Does FDG-PET/CT Obviate the Need for Standard Diagnostic Contrast-Enhanced CT in Patients with Lymphoma?.

Background and Significance: Positron emission tomography using 18-fluoro-2-deoxyglucose (FDG-PET) is useful in the staging and follow up of patients with lymphoma, and has been shown in several studies to be more accurate than computed tomography (CT). These studies have, however, demonstrated a continued role for CT in staging and restaging of lymphoma, and the two modalities are complementary. Increasingly, FDG-PET is performed in conjunction with a low radiation dose, non-contrast CT scan for attenuation correction and localization of lesions. Currently, many patients undergo both FDG-PET/CT and standard diagnostic, contrast enhanced CT, at a significant cost both financially and in terms of radiation exposure. In this study, we evaluated the clinical utility of performing both studies in patients with lymphoma. Study Design and Methods: We retrospectively identified patients with lymphoma who had undergone both FDG-PET/CT and diagnostic, contrast-enhanced CT (a scan pair) for either staging or restaging following treatment. Patients were included if the two imaging studies were performed within 6 weeks of each other with no intervening anti-lymphoma therapy. We compared the results of the two studies, identifying findings that were detected in either FDG-PET/CT or diagnostic CT scan but not both. Discrepancies were considered clinically significant if they were determined to be related either to lymphoma or another disease process which potentially required intervention. Results: Eighty-nine scan pairs which met the criteria were identified in 75 patients. Sixty-one scan pairs were performed for staging, and 28 were performed for treatment follow up. FDG-PET/CT detected additional potentially clinically relevant lesions over CT in 30 patients, of which 11 demonstrated increased clinical stage. Lymphoma therapy was changed based on FDG-PET/CT findings in 2 patients, and in one patient an occult rectal cancer was detected. In contrast, diagnostic CT detected 5 potentially clinically relevant findings, including 2 incidental findings (one definite and one possible venous thrombosis), and 3 patients with splenic lesions. Of the patients with splenic lesions, one was found on follow up to be definitely not related to lymphoma, and the nature of the splenic lesions in the other two patients remained indeterminate. No patient had a change of stage or lymphoma therapy based on diagnostic CT scan, and one patient was treated with anticoagulation based on CT findings. In the subgroup of scan pairs performed for follow up, diagnostic CT added clinically relevant information in none of the patients. Conclusion: In our series of patients, diagnostic contrast-enhanced CT scan did not contribute to staging or restaging of lymphoma when performed concurrently with FDG-PET/CT. Two clinically important incidental findings were detected by CT alone, of which one led to intervention.