First Clinical-Scale Use of a Percutaneous EPR Oxygen Probe (OxyTrack) for Deep-Tissue pO₂ Measurements in a Canine Cancer Patient

Purpose: To assess the safety, feasibility, and performance of a percutaneously insertable L-band electron paramagnetic resonance (EPR) oxygen sensor (OxyTrack) for depth-resolved pO₂ measurement in a spontaneous canine osteosarcoma, and to evaluate its translational potential. Methods: A custom 1.15 GHz continuous-wave EPR spectrometer was used together with two needle-type resonators. An 8-cm, 1-mm-OD OxyTrack measured pO₂ at five depths (2–38 mm) by incremental retraction; a 5-cm, 0.5-mm-OD OxyTrack, mounted on a guiding cannula, enabled alternating measurements in tumor versus adjacent muscle at ∼30-s intervals for 6 min. Data were analyzed in RStudio; means ± SEM were taken from the final minute of each depth. Results: The pO₂ values (± SEM) in the osteosarcoma tumor were 49.26 ± 0.71 mmHg (at 2 mm depth), 50.82 ± 0.25 mmHg (at 8 mm), 19.62 ± 0.47 mmHg (at 18 mm), 4.78 ± 0.05 mmHg (at 28 mm), and 18.15 ± 0.30 mmHg (at 38 mm). The 28-mm site exhibited severe hypoxia. Equilibration followed a mono-exponential model (R² > 0.98) with half-times 0.35–1.17 min. Simultaneous muscle versus tumor measurements gave mean pO₂ of 33.9 ± 2.1 mmHg and 8.0 ± 0.7 mmHg, respectively (ratio ∼4.2:1). No adverse events occurred. Conclusion: OxyTrack provides safe, real-time, quantitative pO₂ assessment at depths > 3 cm, revealing marked intra-tumoral oxygen heterogeneity that single-point methods miss. These findings demonstrate feasibility for clinical translation of EPR oximetry to guide radiotherapy and hypoxia-targeted therapies and support further translational development.