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An interesting look at standard PET surveillance 3-months post-adjuvant oropharyngeal radiation demonstrates an average max SUV of 6.78 in the mucosal targets of 9 patients who received protons versus 4.83 across 10 patients who received photons. We know your first thought: BFD. The thought is that the increased post-treatment mucosal activity is denoting mucosal injury as a result of the decreased beam angles used during proton therapy that in turn result in higher overlapping dose in normal structures. Indeed, the areas of max SUV were correlated with areas receiving the highest proton relative biological effectiveness (at end of Bragg peaks), resulting in as much as 10-30% dose enhancements. While this is all hypothesis generating at best, it fuels clinical investigations into potentially unrealized acute and long term toxicity discrepancies between photon and proton radiation. | Reyes, Int J Radiat Oncol Biol Phys 2022