Inclusion Criteria:

          -  Provision of signed and dated informed consent form.

          -  Stated willingness to comply with all study procedures and availability for the
             duration of the study.

          -  Ability to take oral medication and willing to adhere to the RADFORMIN-regimen.

          -  Male or female, ≥ 18 years of age.

          -  Histological or cytological proven stage III NSCLC after adequate staging with at
             least 18F-2-fluoro-2-deoxy-D-glucose (FDG) PET/CT, contrast enhanced CT-thorax and
             contrast-enhanced CT/MRI brain. (According to 8th edition of TNM classification (=
             tumor, node, metastasis))

          -  Absence of diabetes, (diabetes is defined as fasting plasma glucose >126 mg/dL or
             random plasma glucose >200 mg/dL).

          -  Eastern Cooperative Oncology Group (ECOG) performance score (= World Health
             Organization (WHO) score) of 0-1.

          -  Adequate hematologic, hepatic and renal function as clinically acceptable in the
             opinion of the Primary Investigator.

          -  Adequate pulmonary function in order to be administered definitive radiotherapy. With
             Forced Expiratory Volume (FEV) > 1.2 litres per second or more than 50% of predicted,
             and diffusion capacity of lung for carbon monoxide (DLCO) > 40% predicted. (Values
             without administration of medical bronchodilation. In case of Tiffeneau < 70%
             bronchodilation will be administered)

          -  Having received at least 2 cycles of platinum-based chemotherapy. This according to
             institutional standards and without progression (on a restaging CT-scan within 3 weeks
             after day 1 of the last given cycle, according to RECIST criteria).

        Exclusion Criteria:

          -  Current use of metformin, insulin or other oral antidiabetic drugs
             (thiazolidinediones, sulfonylureas, mitiglinides, alpha-glucosidase inhibitors,
             incretin mimetics, dipeptidyl peptidase-4 inhibitors, amylin analogues,
             sodium-glucose-cotransporter-2 (SGLT-2)-inhibitors) for any reason.

          -  Evidence for metastatic disease.

          -  Conditions associated with increased risk of metformin-associated lactic acidosis: New
             York Heart Association class III or IV congestive heart failure, history of acidosis
             of any type, known kidney injury or disease, alcoholic liver disease or habitual
             intake of 3 or more alcoholic beverages per day.

          -  Known pregnancy or lactating female patients.

          -  Known allergic reactions to components of metformin.

          -  Prior invasive malignancy within the past year (in remission, without evidence for
             current active disease and without maintenance therapy). Except non-melanomatous skin
             cancer, non-invasive carcinoma in-situ of the breast, oral cavity or cervix.

          -  Known acquired immune deficiency syndrome.