In case of ct scan related radiation exposure why is it said that life time effects are cumulative when even a single scan carries risk.

Can't undo damage.: There are 2 biological effects of radiation: the 1st is random DNA damage - any stray radiation can permanently alter DNA & cause a mutation (cancer). The 2nd is "radiation sickness". Every organ has a lifetime limit of radiation that it can absorb; beyond the threshold, damage continues to mount. For the 2nd type, the time between radiation exposure doesn't matter, only the total lifetime dose.