If the a/b of the tumor is lower than that of the OAR, as it very well may be in this case given the RCC and delayed recurrence, then SBRT should work in your favor and I would just get in what you can with 5 fractions
An interesting situation arises when α/β
tumor<α/β
latenormal. If I could still solve partial first order differential equations, I could prove it to you. But by brute force (fortunately you only have to look at 1, 2, 3, 4, or 5 fractions) you can show that a single large fraction is the best option when trying to maximize tumor effect without increasing (i.e. constrained by) late side effect. For example, say a tissue late tolerance is 54 Gy/27 fx, BED Gy3=90 (α/β
normal= 3), and the RCC α/β
tumor =1.5 (BEDGy1.5 thus equals tumor effect BED). Keeping late effects the same, if you give:
One fraction of 15 Gy, BED Gy3=90 and BED Gy1.5=165.
Two fractions of 10.2 Gy, BED Gy3=90 and BED Gy1.5=159.
Three fractions of 8.1 Gy, BED Gy3=90 and BED Gy1.5=155...
and so on and so forth. The one fraction regimen thus has the equivalent late effect profile, superior acute effect profile (a
very superior BEDGy10 profile because its total dose so low) and superior tumor effect profile versus any other multi-fraction regimen conceivable. So why isn't the single fraction regimen used in situations (e.g. prostate?) where we truly think α/β
tumor<α/β
normal? E.g., if one truly thinks this, and one has radiobiology knowledge, one would use one fraction instead of five. (Has been tried in prostate; OK outcomes except maybe in tumors that really weren't α/β
tumor<α/β
normal.) Just a pleasant Sunday morning thought.