Interim positron emission tomography scan in Hodgkin lymphoma: definitions, interpretation rules, and clinical validation.

[18F]-Fluoro-2-deoxy-d-glucose positron emission tomography (18F-FDG PET) scan performed early during the therapy for Hodgkin lymphoma (HL) has a high prognostic value and correlates with survival. Several ongoing trials are under way to investigate the value of treatment adaptation based on early 18F-FDG PET results both in early and advanced-stage HL. In the former, in order to reduce the number of patients in whom involved-field radiotherapy is required for optimal treatment, and in the latter to identify the small subset (about 20%) of patients with a dismal prognosis who should be treated very aggressively while sparing the toxic effects of the therapy in most of them. However some issues still remain unsettled, including the standardization of interpretation rules for the interim-positron emission tomography (PET) scan, the optimal time to perform this examination during therapy, and the impact of a risk-adapted therapeutic strategy on the overall outcome of therapy in HL. International efforts are now underway to reach a consensus among experts on a set of simple, reproducible rules for PET interpretation, and attempts are being made to launch retrospective clinical studies so as to validate these rules.