My dad, his sis, brother and dad died of cancers (brain, stomach, liver) my grandma had colon Cancer. They found polyps in my colon, bad sign at 18?

What type of polyp. That many cancers may prove to be associated with a familial disorder requiring genetic evaluation. Polyps of colon extremely rare unless there is an underlying hereditary disorder. Important to know whether the polyps were inflammatory or adenomatous lesions that could lead to malignant transformation. Should genetic disorder be found, colon surgery may eventually be needed.
Polyps at age 18. means, you need to be under observation, polyps should be removed through colonoscopy If other members has colonic polyps, is indication for genetic testing. Don't worry just follow your doctors advise.

Related Questions

Haven't had solid bowl movement more then 8 months pain & feel mass upper abdomen weight gain dad has colon cancer & aunt has stomach cancer worried?

Don't think cancer. Do think of having a good examination and solid efforts to treat whatever is the cause. Follow the dietary and behavioral recommendations that result and reassure yourself that it is not cancer. This may involve psychotherapy aimed at health anxiety issues. Take it step by step - start with a good GI exam. Good luck.
Lots of issues... You're describing many systemic concerns, but its impossible to tell here if your symptoms are all due to organic/metabolic disease or even related. Granted, your family history is notable, ; may indicate lynch syndrome. A thorough history ; physical with some simple investigatory lab studies ; x-rays would be a great place to start in getting the answers, reassurance ;/or treatment you need.

Can an abdominal/pelvic CT scan incidentally detect colon cancer or polyps?

Incidental. An abdominal ct scan may incidentally demonstrate the suspicion of a fairly large colon cancer but not small lesions or polyps. A conclusive diagnosis of colon cancer requires a tissue diagnosis by biopsy.