My c reactive protein level is 18 .2 should my doctor be checking for cancer ?

Possibly: CRP are elevated in a nonspecific manner. They can track infections and other inflammatory conditions. They can also elevate as a reaction like to a cancer. Screening should be done appropriate for your age. It is not necessary to look beyond screening without signs or symptoms. Ask your physician.

CRP is non-specific : Elevations in CRP are very non-specific, as many things can lead to increased production of the inflammatory blood marker: infections, trauma, burns, inflammatory conditions, cancer, coronary artery disease, etc... http://www.testcountry.org/crp-inflammation.htm.