Repair of Radiation Damage

In mammalian cells consider 3 types of radiation

damage:

- Lethal damage
- damage which is fatal from generation

- Sublethal damage
- damage which can be repaired given enough time

- Potentially lethal damage
- damage that can be prevented with optimal repair conditions
- damage that can be fixed with suboptimal repair conditions

This question is asking about **sublethal damage**. This is the damage which can be repaired given enough time, and is quantified by assessment of instantaneous double dose with increasing temporal separation between the double dose (i.e., test always receives same total dose in same fractions but time between fractions is slowly elongated). If the time is extended too long, there will be an increasing probability of cell production, and so a spurious increase in cell survival.

Although current data suggests that most repair is complete in 8 hours, the majority occurs within the first 3-4 hours.

The first scenario is the double fractions with no time delay, i.e., a single 8Gy fraction. So the cell survival curve will look like this:

That is the 4Gy survival fraction will be doubled. Now I have drawn this using survival curves for illustration but the only relevant bit is a POINT immediately under the 4Gy and its multiples (there is no time display on this graph)!

If there is NO lethal damage, only sublethal damage , and the sublethal damage repair is 100%, then when you come to repeat the dose of 4Gy, you will have all your cells surviving. Of course this never happens, but if you have a large SF_{2} and lots of repair (??melanoma) then you have a radioresistant tumour)

In more reasonable tumours things are not so extreme. The damage of 4Gy which sould result in 0.25 surviving is altered by SLDR, that is the surviving fraction gets larger, so the next shot results in a new survival curve that is slightly above the original 4Gy survival fraction. I could just as easily altered the slope of the line.

Does this make sense?