The scope of the model’s applicability must always be confirmed and attention must be paid to unusual factors that may be important in a minority of patients.

The model was designed in a training cohort that included patients from the AGAMENON oesophageal and gastric cancer registry managed by SEOM in which 42 Spanish university hospitals participate. Patients eligible for this study were adults (age ≥18 years), diagnosed with locally advanced or metastatic unresectable adenocarcinoma of the distal oesophagus, GEJ and stomach, HER2+ who received at least one cycle of chemotherapy and trastuzumab as first-line treatment. HER2 status was studied locally at the centres and defined as IHC 3+ or IHC 2+/FISH+. Subjects who had completed perioperative or adjuvant treatment in the previous 6 months and those with HER2+ cancers that were not treated with trastuzumab first-line were excluded. None of these patients had received immunotherapy. Results were externally validated in a separate cohort from The Christie NHS Foundation Trust (Manchester, United Kingdom), which included consecutive patients who met the same eligibility criteria.

All covariates have to be available at the start of treatment. The covariates considered in this model were: Neutrophil-Lymphocyte ratio (NLR), ECOG-PS, Lauren subtype, HER2 expression, histological grade and tumour burden. Criteria to stratify the overall tumor burden have been used previously by our group. A sensitivity analysis on the entire cohort suggests that these criteria are valid (Annex Figure 1).

Disclaimer: This tool is intended for use by healthcare professionals only. Patients with advanced gastric cancer should seek medical care. Physicians and other healthcare professionals who use the Agamenon HER2 calculator should exercise their own clinical judgment. This app does not provide professional advice. We have exercised great caution in creating this app, based on real-world data. However, medical standards and practice may vary as new information becomes available and professionals should consult a variety of medical sources. While we estimate survival probability for advanced gastric cancer patients undergoing first-line chemotherapy, we in no way endorse any particular management strategy. Your reliance upon predictions obtained through this app is solely at your own risk. We assume no liability or responsibility whatsoever for damage or injury (including death) to you or anyone else from the use of this tool.