Imaging assessment FDG-avid lymphomas. Is Diagnostic CT a necessary adjunct to PET-CT?

Aim: Despite recent local introduction of evidence-based guidelines recommending PET-CT (low dose unenhanced CT) for staging and assessment of treatment response of predictably FDG-avid lymphoma sub-types, in our institution diagnostic CT with intravenous contrast enhancement is still often used in addition to PET-CT at the same time in the course of disease. This study evaluates whether diagnostic CT yields additional diagnostic information for lymphoma patients when performed at same time as PET-CT. Materials & Methods: We carried out a retrospective review of all lymphoma patients referred to our centre for PET-CT imaging over a six month period (February 2008 - July 2008). We determined whether these patients had undergone diagnostic CT at the same time. PET-CT images were classified into ‘staging’ scans and ‘post-treatment’ scans and PET-CT reports were compared with those of diagnostic CT to determine whether there were any differences in diagnostic yield between the two scans. Results: For the 98 PET-CT examinations identified, diagnostic CT had been carried out at the same time in 47 (48%). 22 of these examination pairs (PET-CT vs. diagnostic CT) were for staging, whilst 25 were post-treatment. For staging, there was no difference in diagnostic yield in 10/22 (45%), other than one (5%) incidental finding on PET-CT (uterine fibroids). PET-CT upstaged diagnostic CT in 10/22 (45%) cases and increased confidence in reporting splenic disease on 1/22 occasion (5%). Diagnostic CT upstaged PET-CT on no single occasion. Post-treatment, there was no difference in diagnostic yield in14/25 cases (56%) other than 3 (12%) incidental findings on CT (initial SVC compression, uterine fibroids and interstitial fibrosis). In 4/25 (16%) cases PET revealed evidence of active lymphoma missed by CT whereas; in a further 3/25 (12%) cases PET confirmed no active disease in diagnostic CT residual masses. In one case (4%) of peripheral T-cell Non Hodgkin’s Lymphoma, diagnostic CT demonstrated a possible new non-FDG avid intra-thoracic site of disease which was also seen on the CT component of the PET-CT. No incidental finding (8%) was considered likely to impact on lymphoma management. Conclusion: Our results support the view that FDG-PET-CT obviates the need for additional diagnostic CT in FDG-avid lymphoma. Diagnostic CT may have a remaining role in assessment of lymphoma sub-types that are not predictably FDG avid.