Brain tumours often have low oxygen levels, and that makes them more resistant to radiation therapy. If patients breathe the right mixture of oxygen during treatment, radiation may work better. In this study, patients with brain tumour will undergo a special MRI test while they breathe different mixtures of oxygen and carbon dioxide to find out whether oxygen levels improve in the tumor. Patients will also be asked to repeat this MRI test during the second week of radiation therapy, as well as 3 months, 6 months and 1 year after RT. The MRI test after RT will help us understand how the blood vessels in the normal brain are affected by radiation.

Inclusion Criteria:

• Patients with primary gliomas visible on MRI who are planning to receive radiotherapy or
• Patients with brain metastases>1cm who are planning to receive radiotherapy
• Karnofsky Performance Status >60
• Age 18 years
• Patients must be willing to visit Toronto Western Hospital for at least one MRI scan prior radiotherapy

Exclusion Criteria:

• Seizures not controlled with medications, or non compliance with prescribed anti-seizure medication
• Prior radiation therapy to the brain
• Pregnancy
• Unwilling or unable to co-operate with breathing maneuvers
• Respiratory or cardiac limitations to breathing at 20 L/min
• Medical contra-indications to limited hypercapnia or hypocapnia (known increased intracerebral pressure, metabolic acidosis or alkalosis)
• Contraindication to MRI (patients weighing>136 kgs-weight limit for the scanner tables; allergy to MR contrast agent; patients with pacemakers, cerebral aneurysm clips, shrapnel injury or implantable electronic devices not compatible with MRI)
• Other medical conditions deemed by the PI or associates to make the patients ineligible for protocol procedures