High uptake of L-3-[I-123]iodo-alpha-methyl tyrosine in pilocytic astrocytomas

Despite a favourable prognosis, pilocytic astrocytomas may exhibit signs of malignancy on various neuroimaging modalities. This retrospective analysis was conducted to determine whether scintigraphic features of malignancy ar e also found on single-photon emission tomography (SPET) using L-3-[I-123]i odo-alpha -methyl tyrosine (IMT) as a tracer. Twenty patients with pilocyti c astrocytomas were retrospectively selected from a large series of patient s referred for the evaluation of primary or recurrent brain tumours. IMT SP ET was performed in 16 patients, positron emission tomography (PET) using 2 -[F-18]fluoro-2-deoxy-D-glucose (FDG) was available in 10 of the patients a nd SPET using technetium-99m tetrofosmin or thallium-201 had been performed in 11. Image analysis was performed using standard protocols to determine how many patients exceeded the respective thresholds of malignancy. Feature s of malignancy were found in 7/16 IMT SPET studies, in 7/10 FDG PET studie s and in 7/11 of the residual SPET investigations. A significant correlatio n of tumour size and IMT uptake in primary pilocytic astrocytomas indicated partial volume effects to partly account for the differential uptake behav iour (n=10, r=0.87, P<0.05). Differences in IMT uptake in primaries (1.7<pl us/minus>0.6, n=10) and in recurrent tumours (2.3 +/-0.7, n=6) did not atta in statistical significance. IMT SPET results indicative of malignancy are regularly found in pilocytic astrocytomas, despite their good prognosis. No uptake may be detected in largely cystic or in small tumours.