PET and restaging of malignant lymphoma including residual masses and relapse

Differentiation of post-therapeutic scar tissue from active lymphoma is unsatisfactory when using only morphological imaging approaches. Positron emission tomography (PET) with fluorine-18 fluorodeoxyglucose (FDG) may provide superior clinical information by enabling biochemical tissue characterisation, with enhanced FDG uptake in viable post-therapeutic lymphoma masses and very low uptake in indolent fibrotic tissue. With this in mind, 15 recently published studies reporting the results of differentiation of viable lymphoma from scar tissue in 723 patients were analysed. Sensitivity of FDG-PET for detection of active disease was 71-100%, and the specificity was 69-100%. Accordingly, FDG-PET had a high negative predictive value of 80-100%. In contrast, the specificity and positive predictive value (PPV) of computed tomography were low (4-31% and 19-60%, respectively, except in one study in which the PPV was 82%). Residual masses that were positive on PET were associated with a progression-free survival of 0-40%. Although not perfect, the biochemical approach for characterisation of residual masses in lymphoma with FDG-PET at present seems the most accurate way to differentiate scar tissue from viable residual lymphoma.