1. Uncommon Presentations of Pediatric Posterior Fossa Tumors and Mimics
By: Duy Quang Bui MD, Patrick Sanchez MD, and Matthew Bobinski MD, PhD
University of California, Davis
Neuroradiology
eEdE-206

2. Financial disclosures:
None

3. Part one: Atypical presentations of posterior fossa tumors

4. 14 month old presenting with emesis:a. b. c.
Atypical presentation of medulloblastoma: Axial diffusion (a and b) and coronal T1 (c) show a large and hypercellular tumor predominantly centered within the fourth ventricle with supratentorial extension. Additional site of disease includes the right cerebellopontine angle. There are multiple prominent cysts posteriorly and obstructive hydrocephalus.

5. 8 year old female presenting with posterior headaches, nausea and vomiting:
Anaplastic medulloblastoma: Sagittal T1 (a) shows a hypointense T1 mass within the cerebellum which enhances avidly but heterogeneously (b) with scattered cystic areas on axial T2 (c). Axial ADC (d) shows multiple areas of restricted diffusion typical of medulloblastomas.

6. 5 year old presenting with Emesisb. c.
Typical presentation of medulloblastoma: Axial diffusion (a), ADC (b), and T1 post contrast show a large mass arising from the vermis, expanding the fourth ventricle with characteristic diffusion restriction, and relatively homogenous enhancement. Note a single small cyst posteriorly.

7. Typical vs. Atypical Presentations of Medulloblastoma:
Location:
Midline (typical for children)
Inferior vermis
Eccentric or lateral location
Typically seen in young adults and the desmoplastic variant
Isolated involvement of the cerebellopontine angle
Extension through the foramen of Luschka and Magendie (worse prognosis)
Enhancement:
Homogenous
Focal and patchy
Up to 7.5% lack enhancement
Calcification: No
Up to 20% of cases
Presence of cysts:
Yes, typically small cysts
More likely to have large cysts
Hemorrhage:
Found in up to 5-6% of cases
More commonly seen in anaplastic medulloblastoma

8. 12 month old presenting with emesis:a. b. c. d. e.
Atypical Teratoid/Rhabdoid Tumor (ATRT): This case is very similar to the previous case of atypical medulloblastoma. Axial diffusion (a, b), coronal T1 post contrast (c), and axial T1 (d) shows a large hyper-cellular mass with patchy enhancement involving the majority of the fourth ventricle. Axial T1 shows focal hyperintense T1 signal in the right postero-lateral aspects of the mass with associated susceptibility artifact (e) consistent with hemorrhage.

9. Differentiating at/rt from medulloblastoma
Typically in the midline arising from the lower vermis
Older mean age of presentation (3-7 y.o.)
More likely to involve the cerebellopontine angle
More likely to hemorrhage
Younger age of presentation (1 y.o.)
More aggressive and less sensitive to therapy

10. Bonus Case: a. b. c.
Leptomeningeal spread of medulloblastoma: Axial (a) and coronal T1 post contrast (b) images show diffuse leptomeningeal enhancement. The nodular areas of enhancement show associated diffusion restriction (c) consistent with tumor spread from a medulloblastoma primary. This pattern of spread may also be seen in more atypical variants such as: anaplastic medulloblastoma or medulloblastoma with extensive nodularity.

11. 10 year old boy presenting with nausea and vomiting:c.
ADC was >1700
Atypical presentation of pilocytic astrocytoma mimicking high grade glial tumor: Sagittal T1 post contrast (a) shows a lesion centered in the cerebellum with thick and irregular peripheral enhancement. Axial ADC (b) shows increased diffusivity which would not be expected for a high grade glial tumor. Multiple cystic areas within the tumor on axial T2 (c).

12. 8 year old female presenting with headache:b. a.
Pilocytic astrocytoma: Axial T1 post contrast (a), sagittal T1 post contrast (b), and axial T2 (c) sequences show the classic cyst and nodule appearance of pilocytic astrocytoma arising from the tectum. Note the obstruction of the hydrocephalus.

13. Typical vs. atypical presentations of pilocytic astrocytoma:
Morphology:
Cyst with mural nodule
Solid tumor
Solid tumor with cysts
Solitary cyst without enhancing nodule
Enhancement:
Homogenous
Thick peripheral enhancement with infiltrative margins, mimicking high grade glioma
Cellularity:
Lower cellularity
Higher T2 signal intensity
Higher ADC values (ADC min > 800 x 106 mm2/s
Same
Location:
Cerebellar hemispheres
Optic chiasm, basal ganglia and brainstem are more likely to mimic high grade glial tumors

14. Potential Pitfall:
Sagittal T1 post contrast shows a solidly enhancing mass within the pineal region in this 14 y.o. female. Initially, this was thought to be a germinoma. Upon further examination, the pineal gland is posterior and separate from the mass making this most likely a pilocytic astrocytoma arising from the tectum, which was the final pathologic diagnosis. Pineal region germinomas are overwhelmingly found in males (9:1).
The pineal gland is inferior to the paired internal cerebral veins, at the posterior aspect of the third ventricle, and superior to the tectum (red arrow).

15. 7 year old presenting with Nausea:b.
Focal pontine glioma: Axial T2 (a) shows a focal and relatively well circumscribed mass in the right pons without associated enhancement on axial T1 post contrast (b).

16. 3 year old presenting with ataxia:b. c.
Diffuse infiltrating pontine glioma: Sagittal T1 (a) shows diffuse enlargement of the pons in both craniocaudal and anterior-posterior dimensions with near complete effacement of the fourth ventricle, however without hydrocephalus. The large mass does not enhance on axial T1 post contrast (b) but is markedly hyperintense on axial T2 (c).

17. Typical vs. Atypical presentations of Pontine gliomas:
Morphology:
Infiltrative and occupying 2/3 of the pons, frequently expanding it.
Focal: Occupies less than 50% of the transverse area of the pons
Approximately 5% of brainstem tumors
Margins may be smooth or irregular
Enhancement:
Typically do not enhance prior to treatment and may enhance after.
Exophytic solid nodules of focal pontine gliomas usually enhance.
Presence of hydrocephalus:
Surprisingly no, despite pontine expansion with frequent effacement of the fourth ventricle.
Presence of cysts: No

18. Types of Brainstem gliomas:
Medullary
Pontine
Mesencephalic
Tectal

19. Brief overview of Brainstem Gliomas:
Incidence
Morphology
MRI
Pontine
Most common
Focal:
Dorsal and cranial exophytic extension with mass effect on the inferior cerebellum
Infiltrative
Infiltrates into pons or into spinal cord
Hypointense T1, Hyperintense T2
Typically no enhancement unless treated.
Medullary
Least common
Most commonly infiltrative
Infiltrates and expands medulla and midbrain
Rarely focal
Typically exophytic growth into fourth ventricle
Hypointense T1, hyperintense T2
Mesencephalic
Second most common
Most commonly focal
Less commonly infiltrative
Infiltrative type extends into the cerebral hemisphere and pons.
Hemorrhage and cysts may be present

20. 11 year old male presenting with headache:b. c.
Intraventricular meningioma: Axial T1 post contrast (a) and sagittal T1 post contrast (b) show a heterogeneously enhancing mass centered within the fourth ventricle extending inferiorly through the foramen magnum. The mass is intermediately hyperintense on T2 (c).

21. 15 year old male with skin lesions:b. c.
Meningioma and bilateral vestibular schwannomas: Axial T1 (a) and coronal T1 post contrast images (b) show a homogenously and avidly enhancing extra axial mass arising from the right tentorial leaflet. Additionally, there are bilateral homogenously enhancing masses in the internal auditory canals consistent with vestibular schwannomas. Axial T2 (c) shows an extra-axial mass intermediately hyperintense on T2 without significant adjacent vasogenic edema. This patient was diagnosed with Neurofibromatosis type 2.

22. Typical vs. Atypical presentations of Meningiomas:
Location:
Lateral ventricle: ~78% of cases
Third Ventricle: ~15%
Fourth ventricle: ~7%
More common in pediatric population
Enhancement:
Homogenous
Variable enhancement pattern ranging from homogenous to heterogeneous.
Peritumoral edema:
May be present
Marked peritumoral edema
Imaging features:
Smooth, and well circumscribed
Pediatric meningiomas are characterized by large tumor size and cyst formation
Atypical meningiomas and malignant meningiomas are more locally aggressive

23. 18 year old presenting with headache:b. c. d.
Atypical choroid plexus papilloma: Axial ADC (a) and axial T2 (b) show an intraventricular mass with a peripheral ring of CSF without diffusion restriction. Axial T1 (c) and coronal T1 post contrast (d) show an irregular and lobulated heterogeneous enhancing mass.

24. 10 year old presenting with headache:b. c. d.
Typical choroid plexus papilloma: Axial T1 (a) and axial T1 post contrast (b) show a large avidly enhancing mass within the left atrium which is slightly hyperintense on T2 (c) without diffusion restriction (d).

25. Typical vs. Atypical presentations of Choroid Plexus Papillomas:
Location:
Lateral ventricle (54%) especially in the atrium (left greater than right)
Fourth Ventricle (26%)
Cerebellopontine angle (8%)
Age:
Children
Adolescents and adults
Enhancement:
Avid and homogenous
Heterogeneous
Hydrocephalus:
Yes, secondary to overproduction of CSF and/or obstruction
Same

26. Part two: Potential Mimics of Posterior Fossa Tumors

27. 4 month old baby presenting with emesis. Best guess?c. d. e. f.
Cerebellar abscess: Axial (a), coronal (b), and sagittal T1 post contrast (c) and axial T2 (d) show a large cystic collection with a peripheral rim of enhancement that compresses the fourth ventricle, obstructing the cerebral aqueduct and resulting in hydrocephalus. Axial diffusion (e) and ADC (f) shows diffusion restriction of this large cystic mass.

28. 2 year old presenting with encephalopathy. Best Guess?
Acute disseminated encephalomyelitis: Axial FLAIR (a) shows multiple lesions involving and mildly expanding the medulla without associated enhancement on axial T1 post contrast (b). Sagittal T2 (c) of the cervical spine shows no cord lesions. c.

29. Acute Disseminated Encephalomyelitis:
Typical
Atypical
Distribution of hyperintense focal T2 Lesions:
Subcortical and deep white matter > Periventricular white matter. Often Asymmetric.
Symmetric involvement of bilateral basal ganglia and thalamus.
Additional involvement of the cerebral cortex (30% of cases)
Enhancement: No
Homogenous enhancement
Involvement of spinal cord:
Yes (30%)
May involve gray, white or both gray and white matter
Typically more than 2-3 vertebral bodies in cranial-caudal extent
Swelling and mass effect:
Lesions involving the brainstem, middle cerebellar peduncles
Cerebellar white matter often have significant swelling

30. Adolescent presenting with AMSb.
Rhombencephalitis: Sagittal (a), axial FLAIR (b), and Axial T1 post contrast (c) show hyperintense T2 lesions involving the superior aspects of the pons without associated enhancement.

31. Rhombencephalitis: Typical Atypical Location:
Pons, medulla, cerebellum and upper cervical cord
Midbrain
Cause:
Listeria: most common cause
Viruses: Enterovirus and HSV
Autoimmune: Behcet’s – most common autoimmune etiology
All other bacteria, viruses, and autoimmune diseases
Imaging appearance:
Patchy hyperintense T2 signal with variable enhancement
Same

32. Increasing HEADACHE: b. d. c. e. a.
Hyperacute to acute epidural hematoma: Sagittal T1 (a) shows an isointense epidural fluid collection with significant mass effect on the adjacent right cerebellar hemisphere which may mimic a posterior fossa mass. Axial T2 (b, c) show a hematocrit level due to higher water content layering non-dependently. There is marked susceptibility on axial GRE (d) without associated enhancement (e).

33. The many appearances of hematomas:
Hyperacute
Acute
Early subacute
Late subacute
Chronic
Cellular level
Extravasation
Deoxygenation
Clot retraction with oxidization to met-hb
Cell lysis
Clot broken down by macrophages
Time frame
<12 hours
Hours – 3 days
3-7 days
7 days to 1 month
Weeks to years
State of Hb
Intracellular oxy-Hb
Intracellular deoxy-Hb
Intracellular met-Hb
Extracellular met-Hb
Hemosiderin
T1 weighted imaging
~ or
T2 weighted imaging
Adapted from: Parizel, P., S. Makkat, E. Van Miert, J. Van Goethem, L. Van Den Hauwe, and A. De Schepper. "Intracranial Hemorrhage: Principles of CT and MRI Interpretation." Eur Radiol European Radiology 11.9 (2001):

34. Thank you for going through our presentation!
UC Davis Medical Center

35. References:
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Rath, Tanya J. et al. "Imaging Of Cerebritis, Encephalitis, And Brain Abscess". Neuroimaging Clinics of North America 22.4 (2012): Web.

36. References:
Rossi, Andrea. "Imaging Of Acute Disseminated Encephalomyelitis". Neuroimaging Clinics of North America 18.1 (2008): Web.
Smith, Alice Boyd, James G. Smirniotopoulos, and Iren Horkanyne-Szakaly. "From The Radiologic Pathology Archives 1 : Intraventricular Neoplasms: Radiologic-Pathologic Correlation". RadioGraphics 33.1 (2013): Web.
Zhang, Ti-Jiang et al. "MRI Findings Of Choroid Plexus Tumors In The Cerebellum". Clinical Imaging 35.1 (2011): Web.