In this study, radiation therapy plans will be made for 20 consecutive patients with mediastinal Hodgkin lymphoma or aggressive non-Hodgkin lymphoma. One plan is done using a breathing adapted pre-chemo PET-CT scan fused with a post-chemo planning CT scan and another plan is done using the standard procedure with pre-chemo PET-CT 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.