DEAR DR. DONOHUE: My family has a history of various cancers. My mother's sister had a malignant brain tumor, and my dad had liver cancer. What are the chances I will get cancer, and what can I do about it? - Ms. D.

ANSWER: Yours is a natural concern, but be assured that only a very few cancers are definitely linked to genetic factors, as is for example retinoblastoma, an eye tumor. We have hints of hereditary influence in breast, colon, and perhaps lung cancer, but no specific gene has been found for them. The mere history, then, of multiple cancer occurrences in your family is considered unfortunate coincidence.Having said this, I must add that you have reason to follow the advice meant for all of us; that is have the regular screening exams for various cancers - Pap smears, mammograms, tests for colon cancer, and, for men, prostate checkups. You begin by choosing a family doctor, who can advise you on proper timing for such tests. And need I add that you have to avoid the well-known cancer contributors, like smoking and too high a reliance on fat in diets.

DEAR DR. DONOHUE: I have been bothered by ulcers for several years. Recently, you mentioned bismuth salts as a treatment for ulcers. However, I cannot find them anywhere. Help me find bismuth salts. Thanks. - B.A.

ANSWER: I should not have turned bismuth salts into an arcane item of pharmacology. One of them is our well-known Pepto-Bismol.

The reason bismuth salts have been used for ulcers, particularly duodenal ulcers, is a newly discovered link between such ulcers and a bacterium, helicobacter. Bismuth salts help eliminate helicobacter, often in conjunction with antibiotics. Why not talk this over with your doctor, and see if you could be helped with either the salts alone or the antibiotics? For more on this, see the ulcer report. Write Dr. Donohue/No.25, Box 830, Gibbstown, NJ 08027-9909, enclosing a long, stamped, self-addressed envelope and $2.

DEAR DR. DONOHUE: Maybe you can come up with a rational answer as to why, at age 58, I've come down with psoriasis. I have never had a skin condition. It is limited to the legs mostly. I've heard that drugs can cause it. Do you have any theories? - T.O.

ANSWER: I can't, nor can anyone else, explain why psoriasis should appear at any age. All we know is that it appears most often around age 28. The fact that 2 percent of psoriasis develops in children age 2 and under gives an idea of its timing variation.

The very cause of psoriasis is still a question mark. Genes may play a role, for we do find it in families. Cold, sunless climate has been cited as another possible contributor.

Yes, drugs may be linked to psoriasis, or at least to flare-ups. Beta blockers, lithium, chloroquine and indomethacin are examples of drugs that can cause a worsening of psoriasis in some people.

DEAR DR. DONOHUE: Would you please tell me what causes "trigger finger"? I have it on one hand, and now I am getting the clicking on a finger on the other. - Mrs. D.C.N.

ANSWER: The cause is a swelling of the tunnel that carries the particular tendon. If it swells, the tendon can become hung up, making extension of a finger difficult, hence the clicking sound. Sometimes the finger is drawn back permanently (in a triggering position). If this happens, surgery can be performed to free the tendon. It is not uncommon for this to occur on both hands of the same patient.

Dr. Donohue welcomes reader mail but regrets that, due to the tremendous volume received daily, he is unable to answer individual letters. Readers' questions are incorporated in his column whenever possible.