The study doctors will first make sure that your disease has not spread too much and is not too severe to require immediate treatment with chemotherapy before you can begin treatment on this study. If you are found to be eligible to take part in this study, you will be given Benadryl (diphenhydramine) by vein, and you will be given Tylenol (acetaminophen) by mouth before each dose of rituximab. This is done to help decrease the risk of developing side effects of rituximab. You will then receive 1 dose of rituximab by vein over 6 to 8 hours on Day 1 of treatment. After treatment with rituximab, you will then be given a radioactive antibody, ^111 In Ibritumomab tiuxetan (this is a radioactive agent that binds to rituximab to help with imaging exams), by vein over about 10 minutes. This is so researchers can use a special camera to see where the drug is in your body.