Gastric cancer (GC) remains an oncological challenge, where lymph node involvement is a key prognostic factor. Sentinel lymph node (SLN) mapping may improve staging and reduce surgical morbidity. This study evaluated a dual-tracer technique - superparamagnetic iron oxide (SPIO) and methylene blue - for ex vivo SLN detection in GC, analysed the performance of one-step nucleic acid amplification (OSNA) versus conventional histology and applied pooling analysis with OSNA in non-sentinel nodes.

The aim of this prospective study was, firstly, to evaluate the accuracy of ex vivo SLN detection using a novel dual tracer: SPIO not previously used for this pathology, combined with methylene blue. Secondly, to perform a diagnostic concordance analysis between the results of the conventional histological haematoxylin-eosin (H-E) study versus the OSNA (One Step Nucleic Acid Amplification) molecular method, for the detection of lymph node metastases in patients with gastric cancer undergoing surgery with curative intent and lymphadenectomy.

Observational, prospective and analytical study, carried out in a second level university hospital with 500 inpatient beds.

All patients with an indication for oncological gastric surgery who met the inclusion criteria were consecutively included in the period between August 2017 and March 2022.

All patients were diagnosed, treated and followed up according to the centre's standard clinical practice, following a protocol based on the scientific evidence collected in the European ESMO-ESSO-ESTRO guidelines for the management of gastric cancer Surgical procedures Participation in the study did not alter the indications or standard surgical technique. The only additional intervention consisted of intraoperative administration of SPIO (superparamagetic iron oxide)

Variables collected For each patient included in the study, demographic, clinical and pathological variables were systematically collected: age, sex, type of neoplasm, size, tumour grade, TNM 8ed, stage, neoadjuvant treatment and degree of regression, if any. Perioperative variables such as type of intervention, type of lymphadenectomy, time of infiltration and tracer detection, operative complications, reoperation rate and hospital stay were also evaluated. During follow-up, oncological variables such as recurrence rate, mortality, overall survival and disease-free survival were analysed.

Statistical analysis Data were analysed using SPSS version 26.0 software (IBM Corp., Armonk, NY, USA) and EpiDat version 4.2 (Xunta de Galicia, Spain). A descriptive analysis was performed using measures of central tendency and dispersion for quantitative variables, and absolute and relative frequencies for qualitative variables. The chi-square test with correction was used to compare qualitative variables. Concordance between methods was assessed using Cohen's kappa coefficient. Diagnostic accuracy was determined by calculating sensitivity and specificity, together with their respective 95% confidence intervals according to Hanley&McNeil. Survival curves were estimated using the Kaplan-Meier method, and differences between groups were analysed using the log-rank test. A value of p < 0.05 was considered statistically significant.

Inclusion Criteria:

• Patients over 18 years of age with a diagnosis of gastric adenocarcinoma, undergoing surgery with curative intent by total or subtotal gastrectomy, accompanied by D1, D1+ or D2 lymphadenectomy, and with preoperative positivity for cytokeratin 19 (CK19) in the tumour sample.

Exclusion Criteria:

• Patients with gastric cancer recurrence, those diagnosed with other active neoplasms, metastatic disease at diagnosis, as well as patients with Siewert type I and II gastrooesophageal junction carcinoma. Neoadjuvant treatment was not considered an exclusion criterion.

All patients with an indication for oncological gastric surgery who met the inclusion criteria were consecutively included in the period between August 2017 and March 2022.

It is not yet known whether there will be a plan for the IPD to be made available