RELATIONSHIP BETWEEN TL-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 Tl-201 a ccumulated by the tumoral cells increases in proportion to the degree of tumour malignancy, thus making it possible to differentiate high-gr ade from low-grade gliomas or recurrences from radiation necrosis. How ever, in large areas of tissue such as those examined in Tl-201 SPET s tudies, the uptake of Tl-201 may vary considerably even in tumours wit h the same histological diagnosis, as occurs in glioblastomas (GBMs). In order to evaluate the possible influence of the macroscopic charact eristics of tumours on Tl-201 uptake, we studied a series of 13 patien ts with histologically proven GBMs, comparing magnetic resonance imagi ng (MRI) parameters such as tumour dimensions, perilesional oedema, in tratumoral necrosis and contrast enhancement with the degree of Tl-201 uptake. The patients underwent both Tl-201 SPET and MRI before surger y. The Tl-201 index (tumour/contralateral unaffected brain) was calcul ated using two different region of interest (ROI) methods: the first e mployed irregular large ROIs (3.2+/-13.9 cm(2)) including pixels with more than 50% maximum activity; the second employed regular square sma ll 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 Tl-201 uptake in GBMs when larger ROIs were use d. However, by using small regular ROIs the influence of necrosis on T l-201 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 Tl-201 uptake due to intratumoral necrosis must be carefully evaluated.