Longitudinal monitoring of oxygen saturation with photoacoustic imaging

Assessing the response of cancer treatments non-invasively on an individual (or patient) basis has the potential to impact cancer treatment. The need for a modality that provides multifaceted information about the structural and metabolic changes that occur within tumors is important, especially because of the vasculature and the role it plays in tumor growth. One such modality capable of imaging the anatomy and functionality of vasculature is photoacoustic (PA) imaging. In this study, in vivo PA imaging and estimation of oxygen saturation (SO₂) was performed longitudinally to monitor the efficacy of thermosensitive liposome delivery vehicles. By mapping the distribution of oxygen in 12 animals treated with doxorubicin-loaded thermosensitive liposomes, early SO₂ changes (within hours) were predictive of treatment success as assessed by volumetric tumor growth after 2 weeks. Twelve saline-treated animals showed no observable, early changes in SO₂ and displayed larger intertumoral variability in oxygen distribution compared to the treated group. The longitudinal variations in estimated tumor oxygenation, particularly in the first 5 hours post treatment, demonstrate the unique sensitivity of PA imaging to monitor these functional changes in tumor vasculature.