1. Atypical Teratoid Rhabdoid Tumor of the Third Cranial Nerve (AT/RT)
1R Clarke, 2L Klesse, 3B Braga, 4V Rajaram, 1K Koral
University of Texas Southwestern Medical Center and Children's Health, Dallas, TX
Departments of 1Pediatric Radiology, 2Oncology, 3Neurosurgery and 4Pathology
Control #: 1168
Poster #: EE-29

2. Disclosures
The authors have nothing to disclose.

3. Purpose
AT/RT is a rare, aggressive tumor of the central nervous system
Once thought to represent a more aggressive type of primitive neuroectodermal tumor (PNET), now recognized as a completely separate entity by the World Health Organization since 2000.
Characteristic imaging features of AT/RT are well described, however, when found in uncommon locations these tumors can present difficulty in preoperative diagnosis.
We present this case to highlight that the differential diagnosis of an AT/RT involving the third cranial nerve is rather limited when its location, characteristic imaging features and patient's age are taken into consideration.

4. Material & Methods Case Report:
Previously healthy 13 month-old girl seen by an ophthalmologist for 2 months of outward deviation of the left eye and drooping eyelid
On exam, found to have a left third cranial nerve palsy
MRI showed an enhancing mass in the interpeduncular fossa, related to the midbrain and hypothalamus. The cisternal segment of the left third cranial nerve could not be identified as a separate structure, unlike the normal right third cranial nerve

5. Imaging
A, B: Axial T2 (A) and postcontrast sagittal T1 (B) images show a homogeneously enhancing, relatively T2 hypointense lesion centered in the interpeduncular fossa, inseparable from the left cerebral peduncle.

6. Imaging
C: Coronal reformatted balanced FFE image shows a normal right third cranial nerve (orange arrow) and the relatively T2 hypointense mass (yellow arrows) in the expected location of the left third cranial nerve.

7. Imaging
D: Axial diffusion-weighted image demonstrates peripheral regions of restricted diffusion (white arrows) within the tumor.

8. Results
The patient underwent stereotactic biopsy of the lesion, which was found to be an AT/RT
The patient is currently undergoing aggressive chemotherapy and conformal radiation therapy
Follow-up imaging at 7 months following initial diagnosis has demonstrated stability of the tumor

9. Conclusion
To our knowledge, there are only 4 previously reported cases of CN III AT/RT in the literature, all with striking similarities in both patient presentation and imaging findings
Our case is nearly identical to these
All reported cases of CN III AT/RT, including the present one, resemble AT/RTs found in other CNS locations

10. Conclusion AT/RTs are WHO Grade IV tumors
Most often seen in the pediatric age group, most commonly presenting before the age of 3 years
Aggressive lesions with historically poor prognosis prior to the advent of more targeted therapies
Characteristic imaging features are those of a highly cellular malignant tumor
Relatively iso- or hypointense T2 signal
Avid enhancement
Restricted diffusion
Features are highly consistent among AT/RTs found elsewhere in the CNS

11. Conclusion Differential diagnosis for a tumor in this location
Cranial nerve schwannoma
Exophytic brainstem glioma
Both alternative diagnoses would demonstrate more T2 hyperintensity and would not show restricted diffusion

12. Conclusion
Despite the still bleak prognosis for a child with AT/RT, timely initiation of appropriate treatment may serve to improve prognosis and prolong a patient’s life
Although rarely reported previously, AT/RT should be the leading diagnostic consideration of a tumor arising from a cranial nerve when characteristic imaging features and patient age are taken into consideration

13. References
Rorke LB, Packer RJ, Biegel JA: Central nervous system atypical teratoid/rhabdoid tumors of infancy and childhood: definition of an entity. J. Neurosurg 1996; 85:56-65.
Wykoff CC, Lam BL, Bratwaite CD, et al. Atypical Teratoid/Rhabdoid Tumor Arising From the Third Cranial Nerve. J Neuroophthalmol. 2008; 28(3): 207–211.
Oh CC, Orr BA, Bernardi B, et al. Atypical teratoid/rhabdoid tumor (ATRT) arising from the 3rd cranial nerve in infants: a clinical-radiological entity? J Neurooncol 2015; 124:175–183