Postoperative Chemoradiation or Chemotherapy After Preoperative Chemotherapy for Gastric Cancers

Inclusion Criteria:

1. Must have pathologically-proven adenocarcinoma of the stomach or gastroesophageal (GE)-junction, stage M0, as established by both imaging and surgical pathologic staging.

   Imaging: Clinical stage of M0 will be established by either CT (chest with contrast and abdomen/pelvis with and without contrast), or CT/PET (positron emission tomography) (skull base to mid-thigh). This is standard post-surgery imaging.

   Surgery: Surgical pathologic staging must be M0.
2. Must have completed 3 cycles of neo-adjuvant chemotherapy. Either CAPEOX or FOLFOX is allowed. Dose modifications are allowed, but all 3 cycles must have been completed.
3. Must have undergone a surgical resection with definitive intent, either by open or laparoscopic resection of the primary gastric or GE junction cancer. Patients must have undergone a total gastrectomy, subtotal gastrectomy, or distal gastrectomy (depending on the location of primary gastric lesion) with at least a modified D2 lymphadenectomy.
4. Must be deemed as a good candidate for adjuvant chemotherapy or chemoradiation (to start within 3 months of surgery), in the opinion of the treating investigator. Plan must be to start adjuvant therapy within 90 days of surgery; adjuvant treatment cannot begin more than 90 days after surgery.
5. Must have diagnostic biopsy tissue (pre-neoadjuvant chemo) available for genetic testing.
6. Must have surgical tissue (post-neoadjuvant chemo) available for genetic testing.
7. Must be \> 18 years of age.
8. Must be able to provide informed consent.
9. Must have adequate kidney, liver, and bone marrow function, within 28 days prior to registration, as follows:

   i. Hemoglobin ≥ 8.0 gm/dL

   ii. Absolute neutrophil count (ANC) ≥ 1500 cells/mm3

   iii. Platelet count ≥ 75,000 /mm3

   iv. Calculated creatinine clearance of \> 60 mL/min/m2, calculated as follows:

   For males = ((140 - age \[years\]) x (body weight \[kg\])) / ((72) x (serum creatinine \[mg/dL\])

   For females = 0.85 x male value

   v.Total bilirubin ≤ 1.5 times upper limit of normal (ULN)

   vi.AST (aspartate aminotransferase) (SGOT) and ALT (alanine transaminase) (SGPT) ≤ 3.0 times the ULN
10. Must have life expectancy of greater than 3 months.
11. Must have an ECOG (Eastern Cooperative Oncology Group) performance status 0-2.
12. Male or female patients of childbearing potential must be willing to use contraceptive precautions throughout the trial and 3 months following discontinuation of study treatment. Post-menopausal women must be amenorrheic for at least 12 months to be considered of non-childbearing potential.

Exclusion Criteria:

1. Other than the 3 cycles of neoadjuvant chemotherapy and surgery (mentioned above), must not have received other treatment for their gastric cancer.
2. Female patients who are pregnant, breast feeding, or of childbearing potential without a negative pregnancy test prior to baseline. Women of childbearing potential must have a negative serum pregnancy test as a part of eligibility, within 28 days of registration.
3. Patients unwilling or unable to comply with the protocol, or provide informed consent.
4. Patients with clinical evidence of metastatic disease.
5. Any medical condition that, in the opinion of the investigator, would exclude the patient from participating in this study and treatment plan.