18-FDG-PET for the assessment of residual masses on CT following treatment of lymphomas

Background: The problem of residual masses on post-treatment CT scans is a continuing dilemma for the oncologist treating malignant lymphomas. These masses may contain active disease or represent only necrotic tumour which continues to shrink without further treatment or post-treatment fibrosis which remains stable on continued follow-up. 18-FDG-PET offers a novel metabolic imaging modality, which can differentiate malignant from benign tissue on the basis of increased glycolytic activity. Patients and methods: Thirty-two patients (15 with Hodgkin's disease (HD) and 17 with aggressive histology non-Hodgkin's lymphoma (NHL)) who had residual masses on their posttreatment CT scans underwent 18-FDG-PET. The post-treatment CT and PET scans were compared and the accuracy of the 18-FDG-PET in assessing residual masses was evaluated using clinical and pathological follow-up data. Results: Nine patients had positive post-treatment 18-FDG-PET, eight (89%) of whom have relapsed. Twenty-three patients had negative post-treatment PET with only two relapses in this group. The 2 patients who relapsed had aggressive NHL while none of the 11 HD patients with negative PET relapsed. The median follow-up of patients in continued complete remission is 38 months. Conclusions: 18-FDG-PET can differentiate between residual masses containing viable lymphoma where further treatment will be required to achieve cure and those representing ablated disease, where unnecessary treatment and additional morbidity may be avoided.