Trial to Evaluate Efficacy and Safety of Subcutaneous Mosunetuzumab in Previously Untreated Low Tumor Burden Follicular Lymphoma .

Inclusion Criteria: * at least 18 years old * Histologically confirmed classic FL (cFL) (according to WHO-HEAM4R classification) * Low tumor burden by GELF criteria * No prior therapy except surgery or radiotherapy for disease that was previously localized * Ann Arbor Stage III or IV disease * Bi-dimensionally measurable FDG-avid disease defined by at least one single node or tumor lesion \> 1.5 cm assessed by CT scan and/or clinical examination * Adequate hematologic function defined as follows without growth factors or blood product transfusion within 14 days of first dose of study drug administration: 1. Hemoglobin, without transfusion, 9 g/dL 2. ANC 1.0 109/L 3. Platelet count 75 109/L; * Eastern Cooperative Oncology Group (ECOG) performance status 0 to 2 * Patient can understand and sign the Informed Consent Form (ICF), can communicate with the Investigator, can understand and comply with the requirements of the protocol Exclusion Criteria: * 1\. An active viral infection with hepatitis B virus (HBV) or hepatitis C virus (HCV) defined by detectable viral DNA in the blood by PCR. Patients with HIV are eligible provided an undetectable viral load and a CD4 count \> 200 cell/mcl * 2\. Any of the following laboratory abnormalities: 1. Total Bilirubin or GGT or AST or ALT \> 3 X ULN. 2. Creatinine Clearance calculated by Cockcroft and Gault Formula \< 40 ml/min * Presence or history of CNS involvement by lymphoma * 4\. Prior history of malignancies other than Lymphoma (except for Basal Cell or Squamous Cell Carcinoma of the skin or carcinoma in situ of the cervix or breast) unless the subject has been free of the disease for ≥ 2 years * Contraindication to use Mosunetuzumabor known sensitivity or allergy * Pregnant or lactating females * 7\. Female patients of childbearing potential who cannot or do not wish to use an effective method of contraception, during the study treatment and for 3 months thereafter