Thallium-201 brain SPECT of lymphoma in AIDS patients: pitfalls and technique optimization

Our aim was to examine the 201Tl-SPECT scans in AIDS patients with focal CNS lesions to identify those studies with a false-positive or false-negative result to determine any potential pitfalls in interpretation as well as to suggest methods for technique optimization.

We retrospectively reviewed the charts of 162 AIDS patients with cerebral mass lesions on 201Tl-SPECT studies. One hundred sixty-one patients had CT examinations, of which 50 also had MR studies. One patient had MR imaging without CT. Those patients in whom the diagnosis by 201Tl-SPECT did not correspond with the known pathologic or clinically proved diagnosis were then singled out and their CT, MR, and 201Tl-SPECT studies were reviewed, including blinded interpretation of the 201Tl-SPECT scans alone and alongside the corresponding CT and MR examinations. Studies were examined for lesion morphology, size, location, enhancement pattern, and presence of necrosis. The review of the 201Tl-SPECT studies included both a qualitative approach (subjective analysis of the scans for areas of abnormally increased uptake) and a quantitative approach (comparison of lesion activity versus activity within a reference standard, such as the scalp).

Sensitivity and specificity of 201Tl-SPECT in depicting lymphoma were 100% and 93%, respectively, based on the initial qualitative analysis. Fifty-one patients had positive 201Tl-SPECT results, of whom 43 were determined to have lymphoma (four by biopsy/autopsy, 39 by clinical and radiologic findings). Upon reevaluation with both a quantitative and qualitative approach, those studies initially interpreted as positive in patients without lymphoma (false positives) were found to be negative.

Brain 201Tl-SPECT is an effective study in the diagnosis of CNS lymphoma in AIDS patients. Specificity can be increased by routinely performing a quantitative analysis of all lesions.