Benign tumors can be harmful despite their name. A small benign tumor inside the skull may block the blood supply to the brain, causing a stroke. Larger tumors can be dangerous anywhere in the body - and benign tumors weighing 70 pounds have been recorded. A medium sized or large tumor can deform vital organs and interfere with their functions. Malignant tumor can have the same effects but their real menace lies in metastasis. The process begins when a cancer cell breaks away from the tumor. The malignant cell may enter the bloodstream and be swept to a distant site almost anywhere in the body. Wherever this wandering parasite comes to rest, it is dangerous. Every cancer cell can do many things that normal cell cant. It grows, regardless of local conditions and its descendants reproduce more or less crudely the tissue from which it came. Thus fragments of intestine, bone or stomach will be found in the lung.

Lung cancer is disease which consists of uncontrolled cell growth in tissues of the lung. This growth may lead to metastasis, which is the invasion of adjacent tissue and infiltration beyond the lungs. The vast majority of primary lung cancers are carcinomas, derived from epithelial cells. Lung cancer - the most common cause of cancer-related death in men women - is responsible for 1,3 million deaths worldwide annually as of 2004. The most common lung cancer symptoms are shortness of breath, coughing and weight loss. 

The bloodstream is only one of the pathways of propagation. Cells from a malignant tumor may be absorbed directly by a clear, watery fluid called lymph that is conveyed throughout the body by a network of lymphatic vessels. The malignant cells picked up by lymph may then lodge in filter-like structures called lymph nodes. (Ironically, the lymphatic system normally serves to protect the body against disease). Malignant cells can even work their way through solid tissue; a cancer in the lining of the stomach for example, can grow through the stomach wall to establish colonies elsewhere in the abdominal cavity.

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