Lesion Size Determines Accuracy of Thallium-201 Brain Single-Photon Emission Tomography in Differentiating between Intracranial Malignancy and Infection in AIDS Patients
Robert J. Younga,
Munir V. Ghesania,
Nolan J. Kagetsua and
Andrew J. DeRogatisb
a St. Luke’s-Roosevelt Hospital Center, University Hospital of Columbia, University College of Physicians and Surgeons, New York, NY
b White Plains Hospital, White Plains, NY
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FIG 1. Primary brain lymphoma.
A, Coronal T1-weighted image with contrast shows ring-enhancing mass lesion in right temporal lobe with surrounding edema changes.
B, Coronal thallium-201 SPECT shows focal uptake in right temporal region with thallium index of 4.8, which is consistent with malignancy and was confirmed by brain biopsy to represent primary brain lymphoma.
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FIG 2. Toxoplasma encephalitis.
A, Coronal T1-weighted image with contrast shows heterogeneously enhancing mass in left frontoparietal region with surrounding edema. There is also edema in the right frontoparietal region related to an additional enhancing lesion (not shown).
B, Coronal thallium-201 SPECT shows no corresponding area of focally increased radiotracer uptake. This is most consistent with a nonmalignant, infectious etiology, which was proved to be toxoplasmosis by brain biopsy.
C, Follow-up coronal T1-weighted image, 10 days after antitoxoplasma therapy, shows the size of the enhancing left frontoparietal mass to have decreased and improved surrounding edema.
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FIG 3. False-negative thallium-201 SPECT of lymphoma.
A, Axial CT image with contrast shows a 1.9-cm ring-enhancing mass lesion in the left thalamus with surrounding edema.
B, Axial thallium-201 SPECT shows no corresponding area of focally increased tracer uptake. The slight asymmetry in basal ganglia activity (greater on the left) is due to patient angulation (note also asymmetry of the transverse sinus activity, greater on the right). Although suggestive for an infectious etiology, this lesion falls below our size threshold and, by brain biopsy, was due to lymphoma.
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FIG 4. When using thallium index threshold of 2 for all lesions  2 cm, no malignant lesions were missed and only two false-positive events occurred.
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