We have knowledge on the cellular distribution along the cycle and the variation in radiosensitivity along the cycle (i.e. the most sensitive phase is M, the most radioresistant is S, etc). Radiation affects cell cycle by well-known cell arrest, unlike, for instance, chemotherapy which has a variable effect depending on the administered drug, which can sometimes lead to partial synchronisation.

Larger doses of radiation (hypofractionation) would probably kill larger cell populations in the sensitive phases, but I'm not sure how much influence would have on partial synchronisation (in order to significantly change cellular redistribution). It will also depend on tumour kinetics - such as length of the cell cycle. Run some flow cytometry studies if you have the possibility.

It's an interesting point, nevertheless, so I'll think about it in more depth.

Loredana, thanks for your reply! I agree with your general points, and of course the answer may depend strongly on tumor type. I am interested in the following: During standard multi-fraction radiotherapy (say 50 Gy in 2 Gy fractions), surviving cancer cells can "pile up" in certain cell cycle phases (e.g. S-phase) after say 2 weeks of radiotherapy, and they therefore become somewhat more resistant to subsequent irradiation. In other words, effectiveness of radiotherapy per unit dose decreases somewhat during the last weeks of treatment. Compared to this scenario, how would hypofractionated treatment (fewer fractions, larger doses per fraction, shorter overall treatment time) affect cell cycle redistribution for the same cancer type? Would larger doses per fraction lead to more rapid redistribution? Or, because overall treatment time is shortened, would there be insufficient time for this to occur?

Good point, Igor. Then again, as I said before, cell cycle time would influence your outcome: a tumour with shorter cycle will have a lesser degree of synchronisation (or 'pile up' in the S phase) due to high cell turnover, than a tumour with longer cell cycle time (reason why head and neck cancers - with short cycle times - respond better to hyperfractionation than to standard treatment). Thus the opposite might be valid for tumours with long cell cycle times (and small alpha/beta ratio) such as prostate - when hypofractionation is recommended.

However, redistribution is one of the reasons for fractionation in radiotherapy. Therefore, fewer fractions will limit cellular redistribution and I don't think that larger doses will lead to more rapid redistribution.

Not sure what tumour type are you studying, but there is a good paper on different hypofractionation protocols for prostate written by Fowler et al - Red Journal (IJROBP)- 56(4), pp 1093, 2003 - this might give you some guidance on the radiobiological aspect.

Thanks again, Loredana! I agree with what you are saying about effects of cell cycle length. And of course redistribution is one of the "3 R's" in radiotherapy. Does the following generalization make sense? Small doses per fraction (e.g. 2 Gy) cause relatively short cell cycle delay (e.g. a few hours), so during the intervals between fractions (e.g. 24 hours) many cells can move back from resistant to sensitive cell cycle phases. But large doses per fraction (e.g. 8-12 Gy used for stereotactic radiotherapy) can cause much longer cell cycle delay - perhaps days. After the first fraction, most cells which survive will be in a resistant phase (e.g. S-phase), and will not have time to move out of it before the next fraction is administered. So, cancer cell survival after the whole course of such therapy (e.g. 3-5 fractions) will be determined mainly by survival in the most resistant cell cycle phases, similar to a single acute dose scenario.

It does make sense what you say, Igor. And indeed, if hypofractionation limits cellular redistribution, then more accumulation into resistant phases will occur. So yes, I reckon your generalisation stands.

For quantitative information though, one would need to run some simulations or do lab work. Nice project, Igor :-)

Thanks again, Loredana. So then perhaps the following broader generalization holds as well: Hypofractionation (compared with standard fractionation) reduces cell cycle redistribution, and also reduces cell repopulation (due to shortened total treatment time). But reoxygenation between fractions may still occur - if this is so, even say a 3-fraction dose regimen will have an advantage over a single acute dose in terms of killing hypoxic cells. However, such a regimen's effectiveness relative to standard fractionation (per unit dose or BED) is not clear - maybe more or less, depending on which effect is stronger for a particular tumor type: suppressed cell cycle redistribution (should decrease killing effectiveness) vs suppressed repopulation (should increase killing effectiveness). Does this make sense?

Exactly! That's a good summary, Igor. And for sure, from the R's point of view, any fractionated treatment (even a short 3-fraction dose regimen) should have benefits when compared to a single-dose regimen. Of course, as you also pointed out, the effects will vary as a function of tumour type. In that article I've recommended you before, Fowler suggests at least 5 fractions in a hypofractionated regimen for prostate. For brain tumours the situation could be different.