What is the primary characteristic of cancer as a group of diseases?

The primary characteristic of cancer as a group of diseases is that it involves uncontrolled cell growth. In cancer, normal regulatory mechanisms that control cell division and apoptosis (programmed cell death) are disrupted. This leads to a situation where cells proliferate excessively and fail to undergo the normal processes that would typically eliminate damaged or unneeded cells.

Uncontrolled cell growth can result in the formation of tumors, which may be benign (non-cancerous) or malignant (cancerous), and it can ultimately invade surrounding tissues or spread to other parts of the body through metastasis. The hallmark of cancer is this characteristic of uncontrolled growth, which differentiates it significantly from normal cell processes where growth and division are highly regulated.

In contrast, normal cell division and encapsulation of cells do not capture the essence of cancer. Normal cell division occurs in a tightly controlled manner, ensuring that cells divide only when necessary. Encapsulation, while relevant to some types of tumors, does not define cancer as a whole, as many tumors do not have a clear capsule and are instead infiltrative in nature. Therefore, the defining feature of cancer is its ability to bypass normal growth controls, leading to uncontrolled cell proliferation.