Trastuzumab deruxtecan's (T-DXd's) efficacy and safety has been confirmed in traditional clinical trials, there is a lack of real-world evidence, especially among Chinese patients. The objective of this study is to evaluate real-world effectiveness and safety profile of T-DXd by collecting real-world data treating in patients with locally advanced or metastatic HER2-positive gastric or gastroesophageal junction adenocarcinoma.

The primary objective of this study is to evaluate the effectiveness of T-DXd in real-world setting (real-word time to treatment failure).

The secondary objectives of this study are to assess the safety profile of T-DXd (treatment related adverse events [TRAE] regardless of grade, and physician reported adverse events of special interest [AESI] regardless of grade, etc.) and to evaluate effectiveness of T-DXd (real-world time to next treatment). This is an non-interventional, observational study. No drug will be provided or administered as part of this protocol; however, participants on T-DXd will be enrolled in this study.

Participants must meet all the following inclusion criteria to be eligible for the study:

1. Participants with pathologically diagnosed locally advanced unresectable or metastatic gastric cancer/gastroesophageal junction adenocarcinoma (GC/GEJA).
2. ≥18 years of age at the time of starting the first dose of T-DXd and signing informed consent form (ICF), capable of providing informed consent.
3. HER2-positive status (IHC 3+ or IHC 2+/ISH +).
4. Received prior anti-cancer treatment regimens according to National Medical Products Administration indication or clinical judgement of physician.
5. Decision to newly initiate T-DXd before study ICF signing. If the participants have started the first dose of T-DXd no longer than 21 days before enrollment, they could be enrolled if the signed and dated ICF could be obtained.

Exclusion Criteria：

Participants who meet any of the following criteria will be excluded from the study:

1. Pregnancy or breastfeeding.
2. Participants who at the time of data collection of the study are participating in or have participated in an interventional study that remains blinded.
3. Known hypersensitivity to either the drug substances or inactive ingredients in the drug product.
4. Judged by the investigator to be unfit to participate in the study.

Eligible participants for this study include participants with HER2-positive local advanced or metastatic GC/GEJA.