Inclusion Criteria:

          -  Histologically WHO grade II (diffuse) or III (anaplastic) astrocytoma, IDHmt without
             1p/19q co-deletion (local diagnosis)

          -  Time since diagnostic surgery or first resection ≤ 6 months

          -  No need for immediate radiotherapy followed by chemotherapy

          -  Having seizures only, without functional deficits due to the tumor (but the presence
             of functional deficits due to the resection is allowed)

          -  Patients for whom by local judgment an active surveillance policy is a realistic
             management alternative

          -  The patient is at least 18 years of age on day of signing informed consent

          -  WHO PS 0-2

          -  Adequate hematological, renal, and hepatic function, as follows:

               -  Absolute neutrophil count ≥ 1.5 x 10*9/L

               -  Platelets ≥ 100 × 10*9/L

               -  Serum creatinine ≤ 1.5 times upper limit of laboratory normal (ULN)

               -  Total serum bilirubin ≤ 1.5 × ULN

               -  AST and ALT ≤ 2.5 × ULN

               -  Alkaline phosphatase of ≤ 2.5 × ULN

          -  Presence of at least one paraffin block from the initial diagnosis for pathology
             review and translational research. If a representative formalin-fixed,
             paraffin-embedded (FFPE) block is not available, the collection of optimally 36,
             minimally 24 x 5 µm, unstained slides is required.

          -  At the time of randomization presence only of a non-enhancing tumor on T1 weighted
             contrast enhanced MR images; some faint non-nodular enhancement or enhancement that
             can be ascribed to the surgical resection or peri-operative ischemia is allowed.
             Preoperative enhancement is allowed provided this area is resected as shown on
             postoperative imaging

          -  Ability to take oral medication

          -  Women of child bearing potential (WOCBP) must have a negative serum or urine pregnancy
             test done within 72 hours prior to randomization

          -  Patients of childbearing / reproductive potential must agree to use adequate birth
             control measures, as defined by the investigator, during RT and TMZ treatment and for
             at least 6 months after the last TMZ cycle. A highly effective method of birth control
             is defined as those which result in low failure rate (i.e., less than 1 percent per
             year) when used consistently and correctly

          -  Women who are breast feeding must agree to discontinue nursing prior to the first dose
             of study treatment and until 6 months after the last study treatment

          -  Male patients should be advised not to father a child and not to donate sperm up to 6
             months after receiving the last dose of TMZ, and to seek advice on cryoconservation of
             sperm prior to treatment start

          -  Ability to understand the requirements of the study, provide written informed consent
             and authorization of use and disclosure of protected health information, and agree to
             abide by the study restrictions and return for the required assessments

          -  Before patient registration/randomization, written informed consent must be given
             according to ICH/GCP, and national/local regulations

        Exclusion Criteria:

          -  Presence of signs of increased intracranial pressure after surgery

          -  Requirement of steroids for control of tumor symptoms

          -  Presence of uncontrolled seizures after surgery, defined as having both:

               -  persistent seizures interfering with everyday life activities AND

               -  failed three lines of anti-epileptic drug regimen, including at least one
                  combination regimen

          -  Presence of contra-indications for radiotherapy

          -  Hypersensitivity to dacarbazine (DTIC), to the active substance or to any of the
             excipients used for TMZ capsules

          -  Prior chemotherapy, or prior radiotherapy to the brain

          -  Pregnancy or breastfeeding

          -  Known HIV, chronic hepatitis B, or hepatitis C infection

          -  Inability to take oral medication (e.g., frequent vomiting, partial bowel obstruction)

          -  Concurrent severe or uncontrolled medical disease (e.g., active systemic infection,
             diabetes, hypertension, coronary artery disease, psychiatric disorder) that, in the
             opinion of the investigator, would compromise the safety of the patient or compromise
             the ability of the patient to complete the study

          -  Prior or second invasive malignancy, except non-melanoma skin cancer, completely
             resected cervical or prostate cancer (with PSA of less than or equal to 0.1 ng/mL).
             Other cancers for which the subject has completed potentially curative treatment more
             than 3 years prior to study entry are allowed

          -  Presence of any psychological, familial, sociological or geographical condition
             potentially hampering compliance with the study protocol and follow-up schedule; those
             conditions should be discussed with the patient before registration in the trial