Follicular lymphoma (FL) is a slowly growing cancer of the lymph glands. It often responds well to treatment but has a tendency to come back again (relapse) and therefore needs to be treated more than once. Initial treatment is usually with a 6-month course chemotherapy combined with an antibody drug called rituximab (R+chemo). Most patients who respond to R+chemo are offered further (maintenance) therapy with rituximab alone over a period of 2 years with the aim of delaying relapse. However, there is growing controversy about the routine use of rituximab maintenance after initial R+chemo for the following reasons: (1) It does not prolong survival; (2) It is associated with an increased risk of infection (responsible for 7-8% of all deaths in FL); (3) It prolongs remissions only in the minority of patients whose disease was destined to relapse within 2-3 years. A one-size-fits-all approach to rituximab maintenance is therefore not ideal as many patients will experience complications without deriving any benefit. The PETReA trial will use a new scanning technique called Positron Emission Tomography (PET) to identify which patients are more or less likely to benefit from rituximab maintenance after initial R+chemo treatment. We know that patients whose PET scans return to normal have a low-risk of early relapse, and the trial will therefore investigate if rituximab maintenance can be omitted in this group. In contrast, patients whose PET scans remain abnormal have a high risk of early relapse. The trial will investigate whether this group will benefit from the addition of a drug called lenalidomide to rituximab maintenance. PETReA, which is funded by Cancer Research UK, aims to recruit more than 800 patients from across the UK over a period of 4.5 years and is potentially available for any patient with FL who requires initial R+chemo treatment.