Teaching Case

Patient with neurofibromatosis type-1 diagnosed at the age of eight years, by the presence of cutaneous stigma and optic glioma.
4 years after treatment (chemotherapy with carboplatin plus vincristine), while clinically stable, a follow-up MRI examination disclosed a new focal temporal enhancing lesion, which then had a spontaneous favourable evolution.

Brain MRI performed on an annual basis to follow the tumour, revealed a new left temporal lesion, represented at the beginning by a rounded focus of abnormal low signal on T1-WI, high signal on T2-WI and FLAIR, non-enhancing after intravenous administration of gadolinium.
Two years later, the lesion's appearance had changed, being bigger and enhancing homogeneously.
After another two-year period, without any treatment, MRI showed a benign evolution, the lesion being smaller and with less intense enhancement. Lesion was characterized at this stage by diffusion, perfusion and spectroscopy techniques. It showed high signal on ADC, without any abnormality recognisable on diffusion images (b=1000). Perfusion values were low (rCBV <1.75). Univoxel proton spectroscopy (TE 144ms) revealed an extremely low Cho/NAA ratio (inferior to 0.5), compatible with low-grade glioma, the most probable diagnostic entity considered. There was no glutamate peak, and under short TE evaluation (31 ms) neither lipids or lactate.

NF-1 is a common genetic disease, with various manifestations affecting mainly the skin and the nervous system. Brain abnormalities of NF-1 have been well described in the literature [1].
Besides tumours, namely optic gliomas and other low-grade gliomas, focal parenchymal abnormalities are frequently detected, these representing the so-called UBOs (unidentified bright objects). It is thought that these are due to areas of vacuolated myelin [2], at imaging corresponding to focus of high signal on T2-weighted images, usually without abnormal signal on T1-weighted images, and no mass effect or contrast enhancement [3]. Such areas of altered structure can be considered benign lesions, although correlation with cognitive deficits has been suggested [4], and a proliferative potential postulated, based on the evidence of few cases [3]. Thus, it is advisable to properly follow these areas by imaging [1, 4].
UBOs distribution is diverse. They can be found in the cerebellum, brainstem, basal ganglia, and cerebral hemispheres at several lobes [5].
Our case illustrates a lesion that could be early identified as an UBO, before its enhancing phase. Although the precise nature remains unknown, the main diagnostic possibilities are: low-grade glioma, established as a new lesion or converted from an UBO, or a demyelinating lesion of unusual pattern.
The favourable evolution with spontaneous partial regression should not refute the diagnosis of tumour, as this benign evolution has been noticed in several children with low grade-gliomas, especially pilocytic astrocitomas in the context of NF-1 or not [6, 7, 8].
From the clinical point of view, the patient experimented a worsening period of partial complex seizures when the lesion was with its maximum size at the enhancing phase, documented on electroencephalographic examination, precisely in that temporal region. Prevalence of epilepsy in NF-1 appears to be higher than in general population, which can be explained by tumour masses and occurrence of structural abnormalities [9].
New MR techniques, such as diffusion, spectroscopy and perfusion can be useful in the characterisation of these lesions, helping in the differentiation among clinical entities and monitoring minor changes, particularly using spectroscopy to estimate biochemical markers. To achieve this goal, it is important to know the disease patterns on diffusion imaging and spectroscopy, and several studies have accounted for those abnormalities distinction [5, 10, 11, 12], while experiments with perfusion are sparse.
The presented case serves as an example of the main topics around NF-1 focal lesions of benign behaviour, how to diagnose them better and understand the disease.