First Clinical‐Scale Use of a Percutaneous EPR Oxygen Probe ( OxyTrack ) for Deep‐Tissue pO 2 Measurements in a Canine Cancer Patient

ABSTRACT Purpose To assess the safety, feasibility, and performance of a percutaneously insertable L‐band electron paramagnetic resonance (EPR) oxygen sensor (OxyTrack) for depth‐resolved pO 2 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 pO 2 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 pO 2 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 ( R 2 > 0.98) with half‐times 0.35–1.17 min. Simultaneous muscle versus tumor measurements gave mean pO 2 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 pO 2 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.