People worry about radiation because they cannot feel it. However, nature has a solution - in recent years it has been found that living cells replace and mend themselves in various ways to recover from a dose of radiation.
These clever mechanisms kick in within hours and rarely fail, except when they are overloaded - as at Chernobyl, where most of the emergency workers who received a dose greater than 4,000 mSv over a few hours died within weeks.
However, patients receiving a course of radiotherapy usually get a dose of more than 20,000 mSv to vital healthy tissue close to the treated tumour. This tissue survives only because the treatment is spread over many days giving healthy cells time for repair or replacement.
In this way, many patients get to enjoy further rewarding years of life, even after many vital organs have received the equivalent of more than 20,000 years' dose at the above internationally recommended annual limit - which makes this limit unreasonable.