Multimodal Sentinel Lymph Node Mapping with SPECT/CT and Photoacoustic Tomography

Identification of cancer cells in the lymph nodes surrounding a tumor is important in establishing prognosis. Optical detection techniques such as fluorescence and photoacoustic tomography (PAT) have been reported in preclinical studies for noninvasive sentinel lymph node (SLN) mapping. A method for validation of these techniques is needed for clinical trials. We report the use of a multimodal optical-radionuclear contrast agent as a validation tool for PAT in a preclinical model. Methylene blue (MB) was radiolabeled with 125I for multimodal SLN mapping and used in conjunction with MB to assess the feasibility of multimodal SLN mapping in a rat model by PAT and SPECT. MB provided sufficient contrast for identifying SLNs non-invasively with a PAT system adapted from a clinical ultrasound imaging system. The signal location was corroborated by SPECT using 125I labeled MB. The translation of PAT into the clinic can be facilitated by direct comparison with established imaging methods using a clinically relevant dual SPECT and photoacoustic imaging agent. The new high resolution PAT is a promising technology for sensitive and accurate SLN detection in cancer patients.