This study examines the role of a diagnostic test called Positron Emission Tomography (PET) scanning in patients with malignant lymphoma. The primary goal of this study is to find out how well PET scanning can detect malignant (cancerous) lymphoma, and how often this extra information will result in a change of stage of disease, or will result in a change in treatment management plans of patients with lymphoma.

It is currently unknown whether modifying treatment based on FDG-PET results is appropriate. However, clinicians are increasingly using the FDG-PET result, where obtained, in deciding management. Consequently, it would be premature to mandate PET-based treatment modifications in this protocol. Patients will be treated with existing protocols at the discretion of their treating oncologists, based on available information. Oncologists will be surveyed as to whether FDG-PET scan results could have (or affect patient management, and the intervention will be recorded).

Group A: Newly Diagnosed

Inclusion Criteria:

• Patients > 18 years of age.
• Confirmed diagnosis of HL or NHL (any histology).
• Any Ann Arbor stage.
• Pre-treatment staging including CT of head and neck, chest, abdomen and pelvis, where there is a minimum one equivocal finding on the scan, affecting the assignment of stage or IPI factor (i.e. stage III/IV vs. stage I/II, or a question of extranodal involvement by disease).

Group B: Response Assessment

Inclusion Criteria:

• Patients > 18 years of age.
• Confirmed diagnosis of HL or NHL (any histology).
• Any Ann Arbor stage.
• Pre-treatment staging including CT neck, chest, abdomen and pelvis. If alternate assessment of the neck done by clinical palpitation, ultrasound, or MRI is negative, CT neck is not mandatory.
• Post-treatment staging including CT of head and neck, chest, abdomen and pelvis, at 1 - 6 weeks post-therapy.
• Treated with anthracycline-based chemotherapy, with or without radiation therapy (based on the stage of the disease). For Stage III/IV patients, PET will be done following completion of primary chemotherapy. For stage I/II patients, PET will be done after combined modality therapy, or after chemotherapy alone if this was the primary intention of therapy.
• Patients with residual mass on CT following primary chemotherapy with either "unconfirmed" CR (>75% decrease in size) or PR (³50% decrease in size), based on International workshop criteria.

Exclusion Criteria: Response Assessment

• None Curative treatment Intent
• After initial therapy the response status is: complete response, stable disease, or progressive disease