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What is Cancer?

Cancer is the general name of a large group of diseases characterized by cells that grow out of control and have the potential to spread to other parts of the body.  If left untreated, many forms of cancer lead to serious illness and death.

What causes cancer?

Cancer cells develop when there is damage to a cell's DNA or an alteration to its functioning.  DNA is the genetic material in the cell that directs the cell's activities.  Damage to the DNA can occur for many reasons. 

How does cancer spread?

Cancer cells keep growing and dividing instead of dying and being replaced, like normal cells. Cancer cells can travel through the blood stream or the lymph system to other parts of the body and replace normal tissue. This is called metastasis.

For many cancer types, cancer cells form into a lump or mass, called a tumor.   Other cancer types, like leukemia, do not form tumors, and involve the blood and blood-forming organs. 

What are risk factors for cancer?

Risk factors for cancer include:

Although diet and tobacco use are responsible for the largest proportion of cancer cases, 35% and 30%, respectively, evidence continues to emerge suggesting that the environment plays a role in contributing to cancer.

What are the symptoms of cancer?

There are multiple types of cancer and each cancer is unique in characteristics and symptoms.  However, there are common symptoms that, when present, may warrant an evaluation for cancer:

How is cancer diagnosed?

Cancer can be detected through a positive screening test or by experiencing symptoms.  Diagnostic tests can confirm whether the patient has cancer.  These diagnostic tests can include an examination of the suspected cancerous tissue, also known as a biopsy.

What is a cancer cluster?

Generally speaking, a cancer cluster is defined as a greater-than-expected number of cancer cases that occurs within a group of people in a geographic area over a period of time.  This is done by looking at the number and rates of cancer cases that are occurring (observed) and comparing it to the number and rates that would be expected in the same population.  An apparent excess of cancer may occur due to:

It is important to note that the term "cluster" tends to imply different things to different people, which leads to a large amount of confusion.  For example:

For each of the above definitions, the appropriate methods for statistical analysis and documentation of the cluster are very different.  Interestingly, other diseases and conditions such as preterm birth and asthma are rarely labeled "clusters" even though the above situations as applied to them occur frequently.

Most frequently, questions from citizens about possible clusters are discussed with local or state health department staff and are addressed satisfactorily with education about cancers and cancer risk factors.  However, some situations will require further evaluation to understand what may be going on.  The follow-up evaluation may consist of a routine review of data in the cancer registry, or could require extensive investigation that may take years to complete.  Because cancers are complex diseases, however, in many instances, no clear cause of a true cluster is ever found.

How do we track cancer?

To track cancer in California we use information related to the number of individuals diagnosed with cancer. In our data query, we display the number of individuals diagnosed with each of seventeen types of cancer. We receive these data from the California Cancer Registry.