Response-Adapted Therapy With Copanlisib and Rituximab in Untreated Follicular Lymphoma

Background: The disease follicular lymphoma (FL) develops when the body makes abnormal B-cells. These cells usually build up in the lymph nodes, but can also affect other parts of the body. Researchers want to see if a combination of drugs can attack the cancer cells in people with FL. Objective: To see if copanlisib plus rituximab is effective at slowing the growth of FL. Eligibility: People with FL who have not had prior treatment for their disease Design: Participants will be screened with: * Medical and cancer history * Physical exam * Review of symptoms and ability to perform daily activities * Blood and urine tests * Small amount of bone marrow removed by needle in the hip bone * Scans of the chest, abdomen, and pelvis. Some scans will use a radioactive tracer. Participants will get the study drugs in 28-day cycles for up to 13 cycles. Both are given as an intravenous (IV) infusion. Copanlisib is given over about 1 hour. Rituximab is given over several hours. * For 1 cycle, they will get 3 weekly doses of copanlisib. * For the next cycle, they will get 3 weekly doses of copanlisib and 4 weekly doses of rituximab. * For all other cycles, they will get 2-3 weekly doses of copanlisib and 1 dose of rituximab. Participants will repeat some screening tests during the cycles. They will give a cheek swab and/or saliva sample and may have a tumor sample taken. After treatment, some participants will have a few follow-up visits each year for 5 years, then 1 each year. They will repeat screening tests. Other participants will be contacted by phone every few months.