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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

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

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

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in diagnostic radiology by the American Board of Radiology and specializes

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

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

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

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

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

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

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

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

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

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how I approach those cases and make a differential diagnosis.

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

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

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

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

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

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

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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

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done an MRI on the same child. And

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

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

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

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

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

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

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

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solid with some cystic areas anteriorly and towards the left.

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

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

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

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

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

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

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

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

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at the foramen magnum. And this is another

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

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

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

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

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

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

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

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

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

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

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model enhancing nodule. In my practice, I see

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a lot of predominantly solid pilocytic astrocytomas in clinical practice

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

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

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important in those cases where a pilocytic astrocytoma is predominantly

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

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

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

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

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

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

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

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

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

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this child develops syringohydromyelia in the cervical cord.

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

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

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

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

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

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as well. They can also hemorrhage throughout their course.

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

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

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

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