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

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Abstract
Single-photon emission tomography (SPET) with thallium-201 is used in the assessment of patients with gliomas because the amount of201Tl 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 in201Tl SPET studies, the uptake of201Tl 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 on201Tl 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 of201Tl uptake. The patients underwent both201Tl SPET and MRI before surgery. The201Tl 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 cm2) including pixels with more than 50% maximum activity; the second employed regular square small ROls (2.7 cm2) centered on the maximum activity of the lesion. Of the MRI morphological parameters studied, only necrosis significantly reduced the degree of201Tl uptake in GBMs when larger ROIs were used. However, by using small regular ROIs the influence of necrosis on201Tl 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 of201Tl uptake due to intratumoral necrosis must be carefully evaluated.