Histologically proven intracranial glioblastoma or gliosarcoma at initial surgery
* Patients will be eligible if the original histology was low-grade glioma and a subsequent diagnosis of glioblastoma or gliosarcoma is made (high-grade transformation)

Karnofsky performance status >= 60

Women must not be pregnant or breast-feeding as gadolinium enhanced magnetic resonance imaging (MRI) is contra-indicated

Progression of disease assessed by local site using Revised Assessment in Neuro-Oncology (RANO) criteria, with plan to administer bevacizumab or its biosimilars, either as single therapy or in conjunction with other chemotherapeutic regimens or immunotherapy regimens, in order to treat tumor progression/recurrence per the treating physician; patients receiving bevacizumab or its biosimilars primarily for reduction of edema (i.e. alleviation of symptoms) rather than for tumor treatment are excluded; if the patient’s most recent recurrence occurs while on immunotherapy, this must be judged as true recurrence using immunotherapy (i)RANO criteria

Patients who have not previously received a bevacizumab-containing regimen or its biosimilars (i.e., this must be the first bevacizumab-containing therapy or its biosimilars administered to the patient)

For patients with intratumoral hemorrhage (acute, subacute, or chronic) seen on hemosiderin-sensitive (gradient-echo) MRI, there must be at least 10 x 10 x 10 mm "measurable enhancement" that is not obscured or distorted by magnetic susceptibility blooming artifact

Progressive enhancement (> 25% increase in contrast enhancing volume compared to nadir or a new measurable lesion) on MRI performed within 28 days of registration, and >= 42 days since completion of standard radiation/temozolomide therapy; adjuvant temozolimide is allowable; measurable enhancement is defined as two perpendicular in-plane diameters of at least 10 mm, and at least 10 mm in the 3rd orthogonal direction
* NOTE: If the MRI performed to identify progressive enhancement meets the dynamic susceptibility contrast (DSC)-MRI qualifications (including second dose of Gadavist) for this study and is done on a qualified scanner then this MRI may be used as the baseline (S0) DSC-MRI; bevacizumab or its biosimilars still needs to starts within 3 days of S0 scan and patient must be registered before receiving bevacizumab or biosimilar

Patient must be cleared for administration of bevacizumab or its biosimilars with respect to any recent surgeries, and post-surgical scans must confirm the presence of measurable residual disease

Patients must be able to tolerate brain MRI scans with Gadavist/Gadovist (gadabutrol)
* Ability to withstand 22-gauge intravenous (IV) placement
* No history of untreatable claustrophobia
* No magnetic resonance (MR) incompatible implants/devices or metallic foreign bodies
* No contraindication to intravenous contrast administration
** Adequate organ function, including adequate renal function defined as estimated glomerular filtration rate (eGFR) >= 40 mL/min/1.73 m^2 as calculated per institution standard of care, and meeting local site requirements for intravenous administration of gadolinium-based MRI contrast agents
* No known allergy-like reaction to gadolinium or moderate or severe allergic reactions to one or more allergens as defined by the American College of Radiology (ACR); patient may be eligible if willing to undergo pre-treatment as defined by the institution's policy and/or ACR guidance
* Weight compatible with limits imposed by the MRI scanner table

Patient must be scheduled to receive a treatment regimen containing bevacizumab or its biosimilars; patient can be treated with bevacizumab or its biosimilars alone or in combination with other chemotherapies or immunotherapies; patient may receive treatment with Optune
* NOTE: Bevacizumab or its biosimilar should be given between 5 and 10 mg/kg IV every 2-3 weeks

PRIMARY OBJECTIVE:
I. To determine whether binary changes (increase versus [vs.] decrease) in normalized rCBV within enhancing tumor from baseline scan (S0) to post-dose 1 treatment scan (S1) is associated with overall survival (OS) in recurrent glioblastoma (GBM) patients receiving bevacizumab or its biosimilars for the first time.

SECONDARY OBJECTIVES:
I. To determine whether the baseline S0 normalized rCBV measure alone is associated with OS.
II. To determine whether change in normalized rCBV within enhancing tumor as a continuous variable is associated with OS.
III. To determine whether change in normalized rCBV within enhancing tumor as a continuous variable is associated with OS when adjusting for change in enhancing tumor volume and other potential confounders.
IV. To determine whether baseline cerebral blood flow (CBF) or changes in CBF are associated with OS.
V. To determine the association between standardized rCBV and OS.
VI. To determine whether baseline normalized rCBV or change in normalized rCBV are associated with progression-free survival (PFS).
VII. To determine whether baseline CBF or change in CBF are associated with PFS.
VIII. To determine the association between standardized rCBV and PFS.

EXPLORATORY OBJECTIVE:
I. To measure the repeatability of normalized rCBV and standardized rCBV at baseline (pre-bevacizumab or biosimilar).

OUTLINE:
Patients undergo DSC-MRI within 3 days before bevacizumab initiation and in 12-25 days before their second bevacizumab infusion is given.

After completion of study intervention, patients are followed up every 3 months for 5 years.