Heterogeneous tumor blood oxygenation dynamics during phototherapy deciphered with real-time label-free photoacoustic imaging.

Understanding the heterogeneity of tumor vascular function and oxygenation is key in individualizing treatments, especially with therapies that are ineffective in hypoxic microenvironments. Our previous work has demonstrated that ultrasound-guided photoacoustic imaging (US-PAI)-based blood oxygen saturation (StO₂) measurements can be used as a surrogate marker for predicting the regionalized efficacy of photodynamic therapy (PDT). However, monitoring of StO₂ during therapy could provide additional insights, specifically informing "on the spot" dosing decisions. In this work, we demonstrate the heterogeneous oxygen consumption during PDT by integrating light delivery fibers with the US-PAI transducer and tested the setup on murine tumor models with vascular-targeting benzoporphyrin derivative (BPD) PDT. Besides mapping dose-dependent oxygen utilization in real time, we also show that areas of reoxygenation post-PDT retain vascular function, confirmed with immunohistochemistry. Our results demonstrate the high potential of US-PAI in heterogenous tumoral oxygenation mapping for online dosimetry of cancer therapies such as PDT.