Because of the similarities between the blood vessels in the eye and brain, an eye doctor can occasionally detect an issue taking place in the brain by examining the blood vessels in the eyes. If swelling or shadows in the eye is observed, it may indicate a serious condition in the brain, like a tumor, or clots that might result in a stroke.
Cancers by themselves rarely have manifestations in the eye.  However, there are certain cancers that can spread to the eyes.

Melanoma is one cancer that is primary to the eye, but it can often be found elsewhere on the body. Breast cancer is an example of a cancer that can metastasize to the eye, where it could be found before other tests show the cancer has spread. Breast cancer, leukemia, and other metastatic cancers are occasionally discovered during an eye evaluation. In addition to brain cancer mentioned above, melanoma and basal cell carcinoma (skin cancer) can be detected, and eye doctors can also diagnose lymphoma and other eye tumors. Eye exams save lives.