Introduction: It has been known for over a century that all living systems develop radiation resistance under hypoxic conditions - low pO2. But giving extra radiation - dose painting - or boosting hypoxic tumors regions has not shown enhanced control in mammalian tumors. Pulse EPR quantitative pO2 values can be tomographically imaged as a magnetic resonance measurement with fixed stepped gradients. If there are distinct hypoxic regions, their images can then be extra radiation dose boosts. An isocentric animal irradiation system allowed conformal boosts to tiny resistant tumor subvolumes with 3D printed tungsten loaded plastic blocks. A randomized trial compared radiation boosts to hypoxic tumor vs well oxygenated tumor to determine hypoxic boost control advantage.

Methods: Carcinomas (MCa4) and fibrosarcomas (FSa) in mouse legs with mean volume of 350 μl were treated with an XRAD225Cx animal irradiator to a dose that controlled 15% of tumors (TCD15), determined in separate experiments. Pulse electron paramagnetic resonance (EPR) using Spin Lattice Relaxation (SLR) and a soluble trityl spin probe has been shown to quantitatively image solution and tumor pO2. This and boost treatments were given after EPR pO2 imaging, reconstruction, registration with tumor defining T2 MRI, determination of all tumor hypoxic regions (pO2 ≤10 torr) and 3D printed tungsten loaded plastic aperture fabrication to radiate either well oxygenated or hypoxic tumor regions. Mice were followed for either 180 days (MCa4) or 90 days (FSa) sufficient for 99% of local failures to occur. Treatment of 3D hypoxic volumes was also analyzed for control dependence on surface to volume ratio (SVR).

Results: Tumor control was doubled by radiating hypoxic tumor relative to well oxygenated tumor in 48 MCa4 tumors (p=0.013) and 54 FSa tumors (p=0.04). This was the first demonstration in mammalian tumors of an advantage to targeting hypoxic regions in tumors. It provides the first evidence of image based dose painting within the volume of the tumor. For MCa4 tumors, SVR values below median had significant increased survival (p=0.003) while for FSa tumors below median SVR tumors trended to increased survival (p=0.3).

Clinical Relevance: Modern radiation delivery used intensity modulated radiation therapy (IMRT) for dose gradients outside the tumor volume to reduce doses to critical structures. Dose within the tumor have strict inhomogeneity limits. Allowance of higher dose variation within the tumor volume allows “hot spots” to resistant tumor volumes, sharper gradients outside the volume to protect critical structures. This will enhance the therapeutic ratio.

Citation Format: Howard Halpern, Matthew Maggio, Eugene Barth, Darwin Bodero, Richard C. Miller, Charles A. Pelizzari, Martyna Krzykawska-Serda, Subramanian V. Sundramoorthy, Bulent Aydogan, Ralph R. Weichselbaum, Victor M. Tormyshev, Inna Gertsenshteyn, Boris Epel. Increased tumor control boosting hypoxic regions identified with EPR pO2 imaging in 2 tumor types [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2773.