Let's get pedantic and do some BED calculations with time correction. The time correction factor I'm using, found in Hall, roughly reduces to subtracting 0.5 times the total number of elapsed (tx and non-tx) days over a course of treatment. The time correction factor is only applicable for BED Gy-10 calculations. Someone mentioned the CALGB paper above. I trained with Turrisi. Back in the late 90s we were doing 80.5/35 for Stage I NSCLC with good results (
Conformal high dose external radiation therapy, 80.5 Gy, alone for medically inoperable non-small cell lung cancer: a retrospective analysis. - PubMed - NCBI). Let's calc BED Gy-10 for 70/35, 80.5/35, and a popular SBRT schedule of 60 Gy/4 fx (over about 10 days). We are going to consider time correction because of the patient's long 8 week break.
BED Gy-10 = [70*(1+2/10)]-0.5*49 = 59.5
............... = [80.5*(1+2.3/10)]-0.5*49 = 74.5
............... = [60*(1+15/10)]-0.5*10 = 145
The BED Gy-10 for the patient, who had roughly 5 elapsed weeks (~26 fractions) or 35 elapsed days of treatment followed by an 8 week break for roughly 56+35=91 elapsed treatment days total:
............... = [46.8*(1+1.8/10)]-0.5*91 = 9.7
Clearly she lost a lot of BEDs with the break.
If you want to account for her previous dose and treat her to a 70 Gy/35 fraction equivalent which has a BED Gy-10 equivalent of 59.5 as I showed above, then (
the [(D/2)*1.4] correction factor you see is accounting for the total number of elapsed days of the hypothetical regimen, e.g., if it were a 50 Gy dose where D=50, then the elapsed days would be [(50/2)*1.4]=35 days, or 50 Gy/25 fx over 5 weeks which is 35 days elapsed... 1.4 times the treatment fraction number always equals elapsed treatment days, on average):
59.5 - 9.7 = [D*(1+2/10)]-0.5*[(D/2)*1.4]
D = 58.5 Gy in 2 Gy fraction sizes
If you want to simplify, just round up to 60 Gy/30 fx.
That is to say, based on the 46.8 Gy she has received and an 8 week break afterward, you need to give her an additional 60 Gy/30 fx to get her to a 70 Gy/35 fx BED equivalent.
If you would like to get her to a 80.5 Gy/35 fx dose equivalent at 2.3 Gy fraction sizes,
74.5 - 9.7 = [D*(1+2/10)]-0.5*[(D/2)*1.4]
D = 76 Gy in 2 Gy fraction sizes
That is to say, based on the 46.8 Gy she has received and an 8 week break afterward, you need to give her an additional ~76 Gy/38 fx to get her to a 80.5 Gy/35 fx BED equivalent.
If you would like to get her to a 60 Gy/4 fx SBRT dose equivalent at 2 Gy fraction sizes,
145 - 9.7 = [D*(1+2/10)]-0.5*[(D/2)*1.4]
D = 159 Gy in 2 Gy fraction sizes
That is to say, based on the 46.8 Gy she has received and an 8 week break afterward, you need to give her an additional ~160 Gy/80 fx to get her to a 60 Gy/4 fx BED equivalent.
Finally, let's say you consider strongly hypofractionating e.g. to 5 Gy per fraction, the number of fractions (f) you'd need to give to approach a BED Gy-10 of 100 would be:
100-9.7 = [(f*5)*(1+5/10)]-0.5*f*1.4
f = 13 fractions of 5 Gy apiece
That is to say, based on the 46.8 Gy she has received and an 8 week break afterward, you need to give her an additional 13 fractions of 5 Gy apiece to get up past the 100 BED Gy-10 mark.
Perhaps something like an additional 60 Gy/30 fx would be most feasible, least horrifying, although doesn't really equate to the BEDs of the other regimens does it. At least it gets you 70/35 equivalent which we know works
sometimes.
On the other hand, many would say this is a hopeless case and taking her to anything near these doses would be malpractice... but giving her anything much less than these doses would be fruitless and almost certainly result in 0% local control.
EDIT: you might worry about giving an additional 60 Gy/30 fx on top of the 46.8 Gy. This would worst case work out to a late effect BED Gy-3 of about 178. For an SBRT regimen of 60/4, the late effect BED is ~360, or ~100% more. So as long as the additional 60/30 is conformal-ish, late side effects of the additional dose would not be EXPECTED to be greater than an SBRT treatment in the same patient.
