor's capacity to spread beyond its original location, which distinguishes it from benign tumors that remain localized and do not invade nearby tissues.