Monitoring tissue temperature during photothermal therapy for cancer

Phototherapies offer promising alternatives to traditional cancer therapies. Phototherapies mainly rely on manipulation of target tissue through photothermal, photochemical, or photomechanical interactions. Combining phototherapy with immunotherapy has the benefit of eliciting a systemic immune response. Specifically, photothermal therapy (PTT) has been shown to induce apoptosis and necrosis in cancer cells, releasing tumor associated antigenic peptides while sparing healthy host cells, through temperature increase in targeted tissue. However, the tissue temperature must be monitored and controlled to minimize adverse thermal effects on normal tissue and to avoid the destruction of tumor-specific antigens, in order to achieve the desired therapeutic effects of PTT. Techniques for monitoring PTT have evolved from post-treatment quantification methods like enzyme linked immunosorbent assay, western blot analysis, and flow cytometry to modern methods capable of real-time monitoring, such as magnetic resonance thermometry, computed tomography, and photoacoustic imaging. Monitoring methods are largely chosen based on the type of light delivery to the target tissue. Interstitial methods of thermometry, such as thermocouples and fiber-optic sensors, are able to monitor temperature of the local tumor environment. However, these methods can be challenging if the phototherapy itself is interstitially administered. Increasingly, non-invasive therapies call for non-invasive monitoring, which can be achieved through magnetic resonance thermometry, computed tomography, and photoacoustic imaging techniques. The purpose of this review is to introduce the feasible methods used to monitor tissue temperature during PTT. The descriptions of different techniques and the measurement examples can help the researchers and practitioners when using therapeutic PTT.