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Astrocytoma, IDH-mutant tumors are WHO CNS grade 2, 3 or 4 tumors of the brain found in adults. They are diffuse infiltrating astrocytic tumors where there is no identifiable border between the tumor and normal brain tissue, even though the borders may appear relatively well-marginated on imaging.
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The 5th edition (2021) update to the WHO classification of CNS tumors has substantially changed the classification of astrocytic tumors, building upon the prior (2016) edition that started to define tumors based on molecular characteristics 19.
What would previously in most instances have been known as a diffuse astrocytoma or anaplastic astrocytoma or secondary glioblastoma now all come under the one diagnosis, based on the presence of IDH mutation and the demonstrated absence of 1p19q codeletion, which if present would make the diagnosis of an oligodendroglioma 16.
These IDH-mutant astrocytomas are now graded 2, 3 or 4 based on histological and molecular features, but importantly a grade 4 tumor is no longer a glioblastoma, but rather just an astrocytoma, IDH-mutant WHO CNS grade 4 16.
Glioblastoma is now considered a separate entity and distinct and must be IDH-wildtype, and is therefore discussed separately.
Importantly, the diagnosis of astrocytoma, IDH-mutant is an adult-type diagnosis, distinct from a variety of other pediatric-type diffuse astrocytomas (see astrocytic tumors).
The terms fibrillary astrocytoma and protoplasmic astrocytomas are no longer recognized as separate entities 13. Although gemistocytic astrocytoma are also no longer recognized as distinct entities, gemistocytic tissue pattern remains a histological feature 19.
IDH-mutant adult-type astrocytomas are typically diagnosed in young adults (median age of 36 years for grades 2 and 3 (combined), and 38 years for grade 4) 17. This is substantially younger than glioblastoma IDH-wildtype tumors (median 50-60 years of age) 17.
There is a substantially higher incidence in men of all ages and of all grades tumor (M:F ~1.5) 1,17.
The most common presenting feature (~40% of cases) is a seizure. This is particularly the case in adults. Headaches are often also present. Depending on the size of the lesion and its location, other features may be present, such as hydrocephalus and focal neurological dysfunction, including personality changes.
Adult-type astrocytomas are predominantly composed of a microcystic tumor matrix within which are embedded fibrillary neoplastic astrocytes with mild nuclear atypia and a low cellular density. Often microcystic spaces containing mucinous fluid are present, a typical finding in what was previously known as fibrillary astrocytomas, but even more characteristic and pronounced in what was previously known as protoplasmic astrocytomas.
The occasional occurrence of gemistocytes in a diffuse astrocytoma does not justify the diagnosis of what was previously known as a gemistocytic astrocytoma, which tended to progress more rapidly to higher-grade tumors.
Mitoses, microvascular proliferation and necrosis are absent (if present they suggest a high-grade tumor). Like all tumors derived from astrocytes, diffuse astrocytomas stain with glial fibrillary acid protein (gFAP) 2,3.
It is well recognized that pathological classification has a high interobserver variation and thus imperfectly predicts clinical outcomes 11. More recent studies have shown that the genetic status of these tumors is more reflective of their subtypes than the histologic grading (please refer to isocitrate dehydrogenase (IDH) for a broad discussion on this topic) 11.
The grading of astrocytoma, IDH-mutant is based on histological features, as well as incorporating molecular markers (introduced in the 5th edition (2021) WHO classification of CNS tumors) 19.
well-differentiated, low mitotic activity
no necrosis or microvascular proliferation
CDKN2A/B homozygous deletion absent
anaplasia, significant mitotic activity
no necrosis or microvascular proliferation
CDKN2A/B homozygous deletion absent
microvascular proliferation, and/or
CDKN2A/B homozygous deletion
MRI is the modality of choice for characterizing these lesions, and in the case of smaller tumors, they may be subtle and difficult to see on CT, especially as they tend not to enhance.
Typically low-grade infiltrating astrocytomas appear as isodense or hypodense regions of positive mass effect, often without any enhancement (in fact, the presence of enhancement would suggest higher grade tumors), although tumors with gemistocytic differentiation can demonstrate wispy enhancement.
Cystic or fluid attenuation components are also encountered, particularly in tumors with a gemistocytic component.
Reported signal characteristics include:
isointense to hypointense compared to white matter
usually confined to the white matter and causes expansion of the adjacent cortex
mass-like hyperintense signal that incompletely suppresses on FLAIR: T2/FLAIR mismatch sign
always follow the white matter distribution and cause expansion of the surrounding cortex
cortex can also be involved in late cases in comparison to oligodendroglioma, which is a cortical-based tumor from the start
the "microcystic changes" along the lines of the spread of the infiltrative astrocytoma is a unique behavior for the infiltrative astrocytoma; however, it is only appreciated in a small number of cases
high T2 signal is not related to cellularity or cellular atypia, but rather to edema, demyelination, and other degenerative change 10
typically has facilitated diffusion, with lower ADC values suggesting a higher grade
ADC values correlate with grade 15
grade 4 = 745 ± 135 x 10-6 mm2/s
grade 3 = 1067 ± 276 x 10-6 mm2/s
grade 2 = 1273 ± 293 x 10-6 mm2/s
T1 C+ (Gd)
no enhancement in grade 2 tumors
solid areas of enhancement +/- necrosis suggest higher grade
typically will show elevated choline peak, low NAA peak, elevated choline:creatine ratio
elevated myo-inositol and myo-inositol/creatine ratio
there is a lack of the lactate peak seen at 1:33
the lactate peak represents the necrosis seen in grade 4 tumors
relative cerebral blood volume (rCBV) will depend on the grade
grade 2: no elevation
grade 4: usually elevated especially in enhancing components
has FDG uptake similar to the normal white matter
FDG,18-F-choline and 11C-choline PET is useful for biopsy targeting the most hypermetabolic areas that are most likely the highest grade component
Treatment and prognosis
Treatment depends on clinical presentation, tumor grade, as well as tumor size and location.
Historically low-grade tumors were managed as follows:
biopsy to confirm the diagnosis and observe
surgical resection if feasible
usually radiotherapy at the time of recurrence or progression
In contrast, high-grade tumors have maximal safe resection followed by concurrent chemoradiotherapy (Stupp protocol).
There is an increasing body of evidence that suggests that concurrent chemoradiotherapy, usually reserved for tumors that progressed to higher grades, may be of benefit in lower grade tumors also.
Similarly, there is evidence that resection of low-grade tumors is of benefit and that total resection of the T2 signal abnormality, or even "supramaximal resection" (resection of the normal-appearing brain beyond the margins of T2 signal abnormality) confers survival benefits 20. The extent of resection that is achievable will, however, depend on the location of the tumor and how circumscribed its margins are.
Care must be taken when reviewing survival data as the classification system used (WHO 2007 vs 2016 vs 2021) will dramatically affect the results.
The 5-year survival for adult-type astrocytoma IDH-mutant varies by grade 18:
grade 2 and 3 (combined): 9.3 years *
grade 4: 3.6 years
* NB: It is almost certain that grade 2 tumors do somewhat better and grade 3 do somewhat worse, although little data on this exists on strictly molecular-based diagnosis (post-2016) 18.
Possible imaging differential considerations include:
may be indistinguishable from grade 4 astrocytoma, IDH-mutant
tends to be in older patients
tends to have a less sizable non-enhancing component
infarction: major vascular territory
cerebritis/encephalitis: herpes simplex encephalitis, ADEM
cortical based tumors: oligodendroglioma, angiocentric glioma
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