Inclusion Criteria:

• * Newly diagnosed histologically or cytologically gastric adenocarcinoma. (Siewert III acceptable: the bulk of tumor should be in stomach; gastric tumors with extension to the gastroesophageal junction are permitted.) Patients with T1-3N0-2 are eligible. Patients with N3, or T4 disease are not eligible.
• * Known T-stage defined by EUS. Must have had CT of the chest/abdomen/pelvis with contrast.
• * Medically eligible to receive standard of care chemotherapy.
• * At least 19 years of age
• * ECOG performance status ≤ 2
• Normal bone marrow and organ function as defined below:
• * Absolute neutrophil count ≥ 1,500 cells/mm3
• * Platelets ≥ 100,000 cells/mm3
• * Creatinine clearance \> 50 mL/min
• * The effects of the various chemotherapy agents used in this study on the developing human fetus are unknown. For this reason, women of childbearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control, abstinence) prior to study entry and for the duration of study participation. Should a woman become pregnant or suspect she is pregnant while participating in this study, she must inform her treating physician immediately. Men treated or enrolled on this protocol must also agree to use adequate contraception prior to the study, for the duration of the study, and one month after completion of the study
• * Ability to understand and willingness to sign an IRB approved written informed consent document (or that of legally authorized representative, if applicable).

Exclusion Criteria:

• * Prior surgery, radiation, or chemotherapy for gastric or esophageal cancer.
• * Prior surgery to the esophagus or stomach that would alter the radiation treatment field or stomach motion.
• * Siewert I-II GE junction tumor
• * Any active malignancy within 2 years of enrollment that may alter the course of gastric cancer. (Apparently cured localized malignancy or advanced, but indolent malignancy with significantly more favorable prognosis are allowed).
• * Currently receiving any other investigational agents.
• * A history of allergic reactions attributed to compounds of similar chemical or biologic composition to chemotherapeutic agents used in the study.
• * Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, diabetes, symptomatic congestive heart failure, unstable angina pectoris, or cardiac arrhythmia that are considered clinically significant as determined by the treating physician.
• * Pregnant and/or breastfeeding. Women of childbearing potential must have a negative pregnancy test within 14 days of study entry.
• * Patients with HIV are eligible unless their CD4+ T-cell counts are \< 350 cells/mcL or they have a history of AIDS-defining opportunistic infection within the 12 months prior to registration. Concurrent treatment with effective ART according to DHHS treatment guidelines is recommended. Recommend exclusion of specific ART agents based on predicted drug-drug interactions (i.e. for sensitive CYP3A4 substrates, concurrent strong CYP3A4 inhibitors (ritonavir and cobicistat) or inducers (efavirenz) should be contraindicated).