VSL#3 Versus Placebo in Increasing the Pathological Major Response Rate in Patients With Rectal Cancer

Inclusion Criteria: * Age \> 18 years * Ability to sign informed consent and understand the nature of a placebo- controlled trial. * Ability to complete questionnaire(s) by themselves or with assistance. * Life expectancy ≥ 6 months. * Current diagnosis of cancer that supports the use of continuous definitive or adjuvant external-beam RT to the pelvis to a minimum dose of 4500 cGy delivered with conventional multi-field photon radiotherapy, 3-D conformal photon radiotherapy or intensity modulated photon therapy. * Will receive concurrent administration of chemotherapy (5-FU, capecitabine, cisplatin, oxaliplatin, carboplatin, and/or mitomycin) during pelvic RT * The following laboratory values obtained \> 28 days prior to registration: Hemoglobin \> 10.0 g/dL WBC \> 3,500 Absolute neutrophil count \> 1,500 Platelets \> 100,000 * ECOG Performance Status (PS) of 0, 1 or 2 * Willing to abstain from ingestion of yogurt products and/or any product containing probiotics during study drug treatment. Exclusion Criteria: * Current or prior metastases beyond regional lymph nodes. * Undergone abdominal-perineal resection, Hartmann procedure, or other surgical procedure leaving the patient without a functioning rectum. * Known allergy to a probiotic preparation. * Any history of inflammatory bowel disease. -\> grade 3 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. * Planned use of leucovorin (because of the risk of secretory diarrhea). * Split-course RT is planned. * Prior pelvic RT. * Proton RT. * Any of the following: Pregnant women Nursing women Men or women of childbearing potential who are unwilling to employ adequate contraception * Use of probiotics ≤ 2 weeks prior to registration. * Use of antibiotics ≤ 1 week prior to registration. * Planned continuous antibiotic treatment during RT. * History of gastrointestinal or genitourinary obstruction or porphyria.