Cancer can be difficult to treat, simply because it arises from so many varied roots. Each person's case is unique and must be handled in a method specifically designed to best treat the individual. However, some common treatments are often applied.
The simplest treatment is surgery, in which the malignant tumor is removed. For early-stage cancers, surgery alone may fully resolve the disease, provided all malignant cells are removed before metastasis occurs. However, because it can be difficult to remove cancer cells without harming surrounding tissue and because it can be difficult to ensure all cancerous cells have been removed, surgery is frequently coupled with other therapies.
Radiation therapy is commonly used to kill cancer cells and shrink tumors. High doses of radiation are used to damage the DNA of cancer cells, preventing them from dividing as well as possibly killing them. The source of the radiation can be external, using a focused beam, or internal, often delivered as a solid or liquid. Radiation may cause unpleasant side effects, including sores in the mouth, joint stiffness and aches, fatigue, nausea and vomiting, and hair loss.
Another common treatment is chemotherapy. Chemotherapy is the use of chemical drugs to treat disease, usually cancer. Chemotherapy, like radiation therapy, can be used to kill cancer cells and shrink tumors. Chemotherapy drugs are cytotoxic, meaning they poison living cells. Chemotherapy drugs usually cannot specifically target only cancer cells and thus affect normal cells as well, damaging healthy tissues. Many different medicines with various mechanisms of action are used as chemotherapy drugs, and the specific drug used depends on the patient and the cancer. Occasionally, chemotherapy is the only treatment applied, but usually it is coupled with other forms of treatment. Chemotherapy may cause severe and unpleasant side effects, including fatigue, nausea and vomiting, hair loss, and susceptibility to infection.
A great deal of research has been poured into understanding and treating cancer. In fact, much of what is known about the cell and cell cycle has been uncovered through cancer research while gaining an understanding of the failures of the normal mechanisms. Conversely, as scientists continue to uncover information about healthy cells, they find more and more ways that cancer cells fail to obey the rules that keep healthy cells functioning properly.
The discovery that cancer cells have differing cell surface receptors. A surface receptor is a protein found on the surface of the cell to which substances can bind, has made tailored therapy possible. Cancer can be divided into subtypes based on the cell surface receptors present on the cancer cells. A cancer subtype is a division of a type of cancer based on the properties of the malignant cells.
Much of what is known about cancer subtypes comes from research into breast cancer. For example, there are four major subtypes of breast cancer:
- luminal A
- luminal B
- human epidermal growth factor receptor 2 (HER2)
These four types are characterized by the cell surface receptors present or absent in cancer cells. The important receptors in breast cancer are
- estrogen receptor (ER),
- progestin receptor (PR), and
- human epidermal growth factor receptor 2 (HER2).
Each of the subtypes is characterized by receptors present or absent in the cells—for example, luminal A breast cancer has ER but lacks human epidermal growth factor receptor 2 (HER2) (a receptor that is involved in signaling for apoptosis, or programmed cell death), while triple-negative/basal-like breast cancer lacks all three receptors (hence the name). These subtypes have varying prognoses, which leads to targeted treatment.
For example, luminal A subtype breast cancer has a good prognosis, with high survival rates and low rates of recurrence. In contrast, triple-negative subtype has a poor prognosis, with low survival rates and high rates of recurrence. Thus, treatment for luminal A subtype breast cancer tends to be conservative, while treatment for triple-negative subtype tends to be aggressive.Furthermore, once the cancer subtype is known, therapy can be tailored to suit the tumor. For example, tamoxifen is a cancer drug that is metabolized into molecules that bind to receptors for the hormone estrogen. For ER-positive subtype patients, tamoxifen can be used to stop the growth of tumors and even shrink them. For these tumors, tamoxifen metabolites compete with estrogen for binding to the estrogen receptors on the cell surface. In the absence of tamoxifen, the binding of estrogen to the receptors triggers cell growth and division, in defiance of normal cell cycle checks. Once tamoxifen metabolites bind to estrogen receptors, they suppress gene expression, interrupting the cancer cycle of cell growth and proliferation and leading to apoptosis. However, tamoxifen only works for ER-positive cancer subtypes, such as luminal A. For ER-negative subtypes, such as triple-negative, tamoxifen is not useful.
Action of Tamoxifen
The unique constellations of cell surface receptors on cancer cells offer opportunities for precisely targeted therapies with the fight against cancer. Because radiation and chemotherapy are not precisely targeted, their side effects are severe and unpleasant, and they often do not fully eradicate cancer.
Immunotherapy offers an alternative approach that harnesses the power of the immune system to fight cancer cells. The cell surface antibodies of cancer cells are specific and not present in normal, healthy cells. A monoclonal antibody is a protein that counteracts a specific antigen that is produced from a single cell or from clones of a single cell. Monoclonal antibodies can be produced that will bind antigens in tumor cells and then can be administered to a cancer patient. The patient's own immune system attacks and destroys the cancer cells, while leaving normal cells healthy and intact. Alternatively, the monoclonal antibody may prevent other signals from binding to receptors, inhibiting the pathways those receptors initiate.For example, the drug trastuzumab is a monoclonal antibody that targets HER2. In cancer cells, HER2 overrides the checkpoints in the cell cycle that stop normal cells from undergoing cell division if conditions are not optimal. When trastuzumab binds to HER2 receptors in tumor cells, it inhibits this override, allowing the cells to stop at the usual checkpoints and undergo apoptosis as normal. Trastuzumab may also recruit immune cells to target cancer cells, where they can destroy the cancer.
Action of Trastuzumab
Because cancer can be difficult to detect and treat, many patients receive a combined treatment, adding the power of immunotherapy and treatments based on cell surface receptors to more traditional treatments such as chemotherapy and radiation therapy. These various treatments are nearly always combined with surgery to remove malignant tumors, unless a complicating factor prevents surgery from being a viable option. In addition, prospective cancer treatments are constantly under investigation, and new and exciting therapies remain on the horizon.