The two types of tumors differ in a variety of ways. A benign tumor - the wart is familiar example - is almost always enclosed in a capsule or sheath of fibrous tissue. Malignant tumor is rarely so well confined and tend to invade adjacent tissue. Some exceptions to this rule do exist. Wilms tumor for example, a cancer that strikes at the kidneys in children is encapsulated like a benign growth. This kidney cancer symptoms are palpable mass in the abdomen, hematuria and hydronephrosis. There may also be no signs or symptoms however. A palpable mass is by far the most common sign. The mass typically presents first in the anterior lumbar region, between the margins of the ribs and the crista ilii; it then grows forward to the umbilicus, upwards into the hypochondrium and downwards into the iliac and inguinal regions. In extreme cases it fills the entire belly. The colon and sometimes a portion of the small intestines, lies in front of it. This position of the colon furnishes an important diagnostic mark of all kidney cancers.

When the outward appearance of a tumor does not reveal its nature, the distinction between benign and malignant can be seen through a microscope. If a kidney cancer is benign, the cells within it closely resemble those of the kidney itself in patterning and structure. In Wilms tumor - and in all other cancerous tumors - the cells have only a rough resemblance to those of theing cell tissues in which they originally grew. Instead, they take on characteristics of their own. A normal cell has a single small body - the nucleus - at its center; a cancer cell may have a huge nucleus, or two or even more. Like the nucleus, other components of the cell's interior may be deformed or multiplied. An entire cancer cell is usually misshapen and groups of them grow helter-skelter, lacking the orderly arrangements of normal cells.

There is a third difference between the two types of tumor - the most important distinction of all. Benign tumors are localized; they grow generally quite slowly but they stay at their original sites. Cancer tumors establish outposts elsewhere in the body, by a process called metastasis. Metastasis is the cause of about 90 % of deaths due to breast cancer and roughly 70 % of all patients dying of breast cancer metastasis have evidence of metastatic bone disease. Metastatic breast cancer excrete lysophosphatidic acid (LPA) that binds to receptors on tumor cells, inducing cell proliferation and release of cytokines (IL6 and IL8, potent bone resorptive agents) and stimulating bone resorption.

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