To few participants

Rationale: This project elaborates on a novel finding of the investigators that has not yet been reported in literature, namely the presence of elevated levels of atypical B cells in participants with glioblastoma. ln the period 2015 2018 the investigators analysed the blood immune subset composition of a cohort of 180 participants undergoing neurosurgery. The most relevant finding was the presence of an abnormally elevated level of B cells in the blood of the great majority of participants with glioblastoma. These B cells may be involved in the immunosuppression associated with glioblastoma that makes this tumor refractory to immunotherapy. Multiple regression analysis indicated that the increase in the frequency of atypical B cells in participants' peripheral blood was related with the administration of dexamethasone prior to surgery. However, this study design did not allow the investigators to address the causality of the relationship between dexamethasone and atypic B cell dysregulation. Alternative treatments to dexamethasone exist.

Objective: To investigate the effect of dexamethasone in the dysregulation of atypic B cells in participants with glioblastoma.

Study design: Observational case control pilot study with 20 participants (10 per group).

Study population: Newly diagnosed participants with glioblastoma. Intervention (if applicable): Observational study. Main study parameters/endpoints: Changes in the immune subset composition and functionality in the peripheral blood of participants with glioblastoma upon administration of dexamethasone for neurological signs of peritumoral edema (oral dexamethasone).

Nature and extent of the burden and risks associated with participation, benefit and group relatedness: The investigators will collect blood (28 ml) during the first visit and again (28 ml) at the time of surgery (2 weeks ± 3 days). There will not be additional site visits, physical examinations or any other tests, questionnaires. Blood collection is only a minor discomfort and it does not represent any additional risk.

Inclusion Criteria:

Clinical diagnosis Glioblastoma Patients are 18 years or older at first diagnosis

Exclusion Criteria:

No indication for surgery to confirm radiological diagnosis Not able or willing to give informed consent Allergy or intolerance to dexamethasone

Patients with clinical diagnosis Gliobastoma

Data will be stored in coded form in a Castor database. At this moment data will not be available to other researchers.