In patients with cancer of the head and neck and rectum, knowing the exact location of the tumor is important for designing the radiation field to ensure delivery of high dose of radiation to the tumor while sparing surrounding normal tissues. A new medical imaging method which is a combination of positron emission tomography (PET) and computed tomography (CT) scan, has shown promise in helping the radiation oncologist in defining the exact location and extent of the tumor in certain cancers such as lung cancers. Therefore the purpose of this study is to determine if these imaging methods can be used in combination with the standard radiation treatment planning procedure to improve the accuracy to targeting your tumor with radiation. In addition the PET-CT scan, similar to the PET scan alone with better resolution, can be used to determine whether the tumor has spread to any part of the body outside of the head and neck sites.

Inclusion Criteria:Patients who are scheduled to receive radiation therapy for head and neck cancer at Stanford.

Exclusion Criteria:Non-Stanford patients.

Patients who are scheduled to receive radiation therapy for head and neck cancer at Stanford.