Optimal radiation therapy of these lymphoma patients includes radiotherapy planning using pre-chemotherapy PET-scanning, fused with a post-chemo planning CT-scan. This makes accurate treatment of pre-chemo involved volume possible and also accurate radiation of the post-chemo involved volume possible. Thus, the optimal breathing adapted radiation requires planning using a breathing adapted pre-chemo PET-CT scan fused with a breathing adapted post-chemo planning CT scan. In this study, 20 consecutive patients with mediastinal Hodgkin lymphoma and aggressive non-Hodgkin lymphomas will have combination therapy with chemotherapy and radiation therapy to involved nodes. Two radiation therapy plans will be made for each patient: one using a breathing adapted pre-chemo PET-CT scan fused with a post-chemo planning CT scan, and one using the standard procedure of a pre-chemo PET-CT scan fused with post-chemo planning CT scan. Dose levels to the lymphoma and to the organs of risk will be compared in each patient. The best radiation therapy plan will be used to treat the patient.