Metastasis is a complex process by which the malignant cancer cells from the breast expand into other regions of the body. Once metastasis has occurred, it is much more difficult to effectively treat breast cancer. If breast cancer has metastisized to other areas of the body, it is termed a 'stage IV' breast cancer. Sometimes metastasis has occurred at the same time the original breast cancer is diagnosed. But in other cases the metastasis of breast cancer is found months or even years after the initial treatment. This would be termed a 'recurrent' breast cancer.
Breast cancer cells will typically travel wither through the lymphatic system or through blood vessels to reach distant areas of the body. When breast cancer metastasize to the axillary lymph nodes (under the arm) it is still considered to be a relatively early stage of metastasis, and is potentially curable. But once the breast cancer moves beyond the lymph nodes into other areas of the body, it is usually termed a 'distant' metastasis, and a patient is unfortunately usually not curable at that point.
If breast cancer has metastasized beyond the lymph nodes it is termed a 'systemic' disease. That means that the 'whole body' must be treated and not just a particular spot. The only way to effectively treat the whole body is through chemotherapy and hormonal therapy.
Breast cancer tends to metastasize primarily to the lungs, liver, brain, and regional lymph nodes, and the bone. About 70% of breast cancer deaths are due to breast cancer metastasis to the bone, with the next most common site of breast cancer metastasis is the brain, at close to 10%.
No one really knows what factors will make a certain patient more or less susceptible to breast cancer metastasis. There is growing awareness that part of that susceptibility is due to the 'host factors'; characteristics of the non-malignant cells and the general biological environment surrounding the malignant breast tumour. Sometimes this is referred to as the 'pre-metastatic niche', and it is thought that bone-marrow-derived progenitor cells may directly influence the dissemination of malignant cells to distant areas. Non-neoplastic 'host cells' within the tumour may also play a key role in the regulation of breast cancer metastasis.
Recent breast cancer research suggest that the body's regulatory T cells, which are an integral part of the immune-response system, may play a key role in metastasis. It is speculated that the T cells produce a protein which seems to accelarate the spread of breast cancer cells to other areas of the body. The inflammatory protein RANKL seems to influence the T-cells' ability to spread cancer cells to distant areas of the body. It is believed that by interfering with RANKL's ability to interact with the T-cells, the early metastasis of breast cancer cells can be significantly inhibited.
According to some estimates, between 20% to 30% of all women first diagnosed with localized breast cancer will usually develop it in other areas of the body. In about 10% of original breast cancer diagnoses metatastasis has already occurred. Following breast cancer treatment, it has been estimated that up to 40% of women will experience a relapse and develop a distant metastasis. In the United States in 2011 there are an estimated 16200 women living with metastatic breast cancer.
Some estimates suggest that between 10% to 15% of all diagnoses will be of aggressive forms of the breast cancer which will potentially metastasize within three years following the original diagnosis. However, breast cancer metastasis can take place 10 years or even longer after the original treatment. Metastasized or potentially metastatic breast cancer treatment usually takes the form of adjuvant chemotherapy (also called 'systemic' therapy). While the choice to undergo chemotherapy will always be determined on an individual basis, the fact remains that approximately 80% of all breast cancer patients are treated with some degree of adjuvant chemotherapy. Statistically, the use of adjuvant chemotherapy to treat and prevent the spread of metastatic breast cancer can lead to a 3% to 10% increase in the 15 year breast cancer survival rate overall.
It is important to be realistic about survival of metastatic breast cancer. Approximately 90% of deaths due to breast cancer are from metastasis, and the overall survival rate for metastasized breast cancer is only about 16%. Still, almost 20% of women with metastatic breast cancer may still live more than five years. Above all one should remember that the situation for each woman will be unique, and ultimately statistics are meaningless. No one can predict with any degree of certainty whether or not the body will either experience or be able to recover from metastatic breast cancer.
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