Inclusion Criteria:

          -  Histologically proven diagnosis of newly diagnosed supratentorial high-grade
             astrocytoma (WHO grade III-IV), excluding astrocytoma of brainstem and cerebellum.
             Gliosarcoma or other subvariants are allowed, including the newly defined "diffuse
             astrocytoma, IDH-wildtype, with molecular features of glioblastoma, WHO grade IV"
             (Brat et al., 2018).

          -  Must have recovered from the effects of surgery, postoperative infection, and other
             complications.

             -≥ 18 years of age.

          -  Eligible for and planning to receive standard fractionated RT of 60 Gy with concurrent
             TMZ.

          -  Karnofsky performance status ≥ 60.

          -  Available archival formalin-fixed paraffin-embedded (FFPE) tumor blocks.

          -  Adequate organ and bone marrow function as defined below:

               -  Absolute neutrophil count (ANC) ≥ 1,500 cells/mm3;

               -  Platelets ≥ 100,000 cells/mm3;

               -  Hemoglobin > 9.0 g/dL (Note: the use of transfusion or other intervention to
                  achieve Hgb >9.0 g/dL is acceptable);

               -  Total bilirubin ≤ 1.5 upper limit of normal (ULN)

               -  AST (SGOT) and ALT (SGPT) ≤ 3 x ULN

               -  Creatinine ≤ 1.5 ULN or creatinine clearance ≥ 60 mL/min

               -  If there is history of human immunodeficiency virus (HIV) infection, patients
                  must be on effective antiretroviral therapy, and HIV viral load must be
                  undetectable within 6 months of study enrollment.

               -  If there is history of chronic hepatitis B virus (HBV) infection, patients must
                  have either been treated or are on suppressive therapy (as indicated), and HBV
                  viral load must be undetectable.

               -  If there is history of hepatitis C virus (HCV) infection, patients must have been
                  treated, and HCV viral load must be undetectable.

          -  Females of childbearing potential (defined as a female who is non-menopausal or
             surgically sterilized) must be willing to use an acceptable method of birth control
             (i.e., hormonal contraceptive, intra-uterine device, diaphragm with spermicide, condom
             with spermicide, or abstinence) for the duration of the study. Should a woman become
             pregnant or suspect she is pregnant while participating in this study, she must inform
             her treating physician immediately.

          -  Able to understand and willing to sign an IRB-approved written informed consent
             document (legally authorized representative permitted).

        Exclusion Criteria:

          -  Prior cranial RT or RT to the head and neck where potential field overlap may exist

          -  Gliomatosis, leptomeningeal, or metastatic involvement.

          -  High-grade glioma with known IDH mutation. IDH status could be determined by either
             immunohistochemistry (IDH1-R132H mutation) or sequencing (including other uncommon
             variants of IDH1 and IDH2 mutations) as evaluated routinely for clinical diagnosis
             using a CLIA-approved assay.

          -  Known severe hypersensitivity to tadalafil or other PDE5 inhibitors, including history
             of hypotension, priapism (painful erection > 4 hours duration), blindness, or hearing
             loss during prior treatment with tadalafil or other PDE5 inhibitors.

          -  Concurrent nitrate, alpha-blocker, guanylate cyclase stimulators (eg, riociguat), or
             cytochrome P-450 3A4 (CYP3A4) inhibitor use. CYP3A4 inhibitors include ketoconazole,
             itraconazole, and ritonavir.

          -  Severe, active co-morbidity, defined as follows:

               -  Unstable angina, angina requiring treatment with nitrates, positive cardiac
                  stress test without evidence of subsequent effective cardiac intervention within
                  90 days of planned tadalafil administration

               -  Myocardial infarction, coronary artery bypass graft surgery, or percutaneous
                  coronary angioplasty or stent within the 90 days of planned tadalafil
                  administration

               -  New York Heart Association grade II or greater congestive heart failure within 6
                  months

               -  Serious and inadequately controlled arrhythmia

               -  Hypotension (<90/50 mm Hg) or uncontrolled hypertension (>170/100 mm Hg)

               -  Left ventricular outflow obstructions, such as aortic stenosis

               -  Stroke within the last 6 months

               -  Acute bacterial or fungal infection requiring intravenous antibiotics.

               -  Hepatic insufficiency resulting in clinical jaundice and/or coagulation defects;
                  note, however, that laboratory tests for coagulation parameters are not required
                  for entry into this protocol.

               -  Active peptic ulcer disease.

               -  End-stage renal disease (ie, on dialysis or dialysis has been recommended).

          -  Unilateral blindness, hereditary retinal disorder, including retinitis pigmentosa.

          -  Patients treated on any other therapeutic clinical protocols within 30 days prior to
             registration.

          -  Inability to undergo contrast-enhanced MRI (e.g., due to safety reasons, such as
             presence of a pacemaker, or severe claustrophobia).

          -  Pregnant or breastfeeding. Women of childbearing potential must have a negative
             pregnancy test within 14 days of study entry.

          -  Patients with psychiatric illness/social situations, including alcohol or drug abuse
             that in the investigator's opinion will prevent administration or completion of
             protocol therapy.