PET was more accurate than CT for diagnosing both nodal and extranodal disease. PET resulted in changes in staging in 8% of patients examined for nodal disease and in 16% of patients examined for extranodal disease. PET is more likely to detect otherwise unsuspected organ involvement in the bone marrow than in the liver, spleen, or the digestive tract. The clearest role for PET is in restaging patients following completion of therapy. Restaging PET scanning is performed either for a final response assessment, typically within 6 to 8 weeks of therapy conclusion, or to determine the extent of known or suspected recurrence anytime after therapy. PET is more accurate than CT in this setting, largely related to its superiority in distinguishing between viable tumor and necrosis or fibrosis in residual mass (es).