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learning courses across all key radiology subspecialties. Learn more at

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mrionline.com. Today we're honored to welcome Dr. G for a lecture on pediatric

0:38

posterior fossa tumors. Dr. G is an associate professor of radiology and

0:42

radiological science at the the Johns Hopkins Hospital. She's board certified

0:47

in diagnostic radiology by the American Board of Radiology and specializes

0:51

in pediatric neuroimaging. At the end of the lecture, join Dr.

0:56

G in a Q&A session where she will address your questions you may

0:59

have on today's topic. Please use the Q&A chat feature to submit your

1:04

questions. We'll look at as many as we can before our time is

1:06

up. With that being said, we welcome you. Dr. G, please take it

1:10

from here. Hello. Thanks for having me today here. I'm very excited to

1:15

be here. It's a pleasure. I am a pediatric neuroradiologist. Thank you so

1:19

much for the great introduction. So today I'm going to talk about posterior

1:24

fossa tumors, and I'm going to show cases and explain my approach and

1:29

how I approach those cases and make a differential diagnosis.

1:35

So all of those patients are the children that I have seen in

1:39

the past two years. So my first patient is a three year old

1:42

female who presents with altered mental status. And this unenhanced CT was

1:47

done at an outside institution. And we see a midline posterior fossa mass

1:53

that is hypodense and also extends towards the left cerebellar hemisphere.

1:59

It grows into the fourth ventricle and also enlarges the sphere aspect of

2:03

the fourth ventricle and their supratentorial hydrocephalus. So the critical

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point here on the CT is that this mass is hypodense. Yes, so we have

2:13

done an MRI on the same child. And

2:16

I only start with diffusion weighted imaging if a tumor is arising from

2:21

the fourth ventricle or the cerebellar vermis or the cerebellar hemispheres.

2:26

So the reason for that is that I decide first if a lesion

2:30

is a low grade or a high grade. So on this DWI image, we see no diffusion

2:37

restriction on DWI and increased ADC values, which shows facilitated diffusion.

2:43

And on the T2 weighted image, there is increased signal of this folded

2:51

component of this mass. And also I notice that the mass is predominantly

2:56

solid with some cystic areas anteriorly and towards the left.

3:00

So we have a mass that is predominantly solid in the midline posterior fossa

3:04

that doesn't restrict a diffusion. So these sequences are enough for me

3:10

to make a diagnosis here. And I think this is a juvenile pilocytic astrocytoma

3:15

only by looking at those images. But of course, we have done more

3:20

sequences. We have pre and post contrast imaging. So the first image on

3:25

the left is a pre T1, which is T1 hypointense. The middle image

3:29

shows contrast enhancement. And because of the mass effect and tonsillar

3:35

herniation, there is cord edema because of the alteration of the CSF dynamics

3:40

at the foramen magnum. And this is another

3:46

patient with a different appearance. So again, I start with my diffusion

3:52

weighted images. There's a cystic mass in the midline posterior fossa with

3:56

a solid component at the left lateral aspect, which the white arrow is

4:01

pointing towards on the ADC map. And there's another component inferiorly.

4:07

And the lesion does not show any diffusion restriction. So that means I

4:11

am looking at the cystic mass with a solid nodule that doesn't show

4:16

any diffusion restriction. And the nodule is T2 hyperintense, as the arrow

4:21

is pointing out. So these are pilocytic astrocytomas. And what I want to

4:31

emphasize here is that those lesions can be predominantly solid or,

4:39

as described in many textbooks, they will be a cystic mass with a solid

4:45

model enhancing nodule. In my practice, I see

4:49

a lot of predominantly solid pilocytic astrocytomas in clinical practice

4:57

that shows evident heterogeneous enhancement. And that's what I wanted to

5:02

emphasize in my first patient. So diffusion weighted imaging is also very

5:08

important in those cases where a pilocytic astrocytoma is predominantly

5:13

solid. And pilocytic astrocytoma is the most common pediatric central nervous

5:18

system glial neoplasm. And surrounding vasogenic edema, again, if you look

5:25

at textbook information, is generally absent. But if a pilocytic astrocytoma

5:31

reaches a certain size in a narrow posterior fossa, it will cause sometimes

5:35

significant surrounding brain edema as well. So this should not be something

5:41

that would throw you off diagnosis as well, surrounding brain edema.

5:46

And on the first T2 weighted image, I am showing this cord edema and

5:51

some syrinx formation. Again, because of the tonsillar herniation and the

5:55

mass effect and the CSF, obliteration of the CSF flow at the foramen magnum,

6:01

this child develops syringohydromyelia in the cervical cord.

6:07

So this is another example of a pilocytic astrocytoma with a different appearance.

6:12

So again, there is a hypodense cystic mass that's in the midline posterior

6:16

fossa, but there is a dependent hematocrit level posteriorly where the yellow

6:21

arrow is pointing towards. And pilocytic astrocytomas generally do not hemorrhage,

6:28

but they can hemorrhage. This also includes dorsal brainstem low grade gliomas

6:35

as well. They can also hemorrhage throughout their course.

6:39

But the type of hemorrhage is not usually massive hemorrhage, usually something

6:46

like hematocrit level that's layering or some hemorrhage around the solid

6:50

component. And there is another feature here, is that this curvilinear calcification

6:56

at the anterior aspect of the cystic mass. So pilocytic astrocytoma...

Pediatric Posterior Fossa Tumors, Dr. Melike Guryildirim, 03/10/22

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