Diederik J. Grootendorst

Evaluation of superparamagnetic iron oxide nanoparticles (Endorem) as a photoacoustic contrast agent for intra-operative nodal staging

Detection of tumor metastases in the lymphatic system is essential for accurate staging of malignancies. Commercially available superparagmagnetic nanoparticles (SPIOs) accumulate in normal lymph tissue after injection at a tumor site, whereas less or no accumulation takes place in metastatic nodes, thus enabling lymphatic staging using MRI. We verify for the first time the potential of SPIOs, such as Endorem(®) as a novel photoacoustic (PA) contrast agent in biological tissue. We injected five Wistar rats subcutaneously with variable amounts of Endorem(®) and scanned the resected lymph nodes using a tomographic PA setup. Findings were compared using histology, vibrating sample magnetometry (VSM) and 14 T MR-imaging. Our PA setup was able to detect the iron oxide accumulations in all the nodes containing the nanoparticles. The distribution inside the nodes corresponded with both MRI and histological findings. VSM revealed that iron quantities inside the nodes varied between 51 ± 4 and 11 ± 1 µg. Nodes without SPIO enhancement did not show up in any of the PA scans. Iron oxide nanoparticles (Endorem(®)) can be used as a PA contrast agent for lymph node analysis and a distinction can be made between nodes with and nodes without the agent. This opens up possibilities for intra-operative nodal staging for patients undergoing nodal resections for metastatic malignancies.

Initial results of imaging melanoma metastasis in resected human lymph nodes using photoacoustic computed tomography.

The pathological status of the sentinel lymph node is important for accurate melanoma staging, ascertaining prognosis and planning treatment. The standard procedure involves biopsy of the node and histopathological assessment of its status. Drawbacks of this examination include a finite sampling of the node with the likelihood of missing metastases, and a significant time-lag before histopathological results are available to the surgeon. We studied the applicability of photoacoustic computed tomographic imaging as an intraoperative modality for examining the status of resected human sentinel lymph nodes. We first applied the technique to image ex vivo pig lymph nodes carrying metastases-simulating melanoma cells using multiple wavelengths. The experience gained was applied to image a suspect human lymph node. We validated the photoacoustic imaging results by comparing a reconstructed slice with a histopathological section through the node. Our results suggest that photoacoustics has the potential to develop into an intraoperative imaging method to detect melanoma metastases in sentinel lymph nodes.

First experiences of photoacoustic imaging for detection of melanoma metastases in resected human lymph nodes.

Excision and histological assessment of the first draining node (sentinel lymph node) is a frequently used method to assess metastatic lymph node involvement related to cutaneous melanoma. Due to the time required for accurate histological assessment, nodal status is not immediately available to the surgeon. Hence, in case histological examination shows metastases, the patient has to be recalled to perform additional lymphadenectomy. To overcome these drawbacks we studied the applicability of photoacoustic tomographic imaging as an intra‐operative modality for examining the status of resected lymph nodes.

Detection of melanoma metastases in resected human lymph nodes by noninvasive multispectral photoacoustic imaging

Objective. Sentinel node biopsy in patients with cutaneous melanoma improves staging, provides prognostic information, and leads to an increased survival in node-positive patients. However, frozen section analysis of the sentinel node is not reliable and definitive histopathology evaluation requires days, preventing intraoperative decision-making and immediate therapy. Photoacoustic imaging can evaluate intact lymph nodes, but specificity can be hampered by other absorbers such as hemoglobin. Near infrared multispectral photoacoustic imaging is a new approach that has the potential to selectively detect melanin. The purpose of the present study is to examine the potential of multispectral photoacoustic imaging to identify melanoma metastasis in human lymph nodes. Methods. Three metastatic and nine benign lymph nodes from eight melanoma patients were scanned ex vivo using a Vevo LAZR© multispectral photoacoustic imager and were spectrally analyzed per pixel. The results were compared to histopathology as gold standard. Results. The nodal volume could be scanned within 20 minutes. An unmixing procedure was proposed to identify melanoma metastases with multispectral photoacoustic imaging. Ultrasound overlay enabled anatomical correlation. The penetration depth of the photoacoustic signal was up to 2 cm. Conclusion. Multispectral three-dimensional photoacoustic imaging allowed for selective identification of melanoma metastases in human lymph nodes.

Intra-operative ex vivo photoacoustic nodal staging in a rat model using a clinical superparamagnetic iron oxide nanoparticle dispersion

The ability to accurately detect tumor metastases in lymph nodes is essential for intra-operative staging of various malignancies. Histopathological assessment of nodes has the drawback of a time delay before results are available to the surgeon and a likelihood of missing metastases. Photoacoustic (PA) imaging has been shown to possess the potential to detect melanoma metastases in resected in toto lymph nodes based on intrinsic contrast. To extend application of the method to other malignancies, extrinsic contrast for lymphatic mapping is important. We investigate in a metastatic animal model whether clinically approved superparamagnetic iron oxide (SPIO) nanoparticles, applied for MRI, can help PA imaging for staging in an intra-operative ex vivo setting. Imaging results are compared with 14 Tesla MR images and histology. We observe that irregularities in SPIO distribution in PA images of the nodes and a decrease in contrast correlate with metastatic involvement as seen in MR images and histology. The results show that a PA based imaging technique may be valuable for nodal staging in the field of surgical oncology.

Photoacoustic staging of nodal metastases using SPIOs: Comparison between in vivo, in toto and ex vivo imaging in a rat model

Background and objectives: To determine prognosis and treatment, accurate nodal staging is essential in many tumor types. After injection of clinical grade superparamagnetic iron oxide (SPIO) nanoparticles, it has been shown that metastatic lymph nodes can be distinguished from benign specimens using MR imaging. However, MR does not benefit per-operative nodal staging which requires a non-ionizing, small volume, high resolution, fast imaging technique. In vivo non-invasive photoacoustic (PA) imaging of lymph nodes might facilitate nodal staging during surgery, thereby benefiting both surgeon and patient. Materials and methods: In order to investigate the feasibility of an in vivo nodal staging approach using photoacoustics, six Mat-lylu inoculated Copenhagen rats were photo-acoustically imaged after injection of a new Class IIa medical device SPIO magnetic tracer (Sienna+). Lymph nodes were imaged in vivo, in toto (after euthanization) and ex vivo using multiple wavelength illumination. Results were compared with MRI, immunohistochemistry and photographs of the sectioned nodes. Results: These experiments demonstrate that in an ex vivo setting, the PA contrast of Sienna+ is able to facilitate a distinction between metastatic and benign nodes. A non-invasive distinction between both groups is partially impeded by the low amount of PA contrast generated by the SPIO particles compared to that of endogenous absorbers such as hemoglobins. Conclusions: This comparison between in vivo, in toto and ex vivo PA imaging of lymph nodes after SPIO injection demonstrates that the clinical potential of combined PA/SPIO staging should initially be exploited in an ex vivo setting. Improved distinction between chromophores by for example multi-spectral unmixing might in the near future enable non-invasive assessment of nodal involvement.

Photoacoustic intra-operative nodal staging using clinically approved superparamagnetic iron oxide nanoparticles

Detection of tumor metastases in the lymphatic system is essential for accurate staging of various malignancies, however fast, accurate and cost-effective 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. We evaluate whether a combination of photoacoustic (PA) imaging and a clinically approved SPIO dispersion, could be applied for intra-operative nodal staging. Metastatic adenocarcinoma was inoculated in Copenhagen rats for 5 or 8 days. After SPIO injection, the lymph nodes were photoacoustically imaged both in vivo and ex vivo whereafter imaging results were correlated with MR and histology. Results were compared to a control group without tumor inoculation. In the tumor groups clear irregularities, as small as 1 mm, were observed in the PA contrast pattern of the nodes together with an decrease of PA response. These irregularities could be correlated to the absence of contrast in the MR images and could be linked to metastatic deposits seen in the histological slides. The PA and MR images of the control animals did not show these features. We conclude that the combination of photoacoustic imaging with a clinically approved iron oxide nanoparticle dispersion is able to detect lymph node metastases in an animal model. This approach opens up new possibilities for fast intra-operative nodal staging in a clinical setting.

Optical techniques for the intraoperative assessment of nodal status

The lymphatic system is an important pathway in the metastatic spread of many malignancies and a key prognostic indicator. Nondestructive assessment of the nodal status during surgery could limit the amount of lymph nodes that need to be resected and allow for immediate regional lymphadenectomy during sentinel lymph node biopsy procedures. This review looks into the possibilities of conventional medical imaging methods that are capable of intraoperative nodal assessment and discusses multiple newly developed optical techniques. The physical background behind these techniques is reviewed and a concise overview of their main advantages and disadvantages is provided. These recent innovations show that while the application of optical modalities for intraoperative nodal staging is not yet applied routinely, there is reason enough to expect their introduction in the near future.