Relationship between thallium-201 uptake by supratentorial glioblastomas and their morphological characteristics on magnetic resonance imaging

Single-photon emission tomography (SPET) with thallium-201 is used in the assessment of patients with gliomas because the amount of ²⁰¹Tl accumulated by the tumoral cells increases in proportion to the degree of tumour malignancy, thus making it possible to differentiate high-grade from low-grade gliomas or recurrences from radiation necrosis. However, in large areas of tissue such as those examined in ²⁰¹Tl SPET studies, the uptake of ²⁰¹Tl may vary considerably even in tumours with the same histological diagnosis, as occurs in glioblastomas (GBMs). In order to evaluate the possible influence of the macroscopic characteristics of tumours on ²⁰¹Tl uptake, we studied a series of 13 patients with histologically proven GBMs, comparing magnetic resonance imaging (MRI) parameters such as tumour dimensions, perilesional oedema, intratumoral necrosis and contrast enhancement with the degree of ²⁰¹Tl uptake. The patients underwent both ²⁰¹Tl SPET and MRI before surgery. The ²⁰¹Tl index (tumour/contralateral unaffected brain) was calculated using two different region of interest (ROI) methods: the first employed irregular large ROIs (3.2 ± 13.9 cm ²) including pixels with more than 50% maximum activity; the second employed regular square small ROIs (2.7 cm ²) centered on the maximum activity of the lesion. Of the MRI morphological parameters studied, only necrosis significantly reduced the degree of ²⁰¹Tl uptake in GBMs when larger ROIs were used. However, by using small regular ROIs the influence of necrosis on ²⁰¹Tl uptake was found to be less relevant. Since necrosis is related to tumour proliferative activity and represents a negative prognostic factor in astrocytoma, a possible underestimation of ²⁰¹Tl uptake due to intratumoral necrosis must be carefully evaluated.