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Background: Sentinel lymph node biopsy (SLNB) is a staging procedure dependent on accurate mapping of draining lymphatics via tracers. Robotassisted SLNB enables access to multiple neck levels with a single incision and intraoperative fluorescence guidance to the SLN.Methods: Lymphatic mapping in swine was done using a magnetic tracer and fluorescent dye, injected into the tongue. MRI preoperatively mapped lymphatic spread of the magnetic tracer. Dissection was performed using a da Vinci Xi robot guided by fluorescence-imaging of the dye. Results: Robot-assisted SLNB was successfully performed in all animals (n = 5). A novel MRI protocol differentiated SLNs (n = 6) from lower echelon nodes (n = 11) based on flow progression. Fluorescence imaging provided valuable intraoperative guidance and correlated with magnetic-positive nodes.Conclusions: This study demonstrates preclinical feasibility of a robot-assisted approach to SLNB using magnetic and fluorescent tracers in the head and neck, enabling both preoperative mapping and intraoperative guidance.

Background Gastrectomy with extended (D2) lymphadenectomy is considered standard of care for gastric cancer to provide the best possible outcomes and pathologic staging. However, D2 gastrectomy is a technically demanding operation and reported to be associated with increased complications and mortality. Application of sentinel lymph node (SLN) concept in gastric cancer has the potential to reduce patient morbidity; however, SLN techniques are not established for gastrectomy, in part due to lack of practical tracers. An effective and convenient tracer with enhanced SLN accumulation is critically needed. Methods Mannose-labelled magnetic tracer ‘FerroTrace’ and fluorescent dye indocyanine green (ICG) were injected laparoscopically into the stomach submucosa of 8 healthy swine under general anaesthesia. Intraoperative fluorescence imaging was used to highlight draining lymphatic pathways containing ICG, while preoperative T2-weighted MRI and ex vivo magnetometer probe measurements were used to identify nodes containing FerroTrace. Lymphadenectomy was performed either robotically (n = 2) or via laparotomy (n = 6). Results Mixing ICG and FerroTrace ensured concurrence of fluorescent and magnetic signals in SLNs. An initial trial with robotic dissection removed all magnetic LNs (n = 4). In the subsequent laparotomy study that targeted all ICG-LNs based on intraoperative fluorescence imaging, dissection removed an average of 4.7 ± 1.2 fluorescent, and 2.0 ± 1.3 magnetic LNs per animal. Both MRI and magnetometer detected 100% of SLNs (n = 7). FerroTrace demonstrated high specificity to SLNs, which contained 76 ± 30% of total lymphotropic iron, and 88 ± 20 % of the overall magnetometer signal. Conclusions Through utilisation of this dual tracer approach, SLNs were identified via preoperative MRI, visualised intraoperatively with fluorescence imaging, and confirmed with a magnetometer. This combination pairs the sensitivity of ICG with SLN-specific FerroTrace and can be used for reliable SLN detection in gastric cancer, with potential applications in neoadjuvant therapy. Graphical abstract

337 Background: Gastrectomy with extended (D2) lymphadenectomy is considered as a standard of care for gastric cancer to provide best possible outcomes and pathologic staging. However, D2 gastrectomy is technically demanding operation and reported to be associated with increased complications, including bleeding, pancreatitis, abscess, chylous ascites, and mortality. Application of sentinel lymph node mapping in gastric cancer has the potential to reduce patient morbidity, while facilitating detailed pathology processing required to accurately identify nodal metastases. However, sentinel lymph node biopsy techniques are not established for gastrectomy because of a lack of practical tracers; an effective and convenient tracer that exclusively accumulates in sentinel lymph nodes is critically needed. Methods: A lymphotropic tracer solution consisting of mannose-labelled magnetic nanoparticles (SPIONs) and a fluorescent dye (indocyanine green, ICG) was injected laparoscopically into the stomach submucosa of 6 healthy female domesticated pigs while under general anaesthesia. Intraoperative laparoscopic fluorescence imaging was used to highlight draining lymphatic pathways containing ICG, while a combination of T2-weighted MRI and magnetometer probe detection were used to identify nodes containing SPIONs. Results: Mixing the ICG dye and SPIONs ensured concurrence of fluorescent and magnetic signals in the sentinel nodes. Average tracer accumulation time before dissection was 5 hours. On average 4.5 fluorescent nodes (range 2–7) were removed, of which 2 (average) were also magnetic (range 1–4). Magnetic nodes could be identified using both preoperative MRI and intraoperative magnetometer probe. The mannose label of the SPIONs produced a strong binding affinity to macrophages, and in combination with particle size (80 nm) results in enhanced retention in the sentinel nodes. On average, after 5 hours accumulation time, the sentinel nodes contained 4.1% of the SPION injected dose and 88% of the magnetometer counts. Conclusions: Through the co-injection of fluorescent and mannose-labelled magnetic tracers, lymphatic drainage pathways can be determined preoperatively with MRI, visualised with intraoperative fluorescence cameras, and SLNs confirmed with magnetometer. Although ICG flows through multiple echelons, in doing so it creates a lymphatic roadmap to guide surgeons. The addition of magnetic particles provides sentinel node discrimination, so in combination, these two tracers could greatly enhance the capability to detect SLNs in gastric cancer.