The kidney surgery only looks so low risk (partially) because they only do it on people that passed the CT scan.
(To give an even more extreme example for illustration:
Suppose the scan could perfectly predict who will die from the surgery and who will live without any side effects. Suppose 90% of people fall into the former and 10% of people fall into the latter category. Suppose further that the scan has a 0.1% chance of killing you.
If you scan people beforehand, it will look like the surgery has 0% chance of complications against 0.1% of the scan. But if you dropped the scan, all of a sudden the surgery would have a 90% death rate.)
(To give an even more extreme example for illustration:
Suppose the scan could perfectly predict who will die from the surgery and who will live without any side effects. Suppose 90% of people fall into the former and 10% of people fall into the latter category. Suppose further that the scan has a 0.1% chance of killing you.
If you scan people beforehand, it will look like the surgery has 0% chance of complications against 0.1% of the scan. But if you dropped the scan, all of a sudden the surgery would have a 90% death rate.)