Single Underlying Cause for Multiple Cancers?

A couple anecdotes:

A dear family member of mine had cancer which was successfully treated. Then, a year or two later, she came down with cancer in another part of her body which ultimately killed her.

A friend of a friend had a parotid tumor which was successfully removed and ruled benign. Then a year later she had ovarian cancer which she is currently fighting.

My common sense is telling me that these pairs of cancers are likely to be related. Especially for the second person who is just 35 years old.

My question is this: Do cancers tend to correlate in this manner? If so, is there an accepted explanation? For example, it occurs to me that operating on one cancer might release diseased cells into other parts of the body. Or perhaps chemo-therapy is itself carcinogenic. Or perhaps a lot of cancers are partially caused by a general immune system failure which opens the door to cancers everywhere in the body.

What’s the straight dope?

A benign tumor is not a cancerous tumor. Hence, the later discovered cancer is probably just a coincidence. Was the family member’s cancer completely eliminated? You say “successfully treated.” How was it successfully treated? How many lymph nodes were removed, and did they remove all the nodes affected? Even if they did, some cells may have escaped into the body and metasized to “the other part of the body.” You don’t say what part of the body was “treated” nor did you say what other part of the body was discovered to be cancerous.

I am not a doctor, but I do know several of the answers.

It is possible to get multiple cancers due to the same reason. For example, a large dose of radiation could cause multiple kinds of cancer.

Surgery generally will not spread cancer throughout the body however (see this discussion by a surgeon http://scienceblogs.com/insolence/2007/02/27/surgery-and-the-spread-of-cancer-tumor-a/).

Chemotherapy can be carcinogenic.

General immune system failure can result in multiple cancers (this is known to occur in AIDS)