64Cu DOTA-Trastuzumab PET/CT in Studying Patients With Gastric Cancer

Inclusion Criteria: * Patient must have a histologic diagnosis of gastric or gastroesophageal junction adenocarcinoma * Two patients must be HER2 3+ by immunohistochemistry (IHC) or fluorescence in situ hybridization (FISH) positive * Either the primary tumor or at least one of the metastatic lesions must be \>= 2 cm * Eastern Cooperative Oncology Group (ECOG) performance status of 0-2 * Life expectancy of \>= 3 months * Women of childbearing potential must have a negative serum pregnancy test within 14 days of copper Cu 64-DOTA administrations and must have agreed to use an effective contraceptive method; women of child-bearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control or abstinence) prior to study entry and for four months following duration of study participation; should a woman become pregnant or suspect that she is pregnant while participating on the trial, she should inform her treating physician immediately * CT/magnetic resonance imaging (MRI) scan must be obtained within 4 weeks prior to study entry * Patients must have normal cardiac ejection fraction * All subjects must have the ability to understand and the willingness to sign a written informed consent * All toxicities should recover to grade 0 or 1 prior to day 1 Exclusion Criteria: * Impaired cardiac function including any one of the following: complete left bundle branch block or use of a permanent cardiac pacemaker, congenital long QT syndrome, presence of ventricular tachyarrhythmias, clinically significant resting bradycardia (\< 50 beats per minute), QT interval corrected by Fridericia's formula (QTcF) \> 450 msec on screening electrocardiogram (ECG), or right bundle branch block + left anterior hemiblock (bifascicular block) * Presence of atrial fibrillation * Previous history of angina pectoris or acute myocardial infarction (MI) within 6 months * Congestive heart failure (New York Heart Association functional classification III-IV) * Uncontrolled hypertension (mmHg \> 160 systolic or \> 90 diastolic) * Patients should not have active infections or concurrent neoplastic disease except for skin cancer * Patients may not be receiving any other investigational agents * At the time of enrollment, patients may not have received any biological, chemotherapy, or radiation therapy * Patients who may not have received trastuzumab within the prior 6 months for any other reason * Patients who are pregnant * Subjects, who in the opinion of the investigator, may not be able to comply with the safety monitoring requirements of the study