FDG-PET versus Thallium-201 SPECT in the evaluation of putative CNS lymphoma in AIDS patients

Abstract

1537 Background: The evaluation of CNS lesions in AIDS patients is challenging as both CNS lymphoma and non-neoplastic lesions can have similar clinical presentations and imaging findings. Both Tl-201 SPECT and FDG PET imaging have been used to differentiate malignancy from infection. This study investigates the accuracy of FDG PET compared to Tl-201 SPECT in the diagnosis of CNS lymphoma in patients with AIDS-associated CNS lesions. Methods: Nine patients with AIDS who were found to have rim enhancing brain lesions on MRI underwent both Tl-201 SPECT and FDG PET imaging prior to diagnosis. Diagnosis was made by stereotactic brain biopsy in 6 pts and clinical follow-up in the remaining 3. FDG PET of the brain was performed using a dedicated PET/CT instrument (GE Discovery) 1 hour after 444 MBq FDG. Brain SPECT was performed on a dual head gamma camera (GE Hawkeye) with 185 MBq Tl-201. Images were reviewed along with MRI for anatomic correlation by 2 experienced nuclear medicine physicians blinded to the diagnosis. Results: 43 MRI lesions in 9 patients were evaluated. Clinically, 5 pts (18 lesions) were diagnosed with lymphoma (4 biopsy proven, 1 with known history of lymphoma), and 4 pts (25 lesions) were diagnosed with toxoplasmosis (2 with negative biopsies for malignancy, 2 with clinical course consistent with toxoplasmosis). FDG PET had true positive results for lymphoma in 5 of 5 pts, while Tl-201 SPECT was true positive in only 2 of 5. FDG PET had true negative results in 4 of 4 pts with toxoplasmosis, while Tl-201 had false positive results in 2 of 4. FDG PET had positive and negative predictive values of 100%, while Tl-201 SPECT had PPV 50% and NPV 40%. On a lesion basis, PET showed increased uptake of FDG in 8 of 18 brain lesions in patients with lymphoma, while SPECT showed increased uptake of Tl-201 in 4 of 18 lesions. Tl-201 was positive in 2 lesions in patients diagnosed with toxoplasmosis, while FDG had no false positive lesions. Conclusion: In this limited study, FDG PET proved to be superior to Tl-201 SPECT in the differentiation of CNS lymphoma from toxoplasmosis (overall accuracy 100% vs 44%, respectively). With the increasing availability of FDG PET, this test should probably supplant Tl-201 SPECT as the study of choice following conventional imaging (CT, MRI) of CNS lesions in AIDS patients. No significant financial relationships to disclose.