374. Photoacoustic Nodal Metastases Detection in a Rat Model Using a Clinical Superparamagnetic Iron Oxide Nanoparticle Dispersion

Abstract

Background: Detection of tumor metastases in the lymphatic system is essential for accurate staging of different malignancies, however fast and accurate pre- or intra-operative evaluation of the nodal status remains difficult to perform with common available medical imaging techniques. In recent years, numerous studies have confirmed the additional value of superparamagnetic iron oxide dispersions (SPIOs) for nodal staging purposes, prompting the clearance of different SPIO dispersions for clinical practice. Here we evaluate whether a combination of photoacoustic (PA) imaging and a clinically approved SPIO dispersion, could have the potential for sensitive assessment of the nodal status both pre- and intra-operatively. Materials and methods: Prostate adenocarcinoma cells (MAT-lylu) were inoculated in the footpad of Copenhagen rats. For lymph node imaging animals received a subcutaneous injection of a SPIO dispersion (Endorem®, Guerbet, France) either 5 (n=4) or 8 (n=4) days after tumor inoculation. The resected nodes were photoacoustically imaged at 720 nm using 20 mJ/cm2, 24 hours later, and imaging results were compared to 14 Tesla MR imaging and histology. Control animals were imaged without SPIO injection in the involved leg or received SPIO injections on the contra lateral site harboring no tumor inoculation and hence normal lymph nodes. Results: In the tumor groups clear irregularities, as small as 1 mm, were observed in the PA contrast pattern of the nodes. These irregularities could be correlated to the absence of contrast in the MR images and could directly be linked to metastatic deposits seen on the histological slides. The PA and MR images of the control animals did not show these irregularities. Conclusions: We conclude that the combination of photoacoustic imaging with a clinically approved iron oxide nanoparticle dispersion is able to detect lymph node metastases as small as 1 mm. This approach opens up new possibilities for pre- or intra-operative nodal staging.