 Patients receive ixazomib citrate orally PO once weekly every 4 weeks for up to 24 courses in the absence of disease progression or unacceptable toxicity. Upon completion of 6 courses of ixazomib citrate therapy, patients also receive rituximab intravenously IV once weekly. Treatment repeats every 28 days for up to 2 years in the absence of disease progression or unacceptable toxicity. 