Photoacoustic imaging for the prediction and assessment of response to radiotherapy in vivo
Radiotherapy is commonly used for cancer therapy, although its efficacy is reduced in hypoxic regions of tumours. Photoacoustic imaging (PAI) is an emergent, non-invasive imaging technique that allows the measurement of blood oxygen saturation (sO2) which inversely correlates with hypoxia in tissue. The potential use of PAI as a prognostic tool for radiotherapy outcome was investigated in a head and neck cancer model in vivo. PAI was performed before delivering a single fraction (10, 20 or 30 Gy) treatment. The results show that tumours with pre-treatment higher blood sO2 responded better than those with lower levels in the 10 and 20 Gy groups. For the 30 Gy group, treatment response was independent of blood sO2. The haemoglobin content of the tumours was not correlated with their response to any of the radiation doses studied. Changes in sO2, monitored at 24 h and 96 h following 10 and 20 Gy doses, showed that tumours that were subsequently unresponsive to treatment had an increase in blood sO2 at both time points compared to those which subsequently regressed after radiotherapy. The results suggest that sO2 values measured by photoacoustic imaging can be used before, and shortly after, irradiation to predict subsequent treatment response.