What is the risk for cancer from single head CT scan if CT scan is done on baby?

Probably less than1% First, benefits must outweigh risks when a ct is considered for a baby. Low dose rt from imaging is very relevant in current news. Ct head prob gave <10 to 50 mgy with then cancer induction risks estimated to be <1%.
Little. This appears to be a very low risk situation. If the baby was to get repeated scans that would increase the risk.
Cannot say. Clearly, there is a potential risk of cancer from radiation exposure. The greater the exposure(s), the greater the risk. One cannot quantify the risk, but it is likely quite low. This has to be balanced against the reason for doing the scan. When there is a legitimate reason for scanning, the risk of not doing the scan and missing something vastly outweighs the risks.