Probiotic LGG for Prevention of Side Effects in Patients Undergoing Chemoradiation for Gastrointestinal Cancer

Inclusion Criteria: * Current diagnosis of a gastrointestinal, abdominal, or pelvic cancer for which the use of continuous definitive or adjuvant external-beam RT to the abdomen or pelvis to a minimum dose of 4500 cGy is planned. * Scheduled to receive concurrent administration of fluoropyrimidine chemotherapy (5-FU or capecitabine) during radiation therapy. * Age ≥ 18 years. * Life expectancy ≥ 6 months. * Negative pregnancy test done ≤7 days prior to registration (for women of childbearing potential only). * The following laboratory values obtained ≤ 28 days prior to registration: * Hemoglobin ≥ 9.0 g/dL * WBC ≥ 3,500 * Absolute neutrophil count ≥ 1,500 * Platelets ≥ 100,000 * ECOG Performance Status (PS) of 0, 1, or 2. * Willingness to abstain from ingestion of yogurt products and/or any product containing probiotics during study drug treatment. * Ability to complete questionnaire(s) alone or with assistance. * Ability to understand and willingness to sign informed consent. Exclusion Criteria: * Previous bowel resection which, in the opinion of the investigator, would decrease the benefit of the probiotic. Patients who have undergone recent bowel surgeries which would not decrease the benefit of the probiotic are eligible provided they are more than 30 days from surgery with no serious complications. * Known allergy to a probiotic preparation. * Any history of inflammatory bowel disease. * Grade 3 or 4 diarrhea, rectal bleeding, abdominal cramping, or incontinence of stool ≤7 days prior to registration. * Any medical condition that may interfere with ability to receive protocol treatment. * Prior abdominal or pelvic RT. * Use of probiotics ≤ 2 weeks prior to registration. * Use of antibiotics ≤ 3 days prior to registration. * Planned continuous antibiotic treatment during RT. * History of gastrointestinal or genitourinary obstruction or porphyria. * History of irritable bowel syndrome (IBS). * History of hypersensitivity to all of the following antibiotics: penicillin, erythromycin, clindamycin, and any fluoroquinolone.