Lesion size determines accuracy of thallium-201 brain single-photon emission tomography in differentiating between intracranial malignancy and infection in AIDS patients.

BACKGROUND AND PURPOSE Discrimination between enhancing mass lesions in acquired immunodeficiency syndrome (AIDS) patients with conventional CT and MR imaging remains difficult. We determined the effect of lesion size on thallium-201 brain single-photon emission tomography (SPECT) imaging in differentiating primary brain lymphoma from cerebral toxoplasmosis. METHODS We retrospectively identified 35 AIDS patients with a total of 48 focal enhancing mass lesions on contrast-enhanced brain CT and/or MR images who subsequently underwent thallium-201 brain SPECT imaging. The thallium index of each lesion was evaluated on the basis of the ratio of mean uptake in the lesion compared with the corresponding contralateral side. Receiver operator curves were drawn to determine the optimal thallium index threshold. The effect of lesion size on scan accuracy was evaluated. RESULTS Malignant lesions in 20 patients had a mean thallium index of 2.4 (range, 1-11). Infectious lesions in 15 patients had a mean thallium index of 1.6 (range, 1-3.6). Twenty-five lesions were <2 cm (14 malignant, 11 nonmalignant) and 23 lesions were > or =2 cm (14 malignant, 9 nonmalignant). Thallium index was not a significant predictor of malignancy in the lesions <2 cm by using the logistic regression (P = .27). Receiver operator curve analysis by using thallium index of 2 in small lesions yielded 50% sensitivity and 82% specificity. In contrast, thallium index was a significant predictor of malignancy in lesions > or =2 cm (P < .01), yielding 100% sensitivity and 89% specificity. CONCLUSION Lesion size is a significant determinant of the accuracy of thallium-201 brain SPECT imaging, which should be the initial diagnostic tool for lesions > or =2 cm.