FORWARD Optune and Adjuvant TMZ in Grade II/III Astrocytoma

Inclusion Criteria: * Willing and able to provide written informed consent * Stated willingness to comply with all study procedures and availability for the duration of the study * Life expectancy of at least 3 months * Histologic confirmation of WHO Grade II or III astrocytoma---mixed oligoastrocytomas are permitted * 1p/19q intact per FISH and/or ATRX mutation(s) per immunohistochemistry or next-generation sequencing (e.g. Foundation Medicine, TEMPUS, Caris, or similar CLIA-certified sequencing service) * Mutational identity determined by CLIA-certified sequencing including: 1. IDH1/2 wildtype (i.e. lack of detectable mutations on the sequencing report) and 2. TERT promoter mutation * Karnofsky performance status ≥70% * Maximal safe resection---biopsy alone is allowed * Completed standard chemoradiation with total RT dose of at least 40 Gy and concurrent temozolomide (75mg/m2 daily dose with 80% prescribed dose completed) * Patients with a tumor that was biopsied or resected in the past followed by observation only without definitive chemoradiation and/or chemotherapy given will be eligible, as long as: repeat maximal surgical resection (biopsy only allowed) has been performed, definitive temozolomide/RT treatment meets the criteria above, and adjuvant temozolomide treatment is planned * Candidate for adjuvant high dose temozolomide per investigator's clinical judgement * Adjuvant Temozolomide start date at least 4 weeks, but not more than 6 weeks, from the later of last dose of concomitant temozolomide or radiotherapy * No evidence of early disease progression per RANO criteria at the time of enrollment * Women of childbearing potential (WOCBP) must be using a highly effective method of contraception to avoid pregnancy throughout the study and for at least 24 weeks after the last dose of study drug to minimize the risk of pregnancy. 1. Prior to study enrollment, women of childbearing potential must be advised of the importance of avoiding pregnancy during trial participation and the potential risk factors for an unintentional pregnancy. 2. Refer to section 10.2.1 for guidance on highly effective contraceptive methods acceptable in this study. 3. WOCBP include any woman who has experienced menarche and who has not undergone successful surgical sterilization (hysterectomy, bilateral tubal ligation, or bilateral oophorectomy) or who is not post-menopausal. Post-menopause is defined as: Amenorrhea that has lasted for ≥ 12 consecutive months without another cause, or For women with irregular menstrual periods who are taking hormone replacement therapy (HRT), a documented serum follicle-stimulating hormone (FSH) level of greater than 35 mIU/mL. * Males with female partners of child-bearing potential must agree to use physician-approved contraceptive methods (e.g., abstinence, condoms, vasectomy) throughout the study and should avoid conceiving children for 24 weeks following the last dose of study drug. Exclusion Criteria: * Prior treatment with anti-angiogenic agents including bevacizumab. * Prior treatment with TTFields. * Progressive disease (according to RANO criteria) after temozolomide/RT. * Actively participating in another clinical treatment trial intended to treat the underlying astrocytoma. * Females who are pregnant or breastfeeding. * Significant co-morbidities at baseline (within 2 weeks prior to adjuvant temozolomide start) which would prevent adjuvant temozolomide treatment: 1. Thrombocytopenia (platelet count \< 100 x 103/μL) 2. Neutropenia (absolute neutrophil count \< 1.5 x 103/μL) 3. CTC grade 4 non-hematological toxicity (except for alopecia, nausea, vomiting) 4. Significant liver function impairment - AST or ALT \> 5 times the upper limit of normal 5. Total bilirubin \> 2 times upper limit of normal 6. Significant renal impairment (GFR ≤ 30 ml/min) * Implanted pacemaker, programmable shunts, defibrillator, deep brain stimulator, other implanted electronic devices in the brain, or documented clinically significant arrhythmias. * A skull defect such as missing bone with no replacement * Bullet fragments embedded the skull * Tumors located in the brain stem and/or the cerebellum * History of hypersensitivity reaction to temozolomide, Dacarbazine (DTIC) or hydrogel.