This is a randomized, double-blind, placebo-controlled phase II clinical trial designed to evaluate whether adding omega-3 fatty acids improves outcomes in adult patients with recurrent or metastatic esophageal cancer receiving PD-1 inhibitor therapy.

The main questions are:

Compared with placebo, does omega-3 (EPA+DHA) increase the longitudinal change in skeletal muscle index (ΔSMI) over 6 months? Does it improve clinical outcomes and favorably modulate immune and metabolic biomarkers as well as patient-reported outcomes ?

Participants will be randomized 1:1 to:

Intervention: Standard PD-1 therapy plus oral omega-3 (EPA 2.5 g + DHA 1.25 g per day) for 6 months.

Control: Standard PD-1 therapy plus matching placebo for 6 months. Participants will attend study visits at baseline, 3 months, and 6 months (then every 3 months up to 2 years), undergo body composition and functional assessments, blood sampling, and questionnaires, and have treatment adherence assessed per protocol. Pre-specified exploratory biomarker analyses will be conducted to support mechanism research.

Inclusion Criteria： Age ≥18 years; ECOG 0-2. Histologically confirmed recurrent/metastatic esophageal cancer (AJCC 8th; de novo stage IV or relapse after prior therapy).

PD-1 inhibitor naïve, or prior PD-1 stopped >3 months with subsequent progression (not primary PD-1 resistance).

Adequate organ function per protocol (hematologic, hepatic, renal). Women of childbearing potential: negative pregnancy test and agree to effective contraception.

Signed informed consent.

Exclusion Criteria： PD-1 inhibitor resistance or refractory disease to prior PD-1. Esophageal fistula present or strongly suspected. Active autoimmune disease or immunodeficiency requiring systemic therapy (protocol-defined exceptions allowed, e.g., treated hypothyroidism; controlled type 1 diabetes).

Systemic corticosteroids or other immunosuppressants requiring ongoing use (physiologic or topical steroids allowed).

Interstitial lung disease/pneumonitis history or active pneumonitis on screening CT.

Uncontrolled cardiovascular disease (e.g., NYHA ≥ II heart failure, unstable angina, recent MI, significant uncontrolled arrhythmias).

Serious active infection, including active TB; uncontrolled viral hepatitis (active HBV/HCV per protocol).

Pregnant or breastfeeding. Recent major surgery (per protocol window) or high bleeding risk/therapeutic anticoagulation not suitable for omega-3 use.

Other conditions that, in investigator's judgment, preclude protocol compliance or safety.

De-identified individual participant data (IPD) underlying the primary publication will be shared with qualified researchers for legitimate scientific purposes. Data will be de-identified per HIPAA and local regulations.

Beginning 12 months after primary results publication and for 5 years thereafter.

Researchers must submit a methodologically sound proposal, IRB/ethics approval (or exemption), and a signed data use agreement. Proposals will be reviewed by the sponsor/investigator committee.