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Imaging


Figure 3: Conventional Magnetic Resonance Features of Radionecrosis


AB


calcifications are commonly observed. The central necrotic component has increased signal intensity (SI), while the peripheral, solid portion presents as low SI,1,4,18


peripheral rim of enhancement on T1-weighted imaging with gadolinium.6,7,10


and more diffuse than soap bubble lesions,4,10 with an intense and irregular


Commonly seen enhancement patterns are described as ‘soap-bubble-like,’ ‘Swiss-cheese-like’ and ‘cut green pepper’ (see Figure 3).2,4,7,9,15


Swiss cheese lesions are larger, more variable in size, and are the result of


diffuse necrosis affecting the white matter and cortex with diffuse enhancement of feathery margins and intermixed necrotic foci.


Perfusion Magnetic Resonance C D


Relative cerebral blood volume (rCBV) is the most widely used haemodynamic variable derived from perfusion MR. rCBV has been shown to correlate with primary glioma grade and tumour microvascular density.4,16,22


High-grade primary neoplasms and brain metastases are characterised by high rCBV values (equal to or greater than those of gray matter), which occur owing to increased angiogenesis. RN has low rCBV values, resulting from endothelial cell damage, thrombosis and fibrinoid necrosis.4,5,12,14,16


EF


Normalised rCBV ratios are useful in distinguishing pure RN from pure tumour recurrence. In a study performed by Suhagara et al.,18


the


authors concluded that if the ratio of the enhancing lesion is higher than 2.6 or lower than 0.6, tumour recurrence or RN should be strongly suspected (see Figure 4).


Nevertheless, enhancing masses developing after surgery and ChT and/or RT of high-grade tumours usually demonstrate a combination of both recurrent/persistent neoplasm and RN.2,18


degree of overlapping rCBV values can be observed,4,12,14,16,18


A–C: On axial fluid-attenuated inversion recovery (FLAIR) images, radionecrosis presents as necrotic masses with ill-defined, blurred margins, central high signal intensity (SI) and peripheral (solid-portion) low SI. Typical enhancement patterns are described as ‘soap-bubble-like’ (D, paired image to A), ‘Swiss-cheese-like’ (E, paired image to B) and ‘cut green pepper’ (F, paired image to C).


improve or stabilise; asymptomatic courses may even be seen) and several possible radiological outcomes: lesions may stabilise, regress in size or undergo continued growth with oedema, sometimes with lethal progression.1,4–7,9,10


Imaging of Radionecrosis


Conventional Magnetic Resonance Imaging RN can closely resemble a recurrent tumour because of the following shared characteristics: origin at or close to the original tumour site, contrast enhancement, growth over time, oedema and exertion of mass effect.1–8,10,11,14–20


radiation delivery (in the immediate vicinity of the tumour site and surrounding the surgical cavity of a partially or totally resected tumour), in the periventricular white matter, or within the corpus callosum.1,4,6,10,13,15,19,20


Accordingly, a large and there


is little consensus in the literature on this matter. Thus, rCBV ratios ranging between 0.6 and 2.6 probably represent a mixture of both tumour and necrosis.18


In these cases, rCBV plays only a


complementary role, and follow-up studies are mandatory. Magnetic Resonance Spectroscopy


Structural brain degradation after RT can be predicted by early changes in metabolic activity before the development of neurocognitive symptoms or anatomical changes seen on conventional MR.7 Spectral patterns allow reliable differential diagnosis when either pure tumour or pure RN is found. Unfortunately, in many of the enhancing regions, including those appearing early after treatment, often both tumour cells and radiation injury are present, and the spectral patterns in these cases are less clear.2,4–7,9


If possible, a spectrum of the tumour


should be obtained prior to RT/ChT in order to compare metabolic ratios at follow-up studies.11


A spectrum of normal tissue should RN commonly occurs at the site of maximum always be obtained to provide an internal standard.4


Less common patterns include multiple lesions, lesions in the contralateral hemisphere or arising remotely from the primary tumour, subependymal lesions and temporal-lobe RN (after RT for head and neck tumours).1,4,6,7,10,20


On T2-weighted imaging, RN presents as necrotic masses with ill-defined, blurred margins. Peri-lesional oedema and scattered


56


The main spectroscopic features employed in the distinction between tumour recurrence and RN are as follows (see Figure 4). First, decreasing or unchanging choline (Cho) levels suggest RN.5,12,17


In the


initial stages, Cho levels may be normal, reduced or elevated (resulting from demyelination, therefore mimicking tumour).4 Thereafter, decreasing levels of Cho and creatine (Cr) become evident, reflecting a dilution effect of decreased cell density and oedema. At follow-up studies, a decrease in the abnormal Cho/Cr and Cho/N-acetyl aspartate (NAA) ratios and decreasing or unchanging


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