Thallium-201 Imaging and Estimation of Residual High Grade Astrocytoma

Abstract

Thallium-201 brain imaging was performed on five patients as a method to differentiate persistent and/or recurrent viable Grades III and IV astrocytoma tissue from necrosis or post-therapy changes. Planar images of the head and heart were obtained in order to calculate the ratio of tumor counts to cardiac counts. The heart was chosen as the internal reference organ, as thallium uptake dynamics are reproducible under ordinary circumstances. The numerical estimation of thallium uptake in the brain tumor, expressed in terms of the tumor/cardiac index, correlated well with the clinical course in all five patients. By visual inspection, the initial computed tomographic (CT) and thallium images suggested that the tumors were approximately the same size. Follow-up thallium images were discordant with follow-up CT images. Computed tomography, in general, appeared to depict image alterations suggesting more extensive regrowth of tumor than the actual clinical status or thallium brain scans demonstrated. Histologic examination best correlated with thallium images. In one patient’s course of imaging we were able to detect tumor recurrence, by means of thallium imaging, 4 months prior to its appearance on CT. When performed serially, the tumor/cardiac index may provide an estimate of residual tumor burden, which can help distinguish tumor recurrence from changes secondary to therapy.