Unconstrained.

Top Line: SABR, short for stereotactic ablative body radiation, is the acronym Dr. Timmerman himself prefers over the “duller, less descriptive” stereotactic body radiation therapy (or SBRT).

The editorial: This one is a must-read from top to bottom on the unvarnished history and current philosophy of SABR and related dose constraints from someone who was there for it all. First, let us reflect on the vehement warnings against extreme hypofractionation and the related abandonment of the classical linear-quadratic equation that came from all corners of 20th century rad onc giants. What they couldn’t envision was the vast improvement in technology that would allow us to spare normal tissue, not with shamefully protracted fractionation schedules, but with more accurate beam targeting. SABR constraints were almost exclusively born from “educated” guesses that have since played out to be far too conservative, if anything. To top it off, there are contemporary “Timmerman Tables'' with dose constraints for 10 different fractionation schemes. What will you fail to find in any of them? Mean dose constraints, which are typically “mean-ingless” amidst steep dose fall-offs. To top it off, he predicts we will soon be able to “at least double” many of our dose maximums (currently defined as the hottest ≤0.035 cm3) as our ability to accurately shape dose only improves. Finally, hats off the the HyTEC initiative working to accumulate clinical data—which should, of course, always trump theoretical models—to further inform these ultrahypofractionation constraints.

TBL: “[Progress] won't happen if we constrain such innovation, putting handcuffs on investigators who would otherwise explore better treatments. That's the danger of treating constraints like a religion derived from our current training and experience.” | Timmerman, Int J Radiat Oncol Biol Phys 2021