Joost Jacob Pouw

Pre-operative sentinel lymph node localization in breast cancer with superparamagnetic iron oxide MRI: the SentiMAG Multicentre Trial imaging subprotocol

OBJECTIVE Sentinel lymph node biopsy (SLNB) with a superparamagnetic iron oxide (SPIO) tracer was shown to be non-inferior to the standard combined technique in the SentiMAG Multicentre Trial. The MRI subprotocol of this trial aimed to develop a magnetic alternative for pre-operative lymphoscintigraphy (LS). We evaluated the feasibility of using MRI following the administration of magnetic tracer for pre-operative localization of sentinel lymph nodes (SLNs) and its potential for non-invasive identification of lymph node (LN) metastases. METHODS Patients with breast cancer scheduled to undergo SLNB were recruited for pre-operative LS, single photon emission CT (SPECT)-CT and SPIO MRI. T1 weighted turbo spin echo and T2 weighted gradient echo sequences were used before and after interstitial injection of magnetic tracer into the breast. SLNs on MRI were defined as LNs with signal drop and direct lymphatic drainage from the injection site. LNs showing inhomogeneous SPIO uptake were classified as metastatic. During surgery, a handheld magnetometer was used for SLNB. Blue or radioactive nodes were also excised. The number of SLNs and MR assessment of metastatic involvement were compared with surgical and histological outcomes. RESULTS 11 patients were recruited. SPIO MRI successfully identified SLNs in 10 of 11 patients vs 11 of 11 patients with LS/SPECT-CT. One patient had metastatic involvement of four LNs, and this was identified in one node on pre-operative MRI. CONCLUSION SPIO MRI is a feasible technique for pre-operative localization of SLNs and, in combination with intraoperative use of a handheld magnetometer, provides an entirely radioisotope-free technique for SLNB. Further research is needed for the evaluation of MRI characterization of LN involvement using subcutaneous injection of magnetic tracer. ADVANCES IN KNOWLEDGE This study is the first to demonstrate that an interstitially administered magnetic tracer can be used both for pre-operative imaging and intraoperative SLNB, with equal performance to imaging and localization with radioisotopes.

Comparison of three magnetic nanoparticle tracers for sentinel lymph node biopsy in an in vivo porcine model

Introduction Breast cancer staging with sentinel lymph node biopsy relies on the use of radioisotopes, which limits the availability of the procedure worldwide. The use of a magnetic nanoparticle tracer and a handheld magnetometer provides a radiation-free alternative, which was recently evaluated in two clinical trials. The hydrodynamic particle size of the used magnetic tracer differs substantially from the radioisotope tracer and could therefore benefit from optimization. The aim of this study was to assess the performance of three different-sized magnetic nanoparticle tracers for sentinel lymph node biopsy within an in vivo porcine model. Materials and methods Sentinel lymph node biopsy was performed within a validated porcine model using three magnetic nanoparticle tracers, approved for use in humans (ferumoxytol, with hydrodynamic diameter dH =32 nm; Sienna+®, dH =59 nm; and ferumoxide, dH =111 nm), and a handheld magnetometer. Magnetometer counts (transcutaneous and ex vivo), iron quantification (vibrating sample magnetometry), and histopathological assessments were performed on all ex vivo nodes. Results Transcutaneous “hotspots” were present in 12/12 cases within 30 minutes of injection for the 59 nm tracer, compared to 7/12 for the 32 nm tracer and 8/12 for the 111 nm tracer, at the same time point. Ex vivo magnetometer counts were significantly greater for the 59 nm tracer than for the other tracers. Significantly more nodes per basin were excised for the 32 nm tracer compared to other tracers, indicating poor retention of the 32 nm tracer. Using the 59 nm tracer resulted in a significantly higher iron accumulation compared to the 32 nm tracer. Conclusion The 59 nm tracer demonstrated rapid lymphatic uptake, retention in the first nodes reached, and accumulation in high concentration, making it the most suitable tracer for intraoperative sentinel lymph node localization.

Quantitative Analysis of Superparamagnetic Contrast Agent in Sentinel Lymph Nodes Using Ex Vivo Vibrating Sample Magnetometry

As the first step in developing a new clinical technique for the magnetic detection of colorectal sentinel lymph nodes (SLNs), a method is developed to measure the magnetic content in intact, formalin fixated lymph nodes using a vibrating sample magnetometer (VSM). A suspension of superparamagnetic nanoparticles is injected ex vivo around the tumor in the resected colon segments. A selection of three lymph nodes is excised from the region around the tumor and is separately measured in the VSM. The iron content in the lymph nodes is quantified from the magnetic moment curve using the Langevin model for superparamagnetism and a bimodal particle size distribution. Adverse, parasitic movements of the sample were successfully reduced by tight fixation of the soft tissue and using a small vibration amplitude. Iron content in the lymph nodes is detected with 0.5 μg accuracy and ranged from 1 to 51 μg. Histological staining confirmed iron presence. The current method of measuring intact biological tissue in a VSM is suitable to show the feasibility and merit of magnetic detection of SLNs in colorectal cancer. For clinical validation of magnetic SLN selection in colorectal cancer, a new magnetometer with high specificity for superparamagnetic nanoparticles is required.

Magnetic Detection of the Sentinel Lymph Node in Ex Vivo Tissue with Colorectal Cancer

A new method for sentinel lymph node detection was investigated by ex-vivo experiments in colorectal cancer. A dispersion of superparamagnetic iron oxide (SPIO) nanoparticles was injected in the resected tissue around the tumor. A magnetic probe with a 2 nd order gradiometer configuration was used to localize the sentinel lymph node with accumulated SPIO nanoparticles. Additional analysis of SPIO content was performed on the resected lymph nodes. High-field MRI and conventional microscopy revealed the presence and distribution of SPIO inside the lymph nodes. Magnetometry confirmed SPIO content in lymph nodes and could give an estimate of the amount of SPIO uptake.

Ex vivo sentinel lymph node mapping in colorectal cancer using a magnetic nanoparticle tracer to improve staging accuracy: a pilot study

Nodal status is the most important prognostic factor in colorectal cancer (CRC). Small occult metastases may remain undetected on conventional histopathological examination, potentially resulting in undertreatment. Ex vivo sentinel lymph node mapping (SLNM) can be used to improve the accuracy of nodal staging, but the currently used tracers suffer from drawbacks, which hamper implementation of the technique in routine clinical practice. Magnetic tracers are the optimal size for sentinel lymph node (SLN) retention and allow objective quantitative selection of SLNs; they therefore have great potential for SLNM in CRC. The study evaluates the feasibility of ex vivo magnetic SLNM and compares the performance of this technique with blue dye SLNM.

A Magnetometer Cooled with Liquid Nitrogen for the Characterization and Quantification of Magnetic Nanoparticles in Biological Samples at Room Temperature

For several medical applications of magnetic nanoparticles (MNP) it is desired to know the quantity and characteristics of the particles in the tissue of interest. That can either be necessary to determine how successful a procedure was of how it will be. Therefore a system is built, that is suitable to analyze small intact biological samples at room temperature. The magnetometer is used for the analysis and selection of sentinel lymph nodes in colorectal cancer. In this clinical procedure MNPs are administered in the resected part of the colon to determine the sentinel lymph node. The magnetometer is based on copper wound coils and comprises of two detection coils in series opposition, enclosed by two separately driven excitation coils.

Data-driven gating in PET : Influence of respiratory signal noise on motion resolution

Purpose Data‐driven gating (DDG) approaches for positron emission tomography (PET) are interesting alternatives to conventional hardware‐based gating methods. In DDG, the measured PET data themselves are utilized to calculate a respiratory signal, that is, subsequently used for gating purposes. The success of gating is then highly dependent on the statistical quality of the PET data. In this study, we investigate how this quality determines signal noise and thus motion resolution in clinical PET scans using a center‐of‐mass‐based (COM) DDG approach, specifically with regard to motion management of target structures in future radiotherapy planning applications. Methods PET list mode datasets acquired in one bed position of 19 different radiotherapy patients undergoing pretreatment [18F]FDG PET/CT or [18F]FDG PET/MRI were included into this retrospective study. All scans were performed over a region with organs (myocardium, kidneys) or tumor lesions of high tracer uptake and under free breathing. Aside from the original list mode data, datasets with progressively decreasing PET statistics were generated. From these, COM DDG signals were derived for subsequent amplitude‐based gating of the original list mode file. The apparent respiratory shift d from end‐expiration to end‐inspiration was determined from the gated images and expressed as a function of signal‐to‐noise ratio SNR of the determined gating signals. This relation was tested against additional 25 [18F]FDG PET/MRI list mode datasets where high‐precision MR navigator‐like respiratory signals were available as reference signal for respiratory gating of PET data, and data from a dedicated thorax phantom scan. Results All original 19 high‐quality list mode datasets demonstrated the same behavior in terms of motion resolution when reducing the amount of list mode events for DDG signal generation. Ratios and directions of respiratory shifts between end‐respiratory gates and the respective nongated image were constant over all statistic levels. Motion resolution d/dmax could be modeled as Symbol, with dmax as the actual respiratory shift. Determining dmax from d and SNR in the 25 test datasets and the phantom scan demonstrated no significant differences to the MR navigator‐derived shift values and the predefined shift, respectively. Symbol. No caption available. Conclusions The SNR can serve as a general metric to assess the success of COM‐based DDG, even in different scanners and patients. The derived formula for motion resolution can be used to estimate the actual motion extent reasonably well in cases of limited PET raw data statistics. This may be of interest for individualized radiotherapy treatment planning procedures of target structures subjected to respiratory motion.

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.

Phantom study quantifying the depth performance of a handheld magnetometer for sentinel lymph node biopsy

PURPOSE The use of a magnetic nanoparticle tracer and handheld magnetometer for sentinel lymph node biopsy (SLNB) was recently introduced to overcome drawbacks associated with the use of radioisotope tracers. Unlike the gamma probe, the used magnetometers are not only sensitive to the tracer, but also the diamagnetic human body. This potentially limits the performance of the magnetometer when used clinically. METHODS A phantom, mimicking the magnetic and mechanical properties of the human axilla, was constructed. The depth performance of two current generation magnetometers was evaluated in this phantom. LN-phantoms with tracer uptake ranging from 5 to 500μg iron were placed at clinically relevant depths of 2.5, 4 and 5.5cm. Distance-response curves were obtained to quantify the depth performance of the probes. RESULTS The depth performance of both probes was limited. In the absence of diamagnetic material and forces on the probe (ideal conditions) a LN-phantom with high uptake (500μg iron) could first be detected at 3.75cm distance. In the phantom, only superficially placed LNs (2.5cm) with high uptake (500μg iron) could be detected from the surface. The penetration depth was insufficient to detect LNs with lower uptake, or which were located deeper. CONCLUSION The detection distance of the current generation magnetometers is limited, and does not meet the demands formulated by the European Association for Nuclear Medicine for successful transcutaneous SLN localization. Future clinical trials should evaluate whether the limited depth sensitivity is of influence to the clinical outcome of the SLNB procedure.

Sentinel lymph node identification with magnetic nanoparticles

Most solid malignancies have a tendency to spread through the lymphatic system to locoregional lymph nodes. Presence of metastasis is an important prognostic factor, and is used to determine the optimal treatment of the patient. The sentinel lymph nodes (SLNs) receive direct lymphatic drainage from the tumour area, and are therefore most likely the first site of metastasis, if present. The SLNs therefore play a crucial role in the staging and treatment of cancer. This thesis describes the use of magnetic nanoparticles for SLN identification in colorectal- and breast cancer. In colorectal cancer currently used experimental techniques suffer from limitations, hampering widespread clinical implementation of the technique. In breast cancer, a radioactive tracer is used. However, the worldwide availability of radioisotopes, and therefore the best treatment, is limited. The aim of the research in this thesis is twofold. In colorectal cancer, the objective is to introduce a SLN procedure allowing to improve nodal staging accuracy in a routine clinical workflow after ex vivo administration of magnetic nanoparticles. In breast cancer, we aim to introduce an entirely radioisotope-free method for in vivo SLN Biopsy (SLNB). A commercially available magnetometer and magnetic tracer were used for SLN identification in patients suffering from colorectal cancer to evaluate the feasibility of the technique. Patient, animal and phantom studies were performed to optimize the use of the SLNB technique with magnetic nanoparticles in breast cancer patients. With the work in this thesis we have established the feasibility of using ex vivo magnetic SLNM to improve staging accuracy of colorectal cancer patients, in routine clinical practice. Furthermore, by analysing and resolving shortcomings and pitfalls of the existing intraoperative magnetic technique for SLNB our research made important contributions to make the introduction of an entirely radioisotope-free technique for SLNB in breast cancer patients possible.