A Dose Finding Study of [177Lu]Lu-DOTA-TATE in Newly Diagnosed Glioblastoma in Combination With Standard of Care and in Recurrent Glioblastoma as a Single Agent.

Key Inclusion Criteria:

Common Criteria:

* Participant is \>= 18 years on the day of signing informed consent form
* Histologically confirmed glioblastoma
* Adequate bone marrow, organ function and electrolyte values

Newly diagnosed glioblastoma (Group 1):

* Presence of Gadolinium enhancing tumor in pre-surgery magnetic resonance imaging (MRI)
* Karnofsky Performance Score (KPS) \>= 70 %

Recurrent glioblastoma (Group 3 dose Escalation only):

• Participant has experienced first or second recurrence of their glioblastoma, after standard or experimental therapy that includes prior EBRT

Recurrent glioblastoma (Group 3 dose escalation and expansion):

* Evidence of recurrent disease demonstrated by disease progression using modified Response Assessment in Neuro-Oncology (mRANO) criteria
* KPS \>= 60 %
* \[68Ga\]Ga-DOTA-TATE uptake by positron emission tomography/computed tomography (PET/CT) or PET/MRI at the tumor region
* Presence of Gadolinium enhancement in the tumor region in MRI at the time of diagnosis of tumor recurrence
* A second surgery for glioblastoma is allowed provided that the following criteria are met:

1. Residual and measurable disease post-surgery is not required but surgery must have confirmed the diagnosis of recurrence
2. Surgery completed at least 2 weeks prior to study treatment initiation, with post-surgery recovery without any complications related to surgical procedure

Recurrent glioblastoma (Group 3 Dose Expansion only):
* Patients experiencing first recurrence of their glioblastoma, after standard or experimental therapy that includes prior EBRT

Key Exclusion Criteria:

Common Criteria:

* Participant is receiving additional, concurrent, active therapy for glioblastoma outside of the trial
* Extensive leptomeningeal disease
* History of another active malignancy in the previous 3 years prior to study entry
* Prior administration of a radiopharmaceutical unless 10 or more effective half-lives have elapsed before injection of \[68Ga\]Ga-DOTA-TATE or \[177Lu\]Lu-DOTA-TATE

Newly diagnosed glioblastoma (Group 1):

• Any prior treatment for glioma of any grade

Recurrent glioblastoma (Group 3 dose escalation and expansion):

* Early disease progression prior to 3 months from the completion of radiotherapy
* Previous treatment with bevacizumab for the treatment of glioblastoma with therapeutic intent, or with bevacizumab as supportive therapy (e.g. edema reduction) within 60 days of initiation of study treatment

Recurrent glioblastoma (Group 3 dose escalation only):

• More than 2 prior lines for systemic therapy

Recurrent glioblastoma (Group 3 dose expansion only):

• More than 1 prior line for systemic therapy