Cumulative OAR constraints are a major uncertainty when reirradiating gliomas. This systematic review from the Reirradiation Collaborative Group (ReCog) is the most comprehensive attempt to address this uncertainty. They reviewed 27 studies encompassing a range of reirradiation approaches (mostly hypofractionated, mostly glioblastoma, mostly IMRT/VMAT) and found, unsurprisingly, wild heterogeneity. Of the 24 studies reporting constraints, 17 used flat constraints (applied to the re-RT plan only, ignoring prior dose), 11 used cumulative constraints without tissue recovery factors (TRFs), and none used TRFs. The actual doses delivered to OARs were often well below the stated constraints, making it hard to define true tolerance thresholds. Distilling all of this, the authors offer candidate cumulative near-maximum EQD2 constraints (α/β=2 Gy): optic pathways 75–80 Gy and brainstem 85–100 Gy (both associated with <5% severe toxicity), and normal brain 100–110 Gy (associated with up to ~10% radionecrosis, though larger-volume constraints like D5cc and D10cc are almost certainly needed but remain undefined). | Slevin, Int J Radiat Oncol Biol Phys 2026
