IAC Fundus Enhancing Pseudolesion: First Reported Incidence on Post Contrast 1mm Volumetric T1 SPACE Joseph Probst1, Unni Udayasankar1, Elizabeth Krupinski2, Rihan Khan1 1University of Arizona Medical School Banner - University Medical Center 2Emory University Medical Center Poster #: eP-146 Abstract No: 2891 Submission Number: 2891 ASNR 2016 Washington, D.C.

The authors have no relevant financial disclosures

Purpose To determine the incidence of an apparent pseudolesion in the IAC fundus on 1mm volumetric post contrast T1 SPACE imaging. Small false positive enhancing lesions in the IAC fundus have been described previously (1) but to our knowledge, this is the first report of the incidence of such lesions on 1mm volumetric MRI imaging.

Methods & Materials - update 218 consecutive patients over a 7 month period were retrospectively evaluated for the presence of an enhancing pseudolesion in the IAC fundus on 1mm volumetric T1 SPACE post contrast imaging. 210 patients had the whole brain version of the sequence and 8 had a the small field of view IAC version. 0.1mm/kg of gadolinium were injected for each study. 2 reviewers independently scored the cases for the presence, indeterminate presence, or absence of the pseudolesion in each IAC. After exclusion criteria 202 patients were included involving a total of 398 sides.

Inclusion Contiguous pediatric and adult patients with T1 SPACE post contrast MRI sequence for a seven month period were assessed for a focal enhancing lesion in the IAC fundus on MRI Brain or MRI IAC

Lesion Grading Scale Grade 1 linear Grade 2 linear Grade 0 Grade 1 nodular Grade 2 nodular

Inclusion of two cases of coccidioidomycosis from the same patient Case 1: T1 SPACE with bilateral lesions

High resolution CISS showed no lesion No lesion on T1 FLASH images (3.5mm skip 1.05mm)

Case 2 follow up exam T1 SPACE with bilateral lesions No lesion on T1 FLASH images (3.5mm skip 1.05mm) No CISS for this study but negative on prior

Exclusion criteria History or imaging evidence of leptomeningeal spread of cancer Temporal bone mass entering IAC Temporal bone infection with dural enhancement Temporal bone radiation or surgery with dural enhancement Susceptibility artifact obscuring region

Case of unilateral exclusion Schwannoma Mastoid surgery Susceptibility artifact Mastoid/TMJ infection Meningioma

Cases of bilateral exclusion 7 cases due too excessive motion obscuring IACs 2 cases due to susceptibility artifact from dental braces 2 cases with no contrast 1 case with post op changes and diffuse dural enhancement 1 case of bilateral subdural hematomas and diffuse dural enhancement 3 cases of diffuse leptomeningeal disease (1 breast cancer, 1 high grade astrocytoma, 1 coccidioidomycosis meningitis)

Excluded cases: Leptomeningeal disease Above: Breast cancer leptomeningeal disease Below: High grade astrocytoma leptomeningeal spread of tumor Below: Leptomeningeal disease from Coccidioidomycosis

Excluded case: diffuse dural enhancement with bilateral SDH

Unilateral exclusion: Considered a real lesion Also present on T2 and conventional T1 FLASH with contrast T2 T1 SPACE T1 SPACE T1 FLASH

Results Side No Lesion Indeterminate Lesion Definite Lesion Total Right 151 (74.75%) 23 (11.39%) 28 (13.86%) 202 Left 146 (74.49%) 21 (10.71%) 29 (14.80%) 196 297 (74.62%) 44 (11.06%) 57 (14.32%) 398 Out of 398 possible sides in 202 patients, after consensus reads, 57 were sides were called definitely positive for the pseudolesion (14.3%) and 44 were called indeterminate (11.0%).

Results The majority of cases (74.75% right; 74.49% left) had no lesion reported. Indeterminate lesions were reported 11.39% of the time on the right and 14.80% on the left. Definite lesions were reported on 13.86% and 10.71% of the time on the right and left respectively.

Results There was no significant differences in the distributions of no lesions vs indeterminate vs definite as a function of side (X2 = 0.102, p = 0.9502). There was no significant difference in linear vs nodular (X2 = 0.435, p = 0.5094), with 80% of the linear being 1 lesion and 20% 2, and 73% of the nodular being 1 with 27% 2 lesions. Agreement between the two readers was high (left kappa = 0.898; right kappa = 0.917). The consensus agreement was used for subsequent analyses. The number of cases is in the table above.

Conclusion In our study, an apparent focal enhancing lesion in the IAC fundus on high resolution post contrast T1 SPACE ranges in incidence from 14.32%-25.38%, and is unlikely to be a pathological lesion in the absence of leptomeningeal carcinomatosis. Arriaga et al previously described such enhancing foci with an incidence ranging from 3.5% to 20%.1 Maeta et al had one case of such a lesion that went to surgery, with limited edema and no mass lesion found with the enhancement resolving within two years post surgery. They postulated that arachnoiditis could cause a false positive result.2

Conclusion House et al also had a case that went to surgery with no lesion found, concluding that the finding may represent a non-neoplastic lesion.3 In their literature review they found 9 cases of surgically proven false positive lesions of the IAC, with 6 additional patients that had decreased or resolution of such enhancement. On surgical exploration, the most common findings were the sequelae of arachnoiditis or viral inflammation, and biopsies found hyaline or neuronal degeneration in some cases.3

Conclusion In our study, no patients had surgery as the gold standard, but this incidence is much higher than expected for IAC fundus schwannoma, and the study excluded patients with leptomeningeal carcinomatosis. Consistent with what has been reported in individual cases or small series in the literature with non-high resolution imaging, we believe this entity to be a pseudolesion and not a true mass lesion, based on our large imaging series. Interestingly, despite high resolution 1mm imaging with T1 SPACE, our incidence is in line with that found previously in the literature.

References Arriaga MMA, Carrier, DC, Houston GD. False-Positive Magnetic Resonance Imaging of Small Internal Auditory Canal Tumors: A Clinical Radiologic, and Pathologic Correlation Study. Otolaryngol Head Neck Surg 1995;113(1):61-70. Manabu Maeta, M.D., Ryusuke Saito, M.D., Hideo Nameki, M.D. False-positive magnetic resonance image in the diagnosis of small acoustic neuroma. The Journal of Laryngology & Otology Oct 2001;115:842–844. John W. House, Marc K. Bassim, and Marc Schwartz. False-Positive Magnetic Resonance Imaging in the Diagnosis of Vestibular Schwannoma. Otology & Neurotology 2008;29:1176-1178.