MRI is the standard of care for most neuroimaging indications, and advanced sequences are providing an ever-growing ability for lesion characterisation. PET is widely used in oncology, and the limitations of Fluorine-18 fluorodeoxyglucose in neuro-oncology have been overcome by the development of novel PET tracers. The sequential use of these modalities harnesses the strengths of both, and has led to the development of combined PET-MR systems. This educational exhibit demonstrates the ways in which MRI and PET can be used in a complementary manner for improved lesion characterisation in neuro-oncology, including diagnosis and treatment planning.
This educational exhibit demonstrates the ways in which the complementary strengths of MRI and PET can be combined for improved lesion characterisation and treatment planning in neuro-oncology.
Despite the relative limitations of Fluorine-18 fluorodeoxyglucose (FDG) PET in the brain, due to high background parenchymal uptake, the presence of an unexpected intracranial abnormality on PET is most common on FDG-PET studies. Intracranial pathology may be visualised as either high or low FDG uptake compared to background uptake. MRI then provides further characterisation, and may detect additional lesions.
Novel PET tracers targeting the somatostatin receptor, such as Gallium-68 labelled DOTA-octreotate, are being increasingly used for the diagnosis and management of neuroendocrine tumours (NETs). The differential diagnosis for an intracranial mass demonstrating tracer uptake includes an NET metastasis, meningioma and haemangioblastoma. MRI may then provide a specific diagnosis from this limited differential. PET may also allow a more confident diagnosis of a mass identified on MRI, without the need for craniotomy. For example, in the context of a mass around the skull base or within the carotid space, uptake on DOTA-octreotate PET is strong evidence of a paraganglioma, while such uptake in a durally-based intracranial mass is consistent with a meningioma.
Amino acid tracers such as fluoro-ethyl-tyrosine (FET) can differentiate between malignant intracranial lesions, which demonstrate avid tracer uptake, and non-malignant aetiologies. In contrast to FDG, these tracers do not exhibit significant uptake in normal brain parenchyma, which would otherwise limit characterisation of the lesion. This information combined with conventional and advanced MRI sequences may provide a more confident diagnosis. FET is also useful in distinguishing between recurrent tumour and radiation necrosis, most commonly after stereotactic radiosurgery. Novel PET tracers also have value for treatment planning when the diagnosis is known. For example, combining MRI with DOTA-octreotate PET can be useful for planning radiotherapy to a meningiomas, while amino acid tracers can play a similar role for intracranial gliomas. PET can also be used to predict the response to peptide receptor radionuclide therapy, for example used for treatment of NET metastases.