I feel it is my ethical duty to let my primary care physician know of my initial results of cancer test (+). maybe knows electronically anyway. but, i'm thinking of not informing dental doctors since has to do with stomach area. what do you think?

Not just ethical, but helping the docs help you with best available information. Letting your dentist know would be critical as well as healing may be impaired in the context of treatments and dental procedures have to be scheduled properly in order to have best possible outcome.

Agree with Dr. P. It is in your interest to inform all of your healthcare providers of all of you health issues, medications you take and any procedures you had. Wish you good health!