The investigational part of this study is using a mobile PET/CT scanner to take images of the participants tumor immediately after they are treated with proton radiation. This allows the participant to be treated and imaged on the same bed. The information obtained may improve the accuracy of treatment and may help to minimize the dose delivered unnecessarily to healthy tissue.

For all participants, right after one regular treatment fraction, NeuroPET or NeuroPET II will be wheeled into the patient room. The patient bed will be moved directly into the scan position and a PET scan will be acquired in list-mode for 15 to 20 minutes followed by a CT scan for ~5 minutes. After undergoing their first PET scan, participants will have the option to undergo 1-2 additional scans throughout radiation treatment in the same manner as the first scan.

Inclusion Criteria:

• Adult subjects with brain, head and neck, and skull base tumors receiving external beam proton radiotherapy at the Francis H. Burr Proton Therapy Center at MGH
• Age 18 or over
• ECOG Performance status 0, 1 or 2

Exclusion Criteria:

• Uncontrolled intercurrent illness that, in the opinion of the investigator, would limit the participant's compliance with study requirements.