Is it better to receive a certain dose of radition gradually or all at once?
The period of time over which a dosage is given matters critically. You probably have experienced this in your daily life. If you are out in the bright, summer, sun for 8 consecutive hours on a single day, you are likely to get a sunburn. If you are out in the sun for a total of 8 hours spread evenly over a month, you're very unlikely to get sunburned.

You body has mechanisms that can repair radiation damage or limit it's effect, but these processes take time. A particularly important processes is apoptosis. A cell can detect if its DNA has been critically damaged, and self-destruct before something else goes wrong. If the exposure is acute, there my not be time for the apoptosis mechanism to take effect before it is damaged itself, possibly transforming the cell into a cancerous state. There is considerable evidence that lower dose rates do give rise to fewer cancers.