Average US background radiation in one year: 3 mSv
CxR: 0.02 mSv Pelvis XR: 0.7 mSv
CT Head: 2mSv CT Chest: 8mSv CT A/P: 10mSv
Cardiac PET: 14mSv
So what is the risk for cancer? It varies by study type, age, and sex. So for example, if a 20 yo F gets a CT abdomen and pelvis with contrast, that CT scan increases the patients risk of cancer by 1 in 470 or 0.2%.
Bindman et al. ARCH INTERN MED/VOL 169 (NO. 22), DEC 14/28, 2009